Q1. Explain the structure of the human eye.
Ans. The human eye works like a natural camera, capturing light and turning it into images your brain can understand. Here’s a simple guide to its main parts and how they work:
1. Outer Layer
Cornea: Clear, dome-shaped “window” at the front of the eye.
Bends light to help focus it into the eye (like a camera lens).
Protects the eye from dust and germs.
Sclera: The tough, white outer layer (the “white of the eye”).
Gives the eye its shape and protects the inner parts.
2. Middle Layer
Iris: The colorful part of the eye (blue, brown, green, etc.).
Controls the size of the pupil (the black circle in the center).
Pupil: The hole in the center of the iris.
Acts like a camera’s shutter:
Expands in dim light (to let in more light).
Contracts in bright light (to protect the eye).
Lens: A clear, flexible disc behind the iris.
Changes shape to focus light onto the retina (like adjusting a camera’s zoom).
Ciliary Muscles: Tiny muscles around the lens.
Squeeze or relax to make the lens thicker (for near objects) or thinner (for far objects).
Choroid: A dark layer with blood vessels.
Supplies oxygen and nutrients to the retina.
3. Inner Layer (Retina)
Retina: Acts like the “film” or “screen” at the back of the eye.
Contains light-sensitive cells called rods and cones:
Rods: Work best in dim light (like at night).
Detect shapes, movement, and shades of gray.
Cones: Work best in bright light.
Detect color and fine details (like reading or recognizing faces).
Macula: A tiny yellow spot in the center of the retina.
Packed with cones for sharp, clear vision (e.g., reading or seeing faces).
Optic Nerve: Carries electrical signals from the retina to the brain.
The spot where the optic nerve connects to the retina has no rods or cones—this is your blind spot!
(You don’t notice it because your brain fills in the missing info.)
4. Fluids Inside the Eye 
Aqueous Humor: Watery liquid between the cornea and lens.
Keeps the eye inflated and nourishes the cornea and lens.
Vitreous Humor: Thick, jelly-like gel filling the back of the eye.
Helps the eye keep its round shape.
Q2. Explain working of human eye.
Ans. The human eye works like a super-smart camera, capturing light and turning it into images your brain understands. Here’s a step-by-step breakdown in simple terms:
1. Light Enters the Eye
Cornea: Light first passes through the clear, curved cornea (the eye’s “window”). The cornea bends (refracts) the light to start focusing it.
Pupil: The light then goes through the pupil (the black dot in the center of your eye). The iris (the colored part) adjusts the pupil’s size:
Bright light → Pupil shrinks (to protect the eye from too much light).
Dim light → Pupil expands (to let in more light).
2. Focusing the Light
Lens: Behind the pupil is the lens, a clear, flexible disc. The lens fine-tunes the focus by changing shape:
Ciliary Muscles (tiny muscles around the lens) pull or relax to make the lens:
Thicker → Focuses on near objects (like reading).
Thinner → Focuses on far objects (like a whiteboard).
The lens flips the light rays upside-down and projects them onto the retina.
3. Capturing the Image
Retina: The retina (at the back of the eye) acts like a movie screen. It has millions of light-sensitive cells:
Rods: Detect shapes, movement, and work best in dim light (like at night).
Cones: Detect colors and fine details (like reading or recognizing faces).
Macula: A tiny spot in the retina packed with cones for sharp, clear vision (e.g., reading).
4. Sending Signals to the Brain
Optic Nerve: The rods and cones turn light into electrical signals. These signals travel through the optic nerve (like a cable) to the brain.
Brain Processing:
The brain flips the upside-down image from the retina so you see the world right-side-up.
It combines signals from both eyes to create a 3D image.
The brain also fills in gaps (like your blind spot, where the optic nerve connects to the retina).
5. Adapting to Light and Dark
Bright Light:
Pupil shrinks.
Cones work hard to show colors and details.
Dim Light:
Pupil expands.
Rods take over to help you see shapes and movement (but not colors clearly).
Q3. Explain adjustment of the size of the pupil according to the intensity of light.
Ans. When we are exposed to bright light the iris constricts the pupil partially or we can say that the pupil shortens so that the right amount of light enters and a clear image is formed.
Whereas in a dark room, Pupil expands itself to gather more light in order to obtain a clear image.
