Chapter 1: Light : Reflection and Refraction

Q1. What is Light?

Ans. Light is a form of energy that enables us to see things. Light starts from a source and bounces off objects which are perceived by our eyes and our brain processes this signal, which eventually enables us to see.

Light is a form of energy that enables us to see around us or we can say that it is an electromagnetic wave that can travel through any medium as well as through vacuum.

Q2. What is Rectilinear Propagation of Light?

Ans. Light travels in a straight line in a homogeneous transparent medium, which is known as rectilinear propagation of light.

Cases of light falling on a surface

1. It may get absorbed. The surface that absorbs all the light falling on it appears to be black in colour, or in other words we say that black is a good absorber and emitter of light.
2. When light falls on the surface it may pass through it. But it is not that all light rays would pass. Certain substances allow all rays to pass and few substances allow only certain rays to pass. Those substances which allow all rays to pass appear to be transparent. The process of allowing rays to pass through is called transmit process.

For example: glass is transparent as it allows all rays to pass through it. In this figure we have tinted a plane glass. You can easily notice that tinted glass allows only a few rays to pass and on the other hand, plane glass allows all rays to pass through it.

1. Light falling on a surface may strike the surface and bounce back. This striking and bouncing back of light rays is called reflection. For example: we all often look in the mirror at home when we dress up. We can look at ourselves just because of the phenomenon of reflection.

Q3. What is reflection of light?

Ans. The phenomenon of bouncing back of light into the same medium by the smooth surface is called reflection.

When light ray travelling from one medium bounces back in the same medium after falling on any surface, such a phenomenon is termed as reflection of light.

As you can see the picture given below, that in it the ray is striking the surface and then it bounces back.

There are many surfaces that show reflection but not all surfaces show reflection. To show reflection, there are certain characteristics that must be fulfilled as given below.

Q4. What are Characteristics of Best Reflector?

Ans.

1. It should have a shiny surface
2. It should have a polished surface
3. It should have a smooth surface
4. Out of all, silver metal is the best reflector as it fulfills all the above discussed features.

Q5. Define the terminologies involved in case of reflection

Ans. Look at the figure to understand this in a better way.

1. Incident Ray: It is the ray that strikes the surface.
2. Point of Incidence: It is the point at which the incident ray strikes the surface.
3. Reflected Ray: The ray that bounces back on striking the surface.

1. Normal Ray: The perpendicular drawn to surface.
2. Angle of Incidence: The angle between the incident ray and the normal ray.
3. Angle of Reflection: The angle between the reflected ray and the normal ray.
4. Plane of Reflection: Plane where incident ray reflected ray and normal ray lies.

Q6. What are Laws of Reflection?

Ans. These are certain sets of laws that are obeyed by all surfaces that show reflection.

• 1^st law of reflection: Incident ray, reflected ray and normal ray all lie on the same plane.
• 2^nd law of reflection: Angle of incidence is always equal to the angle of reflection.

i.e [∠i=∠r]

• 3^rd law of reflection The speed, frequency and wavelength of the reflected ray remain unaltered.

Q7. What are Real and Virtual images?

Ans. ∙ An image is formed when the light rays coming from an object meet at a point after reflection from a mirror (or refraction from lens).

The images are of two types

1. 1. 1. Real Images:- Real images are formed when rays of light that comes from an object (or source) meets at a point after reflection from a mirror (or refraction from a lens). Real images can be formed on a screen and can be seen with the eyes.
      2. Virtual images:- Virtual image is an image in which the outgoing rays from an object do not meet at a point. It will appear to meet at a point in or behind the optical device (i.e., a mirror) but they do not actually meet after reflection from a mirror (or refraction from a lens). A plane mirror always forms virtual images

Q8. What are Plane mirror?

Ans.

A plane (flat) glass polished with silver paint on either side is said to be plane mirror. A coat of red paint is required over the silver polish to protect silver layer. Any flat and polished surface that has almost no irregularities on its surface that reflect light is called as a plane mirror.

