Q1. Which part of the human eye acts like a screen?

Answer: The retina acts like a screen on which the image is formed.

Q2. Which part of the eye controls the amount of light entering the eye?

Answer: The iris controls the size of the pupil and regulates the amount of light entering the eye.

Q3. What is the function of the pupil?

Answer: The pupil allows light to enter the eye. Its size changes according to the intensity of light.

Q4. What type of lens is present in the human eye?

Answer: The human eye has a convex lens.

Q5. What is power of accommodation?

Answer: The ability of the eye lens to change its focal length to see nearby and distant objects clearly is called power of accommodation.

Q6. What is the least distance of distinct vision for a normal adult eye?

Answer: The least distance of distinct vision is 25 cm.

Q7. What is the far point of a normal human eye?

Answer: The far point of a normal human eye is infinity.

Q8. Which lens is used to correct myopia?

Answer: A concave lens is used to correct myopia.

Q9. Which lens is used to correct hypermetropia?

Answer: A convex lens is used to correct hypermetropia.

Q10. What is dispersion of light?

Answer: The splitting of white light into its seven constituent colours is called dispersion of light.

Q11. Which colour deviates the most in a glass prism?

Answer: Violet colour deviates the most.

Q12. Which colour deviates the least in a glass prism?

Answer: Red colour deviates the least.

Q13. What is Tyndall effect?

Answer: The scattering of light by colloidal particles is called Tyndall effect.

Q14. Why does the sky appear blue?

Answer: The sky appears blue because blue light is scattered more strongly by small particles in the atmosphere.

Q15. Why are danger signals red?

Answer: Red light has the longest wavelength among visible colours and is scattered the least, so it can be seen from a long distance.

Q1. A person enters a dark room from bright sunlight. Why does it take some time to see objects clearly?

Answer: In bright sunlight, the pupil is small to allow less light into the eye. In a dark room, the pupil needs to expand to allow more light. This adjustment takes some time, so objects are not seen clearly immediately.

Q2. How does the eye lens focus light on the retina?

Answer: The eye lens is a convex lens. It converges light rays coming from an object and forms a real, inverted and diminished image on the retina. The brain interprets this image as erect.

Q3. What is the role of ciliary muscles?

Answer: Ciliary muscles change the curvature and focal length of the eye lens. When they contract, the lens becomes thicker for near vision. When they relax, the lens becomes thinner for distant vision.

Q4. Why can a normal eye see both nearby and distant objects clearly?

Answer: A normal eye can change the focal length of its lens using ciliary muscles. This ability is called accommodation. Due to accommodation, images of both nearby and distant objects are formed clearly on the retina.

Q5. Differentiate between rods and cones.

Q6. What is myopia? How is it corrected?

Answer: Myopia is a defect in which a person can see nearby objects clearly but cannot see distant objects clearly. In myopia, the image of a distant object is formed in front of the retina. It is corrected by using a concave lens of suitable power.

Q7. What is hypermetropia? How is it corrected?

Answer: Hypermetropia is a defect in which a person can see distant objects clearly but cannot see nearby objects clearly. In hypermetropia, the image of a nearby object is formed behind the retina. It is corrected by using a convex lens of suitable power.

Q8. What is presbyopia?

Answer: Presbyopia is an age-related defect of vision caused by weakening of ciliary muscles and reduced flexibility of the eye lens. A person may find it difficult to see nearby objects clearly. Sometimes bifocal lenses are used for correction.

Q9. Why does a prism split white light into seven colours?

Answer: A prism splits white light because different colours of light travel with different speeds in glass and bend by different amounts. This causes dispersion of white light into seven colours.

Q10. What is spectrum?

Answer: The band of seven colours obtained by dispersion of white light is called spectrum. The order is VIBGYOR: violet, indigo, blue, green, yellow, orange and red.

Q11. Why does a rainbow form after rain?

Answer: A rainbow forms because tiny water droplets in the atmosphere act like small prisms. They refract, disperse and internally reflect sunlight, producing a spectrum of colours.

Q12. Why do stars twinkle but planets do not twinkle much?

Answer: Stars are very distant and appear as point sources of light. Atmospheric refraction changes the apparent position and brightness of stars, so they twinkle. Planets appear as extended sources, so the variation in light from different points averages out and they do not twinkle much.

Q13. Why is the sun visible before actual sunrise and after actual sunset?