Q4. Why are we not able to see the things clearly when we come out of a darkroom?
Ans. When we are in darkness, our pupils dilate (expand) to allow more light into the eyes. Upon stepping into a brightly lit area, the pupils must constrict (shrink) to reduce light intake. During this brief adjustment period, vision is temporarily impaired as the eyes adapt.
Similarly, when moving from a brightly lit environment into a dimly lit room, there’s a delay in clear vision. In bright light, the iris contracts the pupil to limit light entry, protecting the retina. In dim conditions, the iris dilates the pupil to maximize light absorption. When transitioning from brightness to darkness, the pupil’s delayed expansion creates a momentary lag in visual clarity until the eyes fully adjust.
This adaptive process, governed by the iris muscles, ensures optimal light regulation for vision but requires time to shift between extremes of light intensity.
Q5. What is accommodation of eye?
Ans. The ability of the eye lens to adjust its focal length to form a clear image on the retina, which can be easily recognized by the brain, is called accommodation.
Focusing on Distant Objects:
When looking at a far-off object, the ciliary muscles relax.
This relaxation causes the lens to become thinner and elongated.
As a result, the focal length of the lens increases, allowing the eye to focus on distant objects clearly.
Focusing on Nearby Objects:
When looking at a nearby object, the ciliary muscles contract.
This contraction makes the lens thicker and shorter.
Consequently, the focal length of the lens decreases, helping the eye focus on close objects.
Least Distance of Distinct Vision:
The least distance of distinct vision is the minimum distance at which an object can be seen clearly. It is also known as the near point of the eye. For young individuals, this distance is about 25 cm.
Far Point of the Eye:
The far point of the eye is the farthest distance at which an object can be seen clearly. For a normal human eye, this distance ranges from 25 cm to infinity.
Q6. What are Defects of the Vision? How are they Corrected?
Ans. Sometimes people cannot see the objects clearly and comfortably. They have blurred vision due to refractive defects of the eye.
The common defects observed in human eye are:
1.Myopia
2. Hypermetropia
3. Presbyopia
Myopia or short-sightedness :
It is a defect in which a person is unable to see far objects clearly but can see nearby objects. The cause for this is that the ciliary muscles do not relax properly, the lens does not elongate properly due to which the focal length does not increase properly. As a result no clear image is formed.
They have far point nearer than the infinity.
The image of distant objects is formed in front of retina (not on the retina).
Causes of myopia:
Excessive curvature of the eye lens, Elongation of the eyeball
Due to this, the image is formed in front of the retina and can’t be identified by the brain.
Correction: Correction can be done by using spectacles containing concave lens that diverge the rays first so that our eye lens can converge them properly on the retina.
- Hypermetropia (Long sightedness): It is a defect in which a person is unable to see nearby objects but can see far off objects clearly. The cause is that the ciliary muscles do not contract properly, the lens does not become thick and short due to which the focal length doesn’t decrease. As a result, the image formed is not clear and can’t be identified by the brain.
Causes of myopia:
Eyeballs being too short
Converging power of the lens being too low
Due to this, the image is formed behind the retina.
Correction:
- By using corrective eyeglasses: Depending on how much the focal length has been altered due to the effects of hyperopia, powered convex lenses can correct this problem. It is also the safest option with minor inconveniences.
- By using contact lenses: Using this has the same effect as that of using eyeglasses but presents a more
convenient and comfortable option. Using contact lenses isn’t safe for everyone, however.
- Refractive surgery: Since the refractive power of the eye is affected by hyperopia, altering this through surgery is another way to counter the defects.
- Presbyopia: When human becomes older (human ageing) the accommodation power of eye starts decreasing. The image of the near point is formed far behind the lens thus make it difficult to see nearby as well as the far objects clearly.
Cause of presbyopia:
Human ageing
Due to weakening of ciliary muscles
Reduced lens flexibility
Correction: Such a defect can be corrected using bi-focal lens. Bifocal lens contains both concave lens and convex lens. Concave in the upper portion and convex in the lower portion of the spectacles 
It can also be corrected by surgery by using suitable contact lens.
Cataract:
- Cataract is a condition of partial or complete loss of vision.
- During old age, the crystalline lens of some people becomes milky and cloudy.