Q9. What are characteristics of image formation by a plane mirror?

Ans. When two rays actually meet or appear to meet at a point then image is formed.

A plane mirror always forms virtual and erect the distance of object from the mirror is same as the distance of the image of the mirror. Size of image and the object is same. A plane mirror forms laterally inverted

Q10. What is lateral inversion?

Ans. When left side of the object appears right side in the image and vice versa is said to be lateral inversion. There are many uses lateral inversion. We see on Ambulance written laterally inverted and we see it actual in our side mirror.

Q11. What are Spherical Mirrors? What are its types?

Ans. Most common type of curved mirrors are spherical mirrors. Mirrors, in which reflecting surfaces are spherical in shape, are known as spherical mirrors. Reflecting surface of a mirror can be curved inwards or curved outwards. The one which is curved inward is known as concave mirror and the one which curved outwards is known as convex mirror.

Types of spherical mirrors:

There are two types of spherical mirrors:

Concave mirror: A spherical mirror whose reflecting surface is curved inwards is said to be concave mirror. It is also called converging mirror as it converges parallel beam of light falling on it.

Convex mirror: A spherical mirror whose reflecting surface is curved outwards is said to be convex mirror. It is also called diverging mirror as it diverges parallel beam of light falling on it.

Q12. Define commonly used terms about Spherical mirrors.

Ans.

Center of curvature: – The reflecting surface of a spherical mirror forms a part of a sphere. This sphere has a center. This point is called the center of curvature of the spherical mirror. It is represented by the letter C. Please note that the center of curvature is not a part of the mirror. It lies outside its reflecting surface. The center of curvature of a concave mirror lies in front of it. However, it lies behind the mirror in case of a convex mirror.

Radius of curvature: – The radius of the sphere of which the reflecting surface of a spherical mirror forms a part, is called the radius of curvature of the mirror. It is represented by the letter R.

Pole of mirror: – The center of a spherical mirror is called its pole and is represented by letter P

Principle axis: – Straight line passing through the pole and the center of curvature of a spherical mirror is called principle axis of the mirror.

Aperture of the mirror: – Portion of the mirror from which reflection of light actually takes place is called the aperture of the mirror. Aperture of the mirror actually represents the size of the mirror. The diameter of the reflecting surface of spherical mirror is called its aperture.

Focus: When parallel light beam is incident on the spherical mirror, then after reflection, the point at which they meet or appears to meet is said to be focus of that mirror.

Focal length: The length of the line joining pole and focus of the spherical mirror is its focal length. It is denoted by the symbol ‘f’ For spherical mirrors of small aperture the radius of curvature is equal to twice the focal length i.e. R = 2f.

Q13. What are the rules for obtaining images formed by spherical mirrors?

Ans. There are certain rules we should follow to make ray diagrams which will further used in understanding light reflection and refraction.

A beam of parallel rays incident on spherical mirror after reflection passes through focus. In case of concave mirror the reflected ray converges to focus but in case of convex mirror the ray appears to diverge from focus.

• 1. The incident ray when passes through focus after reflection it passes parallel to the principal axis. In case of convex mirror the incident ray coming parallel to the polar axis diverges but appears to pass through the focus.

A ray incident on the spherical mirror passing through center of curvature returns back on same path. In case of convex mirror the incident ray appears to be coming from ‘C’ but rebounds back.

1. 1. A ray incident to the pole of the mirror after reflection makes equal angle with the principal axis.

Q14 Explain image formation by concave mirror.

Ans. ∙ The type of image formed by a concave mirror depends on the position of the object kept in front of the mirror.

We can place the object at following places

• Between pole P and focus
• At the focus
• Between focus F and center of curvature C
• At the center of curvature
• Beyond center of curvature
• At far off distances called infinity and cannot be shown in the figures
• Image formation by a concave mirror for different positions of the object is shown below in the table

Position of the object	Position of the image	Size of the image	Nature of the image
At infinity	At the focus F	Highly diminished	Real and inverted
Beyond C	Between F and C	Diminished	Real and inverted
At C	At C	Same size	Real and inverted
Between C and F	Beyond C	Enlarged	Real and inverted
At F	At infinity	Highly enlarged	Real and inverted
Between P and F	Behind the mirror	Enlarged	Virtual and erect

Q15. Explain image formation by convex mirrors.