Answer: Due to atmospheric refraction, sunlight bends as it passes through layers of air of different densities. This makes the sun appear slightly above its actual position. Hence, the sun is visible about 2 minutes before actual sunrise and about 2 minutes after actual sunset.

Q14. Why does the sky appear dark to astronauts?

Answer: In space, there is no atmosphere to scatter sunlight. Therefore, the sky appears dark to astronauts.

Q1. Explain the structure and working of the human eye.

Answer: The human eye is a natural optical instrument that helps us see objects.

• Cornea: A transparent curved surface that allows light to enter the eye and causes most of the refraction.
• Iris: A coloured muscular diaphragm that controls the size of the pupil.
• Pupil: An opening through which light enters the eye.
• Eye lens: A convex lens that focuses light on the retina.
• Ciliary muscles: Muscles that change the curvature of the eye lens to focus nearby and distant objects.
• Retina: A light-sensitive screen containing rods and cones where the image is formed.
• Optic nerve: Carries visual signals from the retina to the brain.

The eye lens forms a real, inverted and diminished image on the retina. The retina converts light into electrical signals, which are sent to the brain through the optic nerve. The brain interprets the image as erect.

Q2. Explain power of accommodation in detail.

Answer: Power of accommodation is the ability of the eye lens to change its focal length so that images of objects at different distances are formed clearly on the retina.

• For distant objects, ciliary muscles relax.
• The eye lens becomes thin.
• Its focal length increases.
• For nearby objects, ciliary muscles contract.
• The eye lens becomes thicker.
• Its focal length decreases.

For a normal adult eye, the near point is 25 cm and the far point is infinity.

Q3. Compare myopia, hypermetropia and presbyopia.

Q4. Explain correction of myopia using a ray concept.

Answer: In myopia, a person cannot see distant objects clearly because rays from distant objects are focused in front of the retina. A concave lens of suitable power is placed before the eye. This lens diverges incoming parallel rays slightly so that the eye lens focuses them exactly on the retina. Therefore, distant objects become clear.

Q5. Explain correction of hypermetropia using a ray concept.

Answer: In hypermetropia, a person cannot see nearby objects clearly because rays from nearby objects are focused behind the retina. A convex lens of suitable power is used. This lens converges the rays before they enter the eye so that the eye lens forms the image on the retina. Thus, nearby objects become clear.

Q6. Explain dispersion of white light through a glass prism.

Answer: When white light enters a glass prism, it refracts at the first surface. Since different colours have different wavelengths and travel with different speeds in glass, they bend by different amounts. Violet bends the most and red bends the least. At the second surface, the colours separate further and a band of seven colours is obtained. This splitting of white light into seven colours is called dispersion.

Order of colours: VIBGYOR

Q7. Explain formation of rainbow.

Answer: A rainbow is formed due to dispersion, refraction and internal reflection of sunlight by tiny water droplets present in the atmosphere.

• Sunlight enters a water droplet and gets refracted.
• White light disperses into seven colours.
• The dispersed light undergoes internal reflection inside the droplet.
• Light refracts again while coming out of the droplet.
• The observer sees a circular arc of seven colours called rainbow.

A rainbow is usually seen when the sun is behind the observer and water droplets are in front.

Q8. Explain atmospheric refraction and its effects.

Answer: Atmospheric refraction is the bending of light due to gradual change in refractive index of different layers of the atmosphere.

Effects of atmospheric refraction:

• Twinkling of stars: The apparent position of a star changes continuously due to atmospheric refraction, causing variation in brightness.
• Advanced sunrise: The sun appears above the horizon before it actually rises.
• Delayed sunset: The sun remains visible for a short time after it actually sets.
• Apparent flattening of the sun: The sun appears slightly flattened near the horizon due to unequal refraction of light from its upper and lower parts.

Q9. Explain scattering of light and its applications.

Answer: Scattering of light is the phenomenon in which light is redirected in different directions by small particles of the medium.

• Very small particles scatter shorter wavelengths like blue and violet more strongly.
• This explains the blue colour of the sky.
• During sunrise and sunset, sunlight travels a longer distance through the atmosphere.
• Blue light gets scattered away, while red and orange light reach our eyes.
• Therefore, the sun appears reddish at sunrise and sunset.
• Red is used in danger signals because it is scattered the least and remains visible from a long distance.