- This causes partial or complete loss of vision; it is possible to restore vision through a cataract surgery.
- In this surgery, the cloudy lens is removed and replace with an artificial lens.
Q7. Explain refraction through glass prism.
Ans. Prism: It is a piece of glass or any transparent material bounded by triangular and three rectangular surfaces. The rectangular surfaces are called refracting surfaces. The angle between two refracting surfaces is called refracting angle or angle of prism.
- The line along which the two refracting surfaces meet is called refracting the edge. Any section of prism which is perpendicular to refracting edge is called principal section of edge.
- Angle between two refracting surface is called angle of refraction or angle of prism. It is represented by A.
Refraction of Light through Prim:
We can see that a ray of light is entering from air to gla.ss at the first surface AB.
The light ray on refraction has bent towards the normal.
At the second surface AC, the light ray has entered from glass to air. Hence it has bent away from normal. The peculiar shape of the prism makes the emergent ray bend at an angle to the direction of the incident ray. This angle is called the angle of deviation. In this case D is the angle of deviation.
Q8. Explain Dispersion of White Light by Glass Prism.
Ans. When light passes through a prism, it splits into a band of colors. This sequence of colors is VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange, and Red). The band of these colors is called the spectrum. The process of splitting white light into its different colors is known as dispersion. This occurs because light rays bend at different angles when passing through the prism. 
-Violet light bends the most.
-Red light bends the least.
A rainbow is a natural example of dispersion, as sunlight splits into different colors when passing through water droplets in the atmosphere.
Colour spectrum: When a beam of white light is passes through a glass prism, the band of seven colours formed is called spectrum of white light or colour spectrum.
Q9. What is recombination of colour spectrum?
Ans.
Isaac Newton was the first to use a glass prism to split sunlight into a spectrum of colors. He then conducted an experiment to further understand this phenomenon:
Newton tried to split the colors further using another similar prism but found that no new colors were formed.
He then placed a second identical prism in an inverted position relative to the first.
As all the colors of the spectrum passed through the second prism, a beam of white light emerged from the other side.
This experiment led Newton to conclude that sunlight is made up of seven colors (VIBGYOR). Any light that produces a similar spectrum is called white light.
The process of combining the seven colors back into white light is known as recomposition of light.
Q10. Explain formation of rainbow:
Ans. 
Rainbows usually appear after rain. They form as a circular arc of colors in the sky when sunlight passes through raindrops during or after a shower.
How is a Rainbow Formed?
Tiny raindrops in the air act like prisms.
When white sunlight enters a raindrop, it undergoes refraction (bending) and dispersion (splitting into different colors).
The different colors bend at different angles.
The light then hits the inner surface of the raindrop and undergoes total internal reflection (bouncing back inside the drop).
Finally, the light is refracted again as it exits the raindrop, and the colors spread out.
Why Do We See a Rainbow?
Due to dispersion and internal reflection, different colors of light reach our eyes from different raindrops.
A rainbow is always seen opposite to the sun in the sky.
Since raindrops act like tiny prisms, a rainbow is a natural example of dispersion.
Q11. What is atmospheric refraction?
Ans. Refraction and Atmospheric Effects
When light travels from one medium to another, it bends due to a difference in the refractive indices of the two media.
Refractive Index and Temperature
Air’s refractive index depends on its temperature.
Cold air is denser, resulting in a higher refractive index.
Hot air is less dense, leading to a lower refractive index.
Seasonal example:
In winter, cold air has a higher refractive index.
In summer, hot air has a lower refractive index.
Light Moving Between Hot and Cold Air
When light travels from hot air (lower refractive index) to cold air (higher refractive index):
The light bends toward the normal (the imaginary perpendicular line at the point of incidence).
This bending occurs because light slows down in the denser medium (cold air).
Atmospheric Refraction
As light from space enters Earth’s atmosphere:
It passes through layers of air with varying temperatures (and thus varying refractive indices).
The continuous change in refractive index causes continuous refraction, bending the light along a curved path.
This phenomenon is called atmospheric refraction.
Examples of Atmospheric Refraction
Twinkling of stars: Caused by fluctuating air layers bending starlight.
Mirages: Occur when light bends due to temperature gradients near hot surfaces.
Q12. Explain certain phenomenon observed in atmospheric refraction
Ans.