Ans. ∙ In order to construct a ray diagram to find out the position, nature and size of image formed by convex mirror , we should remember following path of rays of light.

• A ray of light parallel to the principle axis of a convex mirror appears to be coming from its focus after reflection from the mirror.

• A ray of light going towards the center of curvature of convex mirror is reflected back along its own path.
• Convex mirrors have its focus and center of curvature behind it and no light can go behind the convex mirror and all the rays that we show behind the convex mirror are virtual and no ray actually passes through the focus and center of curvature of the convex mirror.
• Whatever be the position of object in front of convex mirror, the image formed by a convex mirror is always behind the mirror, virtual, erect and smaller than the object.
• Nature, position and relative size of the image formed by a convex mirror is given below in the table

Position of the object	Position of the image	Size of the image	Nature of the image
At infinity	At the focus F, behind the mirror	Highly diminished	Virtual and erect
Between infinity and the pole P of the mirror	Between P and F, behind the mirror	Diminished	Virtual and erect

Q16. What are sign convention used for reflection by spherical mirrors?

Ans. Reflection of light by spherical mirrors follow a set of sign conventions called the New Cartesian Sign Convention. In this convention, the pole (P) of the mirror is taken as the origin. The principal axis of the mirror is taken as the x-axis (X’X) of the coordinate system. The conventions are as follows –

• The object is always placed to the left of the mirror. This implies that the light from the object falls on the mirror from the left-hand side.
• All distances parallel to the principal axis are measured from the pole of the mirror.
• All the distances measured to the right of the origin (along + x-axis) are taken as positive while those measured to the left of the origin (along – x-axis) are taken as negative.
• Distances measured perpendicular to and above the principal axis (along + y-axis) are taken as positive.
• Distances measured perpendicular to and below the principal axis (along -y-axis) are taken as negative.

Q17. What are uses of concave Mirror?

Ans. ∙ It is used in torches: It produces a powerful beam of light rays that converge at infinity.

• It is used in making solar devices: It converges all the rays so when solar rays converge, they increase the temperature at that converging point.
• It is used as shaving mirror: When face is placed between the pole and the focus, it gives virtual, erect and magnified image.
• It is used by dentist to see cavities: When the tooth is placed within the focus it gives virtual and erect image.

Q18. What are uses of convex mirror?

Ans. ∙ Convex mirror is used as the rear view mirror as it covers wide view and the image is always erect.

• In shops it is used as the security mirror. Now CCTV does the same.
• At danger traffic turns it is used to see the opposite comers.

Q19. What is Mirror formula?

Ans. Object distance (u) = In a spherical mirror, the distance of the object from its pole.

Image distance (ν) = Distance of the image from the pole of the mirror.

Focal length (f) = Distance of the principal focus from the pole. Relationship between them is represented as:  1/v+1/u=1/f

Q20. What is Magnification formula?

Ans. Magnification is the ratio of height of image to that of the object. It will take each case of light reflection and refraction either in mirror or lens.

m = Height of image/Height of object= h_i/h_o   Also, m = − v/u

Q21. What are the rules based on magnification?

Ans. ∙ If m is negative then image is real but when it is positive then image is virtual.

• If h = h then m = 1, i.e. image is equal to the object, if h > h then m >1 image is enlarged also if h < h then   i o i o i o    m<1 image is diminished.

• Magnification of plane mirror is always +1.
• If m is +ve and less than 1 then it is convex mirror.
• If m is +ve and more than 1 also when m<1 then it is concave mirror.

Q22. What is refraction of light?

Ans. ∙ We know about light and also know that light travels in a straight line path in a medium or two different  mediums with same density.