Q10. Explain Tyndall effect with examples.

Answer: Tyndall effect is the scattering of light by colloidal particles. It makes the path of light visible.

Examples:

• Path of sunlight becomes visible in a dusty room.
• Path of projector light becomes visible in a cinema hall due to dust or smoke particles.
• Sunlight passing through a canopy of trees becomes visible due to mist and dust particles.
• A beam of light becomes visible when passed through a colloidal solution.

Q1. A myopic person has far point at 80 cm. Find the nature and power of the lens required.

Solution:

For myopia, the corrective lens must form a virtual image of a distant object at the far point of the person.

For distant object, u = ∞ and v = −80 cm = −0.80 m.

f = −0.80 m

P = 1/f = 1/−0.80 = −1.25 D

Answer: A concave lens of power −1.25 D is required.

Q2. A person cannot see objects beyond 2 m clearly. Find the power of the lens required.

Solution:

The person is suffering from myopia. Far point = 2 m.

f = −2 m

P = 1/f = 1/−2 = −0.5 D

Answer: A concave lens of power −0.5 D is required.

Q3. A hypermetropic person has near point at 75 cm. Find the power of lens required to read at 25 cm.

Solution:

Correcting lens should form a virtual image at the person’s near point.

Object distance, u = −25 cm; image distance, v = −75 cm.

1/f = 1/v − 1/u

1/f = 1/−75 − 1/−25

1/f = −1/75 + 1/25 = 2/75

f = 37.5 cm = 0.375 m

P = 1/0.375 = +2.67 D

Answer: A convex lens of power approximately +2.67 D is required.

Q4. A person needs a convex lens of focal length 50 cm for reading. Find its power.

Solution:

f = +50 cm = +0.50 m

P = 1/f = 1/0.50 = +2 D

Answer: Power of the lens is +2 D.

Q5. A corrective lens has power −4 D. Find its focal length and the defect it corrects.

Solution:

P = 1/f

f = 1/P = 1/−4 = −0.25 m = −25 cm

Answer: Focal length is −25 cm. Negative power means it is a concave lens used to correct myopia.

Q1. A student says that the sky appears blue because the atmosphere absorbs all colours except blue. Is the statement correct?

Answer: No. The sky appears blue mainly because blue light is scattered more strongly by small particles in the atmosphere. It is not because the atmosphere absorbs all other colours.

Q2. A person can read a book clearly but cannot see the blackboard clearly. Identify the defect and correction.

Answer: The person is suffering from myopia. Distant objects are not seen clearly. It is corrected by using a concave lens of suitable power.

Q3. A person can see the blackboard clearly but cannot read a book held at 25 cm. Identify the defect and correction.

Answer: The person is suffering from hypermetropia. Nearby objects are not seen clearly. It is corrected using a convex lens of suitable power.

Q4. Why does the sun appear reddish during sunrise and sunset but white at noon?

Answer: During sunrise and sunset, sunlight travels a longer distance through the atmosphere. Shorter wavelengths like blue and violet are scattered away. Longer wavelengths like red reach our eyes, so the sun appears reddish. At noon, sunlight travels a shorter distance, so most colours reach our eyes together and the sun appears nearly white.

Q5. If a prism is placed upside down in the path of a spectrum produced by another prism, what may happen?

Answer: The second inverted prism can recombine the seven colours to form white light again. This shows that white light is made up of seven colours.

Q6. A star appears slightly higher than its actual position. Which phenomenon explains this?

Answer: This is due to atmospheric refraction. Light from the star bends as it passes through layers of atmosphere of different densities, making the star appear higher than its actual position.

Q7. Why does a beam of light become visible in fog but not in clean air?

Answer: Fog contains tiny water droplets that scatter light. This scattering makes the path of light visible. Clean air has very few particles, so the beam is not easily visible.

Q8. Why is violet at the bottom and red at the top in a primary rainbow?

Answer: Different colours deviate by different amounts inside water droplets. Violet deviates more and red deviates less. Due to the geometry of refraction and internal reflection, red appears on the outer side and violet on the inner side of a primary rainbow.

Q9. Why can’t we see clearly when we move suddenly from a dark room to bright sunlight?

Answer: In a dark room, the pupil is large to allow more light. In bright sunlight, the pupil must contract to reduce the light entering the eye. This adjustment takes a short time, so vision is uncomfortable or unclear initially.