1. Twinkling of Stars (Scintillation)
Cause: Starlight passes through layers of air with varying temperatures and densities. These fluctuations cause continuous refraction, bending the light in random directions.
Effect: The apparent position and brightness of the star change rapidly, making it “twinkle.”
Why planets don’t twinkle: Planets are closer and appear as extended objects (not point sources). The light from different parts of the planet averages out the refraction effects, minimizing twinkling.
2. Mirages
Cause: Extreme temperature gradients near hot surfaces (e.g., deserts or roads) create layers of air with sharply different refractive indices. Light bends gradually, and at a critical angle, it undergoes total internal reflection.
Effect:
Inferior mirage (common on roads): Light from the sky bends upward, creating the illusion of a reflective water-like surface.
Superior mirage (over cold surfaces): Light bends downward, making distant objects (e.g., ships) appear to float above the horizon.
3. Apparent Position of Celestial Objects
Cause: Atmospheric refraction bends light from the sun, moon, or stars as they near the horizon.
Effect:
Early sunrise: We see the sun about 2 minutes before it actually rises above the horizon.
Delayed sunset: The sun remains visible for about 2 minutes after it has dipped below the horizon. 
Flattened sun at sunrise/sunset: Refraction is greater at the horizon, distorting the sun’s shape.
4. Advanced Sunrise and Delayed Sunset
Mechanism: The Earth’s atmosphere acts like a prism, bending sunlight toward the observer even when the sun is geometrically below the horizon.
Outcome: Daylight lasts slightly longer than it would in the absence of an atmosphere.
Cause: During sunset or sunrise, shorter wavelengths (green/blue) refract more than longer ones (red). When the air is exceptionally clear and stable, the green light briefly becomes visible.
Effect: A fleeting green spot or flash appears at the top edge of the sun’s disk.
6. Why Stars Near the Horizon Appear Reddish
Cause: Light from stars near the horizon travels through more atmosphere. Shorter wavelengths (blue/green) scatter away (Rayleigh scattering), leaving longer wavelengths (red/orange) to dominate.
Effect: Stars close to the horizon appear redder and dimmer.
Q13. What is scattering of the light?
Ans. Scattering of the light is a very important phenomenon of our daily life.
Spreading of light in different directions on passing through regions of small particles is called scattering of light.
This scattering is the result of the presence of various particles in the atmosphere.
Our planet earth consists of various mixtures of particles like smoke, molecules of air, dust particles and water drop- lets. These diffused particles reflect the light before it reaches the earth.
Q14. List some applications of Scattering of light:
Ans. The sky appears blue: 
Atmosphere contains fine particles which are much smaller in size as compared to the wavelength of visible light. These finer particles effectively scatters blue light rather than red light as red light has wavelength of 8 times greater than that of blue light. As sunlight passes through the atmosphere, the fine particles in air scatter the blue colour. If there would be no atmosphere the sky would have looked dark.
The danger signal lights are red in colour
The danger signal lights are red in colour because the red light having high wavelength is least scattered by fog or smoke. Therefore, it can be seen in the same colour at a distance.
Colour of the Sun at Sunrise and Sunset:
During sunrise and sunset, light from the Sun near the horizon passes through thicker layers of air and larger dis- tance in the earth’s atmosphere before reaching our eyes. Light from the Sun overhead would travel relatively shorter distance, resulting in white appearance of sun. Near the horizon, most of the blue light and shorter wavelengths are scattered away by the particles. Therefore, the light that reaches our eyes is of longer wavelengths, hence the reddish appearance.
Text Book Questions
Q1. What is meant by the power of accommodation of the eye?
Ans. The power of accommodation refers to the ability of the eye’s lens to adjust its focal length by changing its shape (via the ciliary muscles). This allows the eye to focus clearly on both distant objects (by flattening the lens) and near objects (by thickening the lens), ensuring sharp images form on the retina.
Q2. A person with a myopic eye cannot see objects beyond 1.2 m distinctly. What type of corrective lens should be used?
Ans. Myopia (short-sightedness) is corrected using a concave (diverging) lens with a focal length equal to the eye’s far point (1.2 m in this case). The lens diverges incoming light rays, reducing the eye’s excessive focusing power and allowing the image to form correctly on the retina.