• Now a question arises what happens when light travels from one medium to another with different densities for  example from air to glass.

• When light ray is made to travel from one medium to another say from air to glass medium then light rays bend at the boundary between the two mediums.
• So, the bending of light when it passes from one medium to another is called Refraction of light.
• The refraction of light takes place on going from one medium to another because the speed of light is different in two media.

• Medium in which speed of light is more is called optically rarer medium and medium in which speed of light is less is known as optically denser medium. For example glass is an optically denser medium than air and water.

NOTE:- When light goes from rarer medium to denser medium it bends towards the normal and when it goes from denser medium to rarer medium it bends away from the normal.

Types of medium

We have two types of mediums:

• Rarer medium
• Denser medium
• Rarer medium- We can define rarer medium as the medium which has less density and speed of light is more.

Example: air is rarer than water.

• Denser Medium- It is that which has more density and speed of light is lesser in it. Example: water is denser than air.

Please note the type of medium is comparative study as one medium can be rarer in one case and can be denser in another case. For example: water is denser than air but water is rarer in comparison to glass.

Q23. Explain refraction through a rectangular glass slab

Ans. ∙ To understand the refraction of light through a glass slab consider the figure given below which shows the refraction of light through a rectangular glass slab.

• Here in this figure AO is the light ray traveling in air and incident on glass slab at point O.
• Now on entering the glass medium this ray bends towards the normal NN’ that is light ray AO gets refracted on entering the glass medium.
• After getting refracted this ray now travels through the glass slab and at point B it comes out of the glass slab as shown in the figure.

• Since ray OB goes from glass medium to air it again gets refracted and bends away from normal N N’ and goes in direction BC.

• Here AO is the incident ray and BC is the emergent ray and they both are parallel to each other and OB is the refracted ray.

• Emergent ray is parallel to incident ray because the extent of bending of the ray of light at the opposite parallel faces which are PQ (air-glass interface) and SR (glass-air interface) of the rectangular glass slab is equal and opposite.
• In the figure i is the angle of incidence, r is the angle of refraction and e is the angle of emergence.
• Angle of incidence and angle of emergence are equal as emergent ray and incident ray are parallel to each other.
• When a light ray is incident normally to the interface of two media then there is no bending of light ray and it goes straight through the medium.

Q24. Define some important Terms used for refraction of light

Ans. Look at the figure carefully then you will understand the terminology involved:

• Incident Ray: The ray that strikes the surface.

• Point of Incidence: Point where the incident ray strikes the surface.
• Refracted Ray: The ray that changes its path when it travels from one medium to another.
• Normal Ray: The perpendicular drawn to the point of incidence.
• Angle of Incidence: Angle between incident ray and the normal ray.
• Angle of Refraction: Angle between refracted ray and the normal ray.

Q25. What are laws of refraction of light?

Ans. Refraction is due to change in the speed of light as it enters from one transparent medium to another.

Experiments show that refraction of light occurs according to certain laws. So Laws of refraction of light are:

1. The incident ray, the refracted ray and the normal to the interface of two transparent media at the point of incidence, all lie in the same plane.
2. The ratio of sine of angle of incidence to the sine of angle of refraction is a constant, for the light of a given color and for the given pair of media. This law is also known as Snell’s law of refraction.
   • If i is the angle of incidence and r is the angle of refraction then sini/sinr = constant =n (1)

This constant value is called the refractive index of the second medium with respect to the first.

Q26. What is reversibility of path of light Ans.

We have noticed that the path of light is reversible. For example: If light enters as shown in the figure, it undergoes refraction as shown and also, if the light enters in the opposite manner, it will follow the same sequence. The ray that moves out to its original medium after passing through certain medium is emergent ray and it can be defined as:

Emergent Ray: The ray that passes through a certain media and then returns to its original media is known as the emergent ray.

Angle of Emergence: The angle between a normal ray and an emergent ray is known as angle of emergence.

Q27. What is Lateral Displacement?

Ans. An interesting phenomenon that we notice while studying reversibility of light is lateral displacement. It can be defined as the perpendicular shift in the path of incident ray when it travels through a certain medium and then returns to its original medium.

Q28. List some Consequences of Refraction:

Ans. ∙ A stick immersed in water appears to be bent: A stick immersed in water reflects light rays. These rays when travelling in water travel in a straight line path but when they go from water to

air, they deviate from their path and get deflected away from the normal. When these refracted rays are produced, they appear to meet at point “I” which is higher than the actual point that is ‘O’. Therefore, a stick immersed in water appears to be bent due to refraction.

• The water level appears to be raised: When rays moves out from water to air, they will bend away from normal and when produced, they meet at point “I” which is above actual point ‘O’. So, apparent image is at ‘O’ and actual image is at ‘I’.

Q29. What is Refractive Index?

Ans. When light passes from one medium to another medium, it changes its direction. The extent to which the direction of light changes; is expressed in terms of refractive index. The value of refractive index is dependent on the speed of light in two media; v₁ is the speed of light in medium 1 and v₂ is the speed of light in medium 2. The refractive index of medium 2 with respect to medium 1 is represented as n₂₁.

If medium 1 is vacuum or air, then the refractive index of medium 2 with respect to vacuum is known as absolute refractive index of the medium.

Where c is the speed of light in air, v is the speed of light in other medium and n_m is the refractive index of the medium.

Refractive index depends upon the nature of materials, density of medium and wavelength of the color of light. Refractive index of some material media are

• Benzene = 1.50
• Water = 1.33
• Turpentine oil = 1.47
• Diamond = 2:42
• Carbon disulphide = 2.64.

Q30. What is total internal reflection?

Ans.

• When the light goes from a denser to a rarer medium it bends away from the normal. The angle at which the incident ray causes the refracted ray to go along the surface of the two media in parallel direction is called critical angle.
• When the incident angle is greater than the critical angle, it reflects inside the denser medium instead of refracting. This phenomenon is known as Total Internal Reflection.

Q31. List some examples of Total Internal Reflection:

Ans. Following are the examples of total internal reflection:

Diamond: When the incident ray falls on every face of the diamond such that the angle formed, the ray is greater than the critical angle. The critical value of the diamond is 23°.

This condition is responsible for the total internal reflection in a diamond which makes it shine.

Mirage: It is an optical illusion that is responsible for the appearance of the water layer at short distances in a desert or on the road. Mirage is an example of total internal reflection which occurs due to atmospheric refraction.

Optical Fibre: When the incident ray falls on the cladding, it suffers total internal reflection as the angle formed by the ray is greater than the critical angle. Optical fibres have revolutionised the speed with which signals are transferred, not only across cities but across countries and continents making telecommunication one of the fastest modes of information transfer. Optical fibres are also used in endoscopy.

 

Q32. What is Spherical Lens? What are its types?

Ans.

• Lens – A lens is a transparent refracting medium bounded by two curved surfaces which are generally spherical or one of the surfaces is plane.
• Biconvex lens – A lens having its two surfaces bulging outwards is called biconvex lens.
• A double concave lens is bounded by two spherical surfaces, curved inwards.
• Convex lens are called converging and concave lens are called diverging lens.

Types of spherical lens

There are two types of lens the first one is concave lens and the other one is convex lens.

When both the two spherical surfaces bulge outwards, it is known as convex lens. It converges the light rays.

When the two spherical surfaces bulge inwards, they are known as concave lens. They are known as diverging lens.

Note: The rays that fall on convex lens, after refraction, meet at one point so it is a converging lens. On the other hand,a concave lens splits all refracted rays in different directions, so it is a diverging lens.

Q33. Define some terminologies used in spherical lenses

Ans.