Q3. What are the far point and near point of a human eye with normal vision?
Ans.
Near Point: The closest distance at which an object can be seen clearly without strain. For a normal eye, this is 25 cm.
Far Point: The farthest distance at which objects can be seen distinctly. In a normal eye, this is infinity.
Q4. A student has difficulty reading the blackboard while sitting in the last row. What defect is this, and how is it corrected?
Ans. The student has myopia (short-sightedness), where distant objects appear blurry. Correction involves using a concave lens of appropriate power. This lens diverges light rays before they enter the eye, ensuring the image focuses precisely on the retina instead of in front of it.
Q5. A person needs a lens of power -5.5 dioptres for correcting his distant vision. For correcting his near vision he needs a lens of power +1.5 dioptre. What is the focal length of the lens required for correcting (i) distant vision, and (ii) near vision?
Ans. The power (P) of a lens of focal length f is given by the relation
Power (P) = 1/f
-
- Power of the lens (used for correcting distant vision) = – 5.5 D Focal length of the lens (f) = 1/P
f = 1/-5.5
f = -0.181 m
The focal length of the lens (for correcting distant vision) is – 0.181 m.
-
- Power of the lens (used for correcting near vision) = +1.5 D
- Power of the lens (used for correcting near vision) = +1.5 D
Focal length of the required lens (f) = 1/P
f = 1/1.5 = +0.667 m
The focal length of the lens (for correcting near vision) is 0.667 m.
Q6. The far point of a myopic person is 80 cm in front of the eye. What is the nature and power of the lens required to correct the problem?
Ans. To correct the myopia the person concerned should use concave lens of focal length (f) = -80 cm = -0.80 m Power of lens (P) = 1/f(m) = 1/-0.80 = 100/-80 = -1.25 D.
Q7. Make a diagram to show how hypermetropia is corrected. The near point of a hypermetropic eye is 1 m. What is the power of the lens required to correct this defect? Assume that near point of the normal eye is 25 cm.
Ans. Diagram representing the correction of hypermetropia is a follows:
Near point of defective eye is 1 m and that of normal eye is 25 cm. Here, u = -25 cm, v = -1m = 100 cm.
Using lens formula
| 1/f | = | 1/v – 1/u |
|---|---|---|
| 1/f | = | 1/-100 + 1/25 = 3/100 |
| f | = | 100/3 cm = 1/3m. |
| P | = | 1/f(m) = 1/0.33 = +3.0 D. |
Q8. Why is a normal eye unable to see objects placed closer than 25 cm clearly?
Ans. A normal eye cannot focus on objects closer than 25 cm (the near point) because the ciliary muscles and lens have physical limits. To focus on near objects, the lens must become more convex (rounded). Beyond 25 cm, the ciliary muscles cannot contract further to thicken the lens enough, and the light rays diverge too sharply to converge on the retina.
Q9. What happens to the image distance in the eye when the object’s distance increases?
Ans. The image remains focused on the retina regardless of the object’s distance. When viewing distant objects, the ciliary muscles relax, causing the lens to flatten (reducing curvature). This increases the lens’s focal length, allowing the eye to focus on faraway objects without changing the image distance.
Q10. Why do stars twinkle?
Ans. Stars twinkle (scintillation) due to atmospheric refraction. As starlight passes through Earth’s turbulent atmosphere, varying air density and temperature cause rapid, random bending of light. This distorts the star’s apparent position and brightness, creating the twinkling effect.
Q11. Why don’t planets twinkle?
Ans. Planets do not twinkle because they are closer to Earth and appear as extended disks (not point sources like stars). Light from different parts of a planet averages out atmospheric distortions, minimizing fluctuations in brightness or position.
Q12. Why does the Sun appear reddish early in the morning?
Ans. At sunrise/sunset, sunlight travels through a thicker layer of atmosphere. Shorter wavelengths (blue/green) scatter away via Rayleigh scattering, while longer wavelengths (red/orange) pass through. This leaves predominantly red light reaching our eyes, making the Sun appear reddish.
Q13. Why does the sky appear dark to astronauts?
Ans. In space, there is no atmosphere to scatter sunlight. On Earth, Rayleigh scattering by air molecules causes the blue sky. Astronauts see darkness because without atmospheric particles, light travels straight and unscattered, leaving the sky black.