• Centre of curvature: The centre of these spherical surfaces is known as centre of curvature, represented by C.
• Principal axis: Any imaginary straight line passing through the centre of curvature of a lens is known as principal axis.
• Optical centre: The central point of the lens is known as optical centre.
• Aperture: The effective diameter of the spherical lens is known as aperture.
• Focus: Point where refracted rays meet or appear to meet. A lens has two principal foci.
• Focal length: The distance between the focus and the optical centre.
• Paraxial Ray: A ray close to principal axis and also parallel to it.
• Pole (P): The midpoint or the symmetric centre of a spherical lens is known as its Optical Centre. It is also called as the pole.

Q34. What are rules for drawing the ray diagrams?

Ans. ∙ A ray of light parallel to principal axis passes/appears to pass through the focus.

A ray passing through the optical centre undergoes zero deviation.

Q35. Explain image formed by the Convex Lens for various positions of the object.

Ans.

Image formation by a convex lens for different positions of the object is shown below in the table

Position of the object	Position of the image	Relative size of the image	Nature of the image
Infinity	At focus F2	Highly diminished, point sized	Real and inverted
Beyond 2F1	Between F2 and 2F2	Diminished	Real and inverted
At 2F1	At 2F2	Same size	Real and inverted
Between F1 and 2F1	Beyond 2F2	Enlarged	Real and inverted
At focus 2F1	At infinity	Infinitely large or highly enlarged	Real and inverted
Between F1 and optical center O	On the same side of the lens as the object	Enlarged	Virtual and erect

Analysis of images formed by convex lens:

Object between O and F (used in case of simple microscope)

Characteristics of image in front of the lens, magnified and virtual-erect.

Object at F (used in case of search lights)

Characteristics of image: formed at infinity, real and inverted and highly magnified.

Object between f and 2f (used in case of optical projector).

Characteristics of image: formed beyond 2f, real inverted and magnified.

Object at 2f (used in case of terrestrial telescope).

Characteristics of image: image formed at 2f ,same size and real inverted.

Object beyond 2f (used in case of photographic camera).

Characteristics of image: image is formed between f and 2f , real, inverted and diminished.

Object at infinity (used as burning glass)

Characteristics of image: formed at f, highly diminished, real and inverted

Q36. Explain image formed by the Concave Lens

Ans.

Nature, position and relative size of the image formed by a concave lens for various positions of the object is given below in the table

Position of the object	Position of the image	Relative size of the image	Nature of the image
At infinity	At focus F	Highly diminished, point- sized	Virtual and erect
Between infinity and opti- cal center O of the lens	Between F1 and optical center O	Diminished	Virtual and erect

Analysis of image formed by concave lens

Object anywhere on p.axis (used in case of correcting myopic eye) Characteristics of image formed: within focus, virtual-erect and diminished.

Object at infinity (used in case of Galilean telescope)

Q37. What are Sign Convention used for Spherical Lenses?

Ans.

• All the distances are measured from the optical center of the lens.
• The distances measured in the same direction as that of incident light are taken as positive.
• The distances measured against the direction of incident light are taken as negative.
• The distances measured upward and perpendicular to the principle axis are taken as positive.
• The distances measured downwards and perpendicular to principle axis is taken as negative.

Q38. What is Lens Formula and Magnification?

Ans. Lens Formula gives the relationship between object distance (u), image image-distance (v) and the focal length (f ) and is expressed as  1/f=1/v−1/u

this formula is valid in all situations for any spherical lens.

• The magnification produced by a lens is defined as the ratio of the height of the image and the height of the object.

• Magnification produced by a lens is also related to the object-distance u, and the image-distance v and is given by m=v/u

Q39. What is Power of lens?

Ans. The ability of lens to converge or diverge incident ray depends upon its focal length. The reciprocal of focal length (in meter) is said to be power of lens. The power P of a lens of focal length f is given by P=1/f

• Power of a convex lens is positive and that of a concave lens is negative.
• The SI unit of power of a lens is ‘dioptre’. It is denoted by the letter D.
• 1 dioptre is the power of a lens whose focal length is 1 meter so, 1D=1m^–1
• Power of a lens is inversely proportional to thickness of the lens.
• The power of combination of n number of lenses is P = P₁ + P₂ + P₃ + P₄ + P_n.

TEXT BOOK QUESTIONS

Q1. Define the principal focus of a concave mirror.

Ans. Light rays that are parallel to the principal axis of a concave mirror converge at a specific point on its principal axis after reflecting from the mirror. This point is called the principal focus of the concave mirror.

Q2. The radius of curvature of a spherical mirror is 20 cm. What is its focal length?

Ans. Radius of curvature (R) = 20 cm

Radius of curvature of the spherical mirror = 2 × Focal length (f)

R = 2f

f = R/2 = 20 / 2 = 10

Therefore, the focal length of the spherical mirror is 10 cm.

Q3. Name the mirror that can give an erect and enlarged image of an object.

Ans. The mirror that can give an erect and enlarged image of an object is Concave Mirror.

Q4. Why do we prefer a convex mirror as a rear-view mirror in vehicles?

Ans. Convex mirror is preferred as a rear-view mirror in cars and vehicles as it gives a wider field of view, which helps the driver to see most of the traffic behind him. Convex mirrors always form an erect, virtual, and diminished image of the objects placed in front of it.

Q5. Find the focal length of a convex mirror whose radius of curvature is 32 cm.

Ans. Radius of curvature (R) = 32 cm

Radius of curvature = 2 × Focal length (f)

R = 2f

f = R/2 = 32/2 = 16

Therefore, the focal length of the given convex mirror is 16 cm.

Q6. A concave mirror produces three times magnified (enlarged) real image of an object placed at 10 cm in front of it. Where is the image located?

Ans Let, ho = h

where,h_i is the height of the image ho is the height of the object

It is given that three times the enlarged real image of the object is produced.

So, hi = −3h (− due to real image formation)

⇒ m =

⇒ 3 =

⇒ v = −30cm

(negative sign due to the formation of inverted image)

Therefore, the location of image from the mirror is at a distance of 30cm and the nature of the image is inverted.

Q7. A ray of light travelling in air enters obliquely into water. Does the light ray bends towards the normal or away from the normal? Why?

Ans. The light ray bends towards the normal. When a light ray enters from an optically rarer medium (which has low refractive index) to an optically denser medium (which has a high refractive index), its speed slows down and bends towards the normal. As water is optically denser than air, a ray of light entering from air into water will bend towards the normal.

Q8. Light enters from air to glass, having a refractive index 1.50. What is the speed of light in the glass? The speed of light in vacuum is 3 x 10⁸ ms^-1.

Ans. Refractive index of a medium (nm) = Speed of light in vacuum/Speed of light in the medium

Speed of light in vacuum (c) = 3 × 10⁸ m/s Refractive index of glass (ng) = 1.50

Speed of light in the glass (v) = Speed of light in vacuum/ Refractive index of glass

= c/n_g

= 3 × 10⁸/1.50 = 2x 10⁸ ms^-1.

Q9. The refractive index of diamond is 2.42. What is the meaning of this statement?

Ans. Diamond has a refractive index of 2.42 which means that the speed of light in diamond will reduce by a factor of 2.42 as compared to its speed in the air.

In other words, the speed of light in diamond is 1/2.42 times the speed of light in vacuum.

Q10. Define 1 dioptre of power of a lens.

Ans. Dioptre is the SI unit of power of lens is denoted by the letter D. 1 dioptre can be defined as the power of a lens of focal length 1 metre.

Q11. A convex lens forms a real and inverted image of a needle at a distance of 50 cm from it. Where is the needle placed in front of the convex lens if the image is equal to the size of the object? Also, find the power of the lens.

Ans. Distance of image from convex lens,v=50cm Distance of object in front of lens,u=?

The image formed is real and inverted. So, the magnification of the lens is −1−1.

It is known that,⇒ −1 =

⇒ −1 =

⇒ u = −50cm

f is the focal length of the lens.

⇒ =

⇒ =

⇒ =

⇒ f =

⇒ f = 25cm = 0.25m

It is known that,

⇒ P =

⇒ P = +4D

Therefore, the object distance from the lens is u=−50cm and power of the lens is P=+4D.

Q12. Find the power of a concave lens of focal length 2 m.

Ans. The focal length of the concave lens (f) = 2 m

Power of lens (P) = 1/f = 1/ (-2) = -0.5D

Q13. Name the type of mirror used in the following situations.

1. Headlights of a car
2. Side/rear-view mirror of a vehicle
3. Solar furnace

Support your answer with reason.

Ans. (a) Concave Mirror: Because concave mirrors can produce a powerful parallel beam of light when the light source is placed at their principal focus.

1. Convex Mirror: Because of its largest field of view.

1. Concave Mirror: Because it concentrates the parallel rays of the sun at a principal focus.

Q14. One-half of a convex lens is covered with a black paper. Will this lens produce a complete image of the object?

Verify your answer experimentally. Explain your observations.

Ans. Yes, the lens produces a complete image of the object with less intensity.

Consider the following two cases:

In the first case the lower half of the lens is covered with black paper. Light rays coming from the object are refracted only from the upper half and the image is formed, whereas in the lower half the light rays are blocked.

In the second case the upper half of the lens is covered with black paper. Light rays coming from the object are refracted only from the lower half and the image is formed, whereas in the upper half the light rays are blocked.

Therefore, change in intensity of the image is observed i.e., the intensity of the image is less and the complete image is formed.

This can be verified experimentally by observing the image of a distant object like a tree on a screen when the lower half of the lens is covered with a black paper.

Q15. A concave lens of focal length 15 cm forms an image 10 cm from the lens. How far is the object placed from the lens? Draw the ray diagram.

Ans. f= -15 cm, v= -10 cm

1/v -1/u = 1/f

1/u = 1/15 – 1/10 = -1/30

u = -30 cm.

Ray diagram as follows:

Q16. The magnification produced by a plane mirror is +1. What does this mean?

Ans. The positive sign means an image formed by a plane mirror is virtual and erect. Since the magnification is 1 it means that the size of the image is equal to the size of the object.

Q17. An object 5.0 cm in length is placed at a distance of 20 cm in front of a convex mirror of radius of curvature 30 cm. Find the position of the image, its nature and size.

Ans. Radius of curvature (R) = 30 cm

f = R/2 = 30/2 = 15 cm u = –20 cm, h= 5 cm.

1/v +1/u = 1/f

1/v = 1/15+ 1/20 = 7/60

v = 60/7 = 8.6 cm. image is virtual and erect and formed behind the mirror.

h_i/h₀ = v/u h_i/5 = 8.6/20

h_i = 2.2 cm.

Size of image is 2.2 cm.

Q18. An object of size 7.0 cm is placed at 27 cm in front of a concave mirror of focal length 18 cm. At what distance from the mirror should a screen be placed, so that a sharp focused image can be obtained? Find the size and the nature of the image.

Ans. u = – 27 cm, f = – 18 cm. h_o= 7.0 cm

1/v = 1/f- 1/u

1/v = -1/18 + 1/27 = -1/54

V = – 54 cm.

Screen must be placed at a distance of 54 cm from the mirror in front of it.

h_i/h₀ = v/u h_i/h₀ = v/u

h_i/7 = +54/-27

h_i = -2 x 7 = -14 cm.

Thus, the image is of 14 cm length and is inverted image.

Q19. Find the focal length of a lens of power -2.0 D. What type of lens is this?

Ans. Power of lens (P) = -2.0 D

P = 1/f or f = 1/m

f = 1/-2.0 = -0.5 m.

(-ve) sign of focal length means that the lens is concave lens.

Q20. A doctor has prescribed a corrective lens of power +1.5 D. Find the focal length of the lens. Is the prescribed lens diverging or converging?

Ans. P = +1.5 D

f = 1/P = 1/+1.5 = 0.67 m.

As the power of lens is (+ve), the lens is converging lens.