Q10. A person with normal vision grows old and starts having difficulty reading small print. What is the most likely reason?

Answer: The most likely reason is presbyopia, caused by weakening of ciliary muscles and reduced flexibility of the eye lens with age.

Q11. Why is red light preferred in traffic signals instead of blue light?

Answer: Red light has a longer wavelength and is scattered less by dust, fog and smoke. Therefore, it remains visible from a longer distance.

Q12. The image on retina is inverted. Why do we see objects erect?

Answer: The retina sends electrical signals to the brain through the optic nerve. The brain processes these signals and interprets the image as erect.

Q1. Why does the eye lens need to change focal length if the distance between lens and retina is fixed?

Answer: The distance between eye lens and retina cannot change. Therefore, to focus objects at different distances on the retina, the eye lens changes its focal length by changing its curvature.

Q2. Why does myopia occur if the eyeball is too long?

Answer: If the eyeball is too long, light from distant objects is focused before reaching the retina. Therefore, the image becomes blurred and the person cannot see distant objects clearly.

Q3. Why is a concave lens used for myopia and not a convex lens?

Answer: In myopia, rays are focused in front of the retina. A concave lens diverges the rays before they enter the eye, shifting the image back onto the retina. A convex lens would converge rays more and worsen the defect.

Q4. Why is a convex lens used for hypermetropia?

Answer: In hypermetropia, rays from nearby objects are focused behind the retina. A convex lens converges the rays before they enter the eye, helping the image form on the retina.

Q5. Why do all colours not bend equally in a prism?

Answer: Different colours have different wavelengths and travel with different speeds in glass. Therefore, their refractive indices are different and they bend by different amounts.

Q6. Why does the sky not appear violet even though violet scatters more than blue?

Answer: The sun emits less violet than blue, much of violet is absorbed in the upper atmosphere, and human eyes are more sensitive to blue than violet. Therefore, the sky appears blue.

Q7. Why does the sun appear flattened near the horizon?

Answer: Light from the lower part of the sun passes through denser layers of atmosphere and bends more than light from the upper part. This unequal atmospheric refraction makes the sun appear flattened.

Q8. Why can’t we see a rainbow every time after rain?

Answer: A rainbow is visible only when sunlight, water droplets and the observer are in a suitable position. Usually, the sun must be behind the observer and water droplets must be in front.

Q9. Why does the path of light become visible in a dusty room?

Answer: Dust particles scatter light. This scattering makes the path of light visible. This is an example of Tyndall effect.

Q10. Why do clouds appear white although scattering occurs in them?

Answer: Clouds contain relatively larger water droplets that scatter all colours of white light almost equally. Hence, clouds appear white.

Q1. Define power of accommodation. Write the near point and far point of a normal eye.

Answer: Power of accommodation is the ability of the eye lens to change its focal length to focus nearby and distant objects on the retina. The near point of a normal eye is 25 cm and the far point is infinity.

Q2. What is meant by dispersion of light? Name the colour that deviates the most and the least.

Answer: Dispersion is the splitting of white light into seven colours. Violet deviates the most and red deviates the least.

Q3. Explain why stars twinkle.

Answer: Stars twinkle due to atmospheric refraction. Since stars are point sources, small changes in the atmosphere change the path and apparent brightness of their light continuously, causing twinkling.

Q4. Explain why the sky appears blue.

Answer: The molecules and tiny particles in the atmosphere scatter shorter wavelengths of light more strongly. Blue light is scattered more than red light, so the sky appears blue.

Q5. What is cataract?

Answer: Cataract is a condition in which the eye lens becomes cloudy or milky, causing blurred vision. It usually occurs in old age and can be treated surgically.

Q6. Why are red signals used as danger signals?

Answer: Red light has the longest wavelength and is scattered the least by fog, smoke and dust. Therefore, it can be seen clearly from a long distance.

Q7. Explain why planets do not twinkle like stars.

Answer: Planets are closer to Earth and appear as extended sources of light. The changes in brightness from different points of the planet average out. Therefore, planets do not twinkle much.

Q8. Why does the sun appear reddish at sunrise and sunset?

Answer: At sunrise and sunset, sunlight travels a longer path through the atmosphere. Shorter wavelengths are scattered away, while red light reaches our eyes. Hence, the sun appears reddish.

Q1. A person’s far point is 50 cm. What type of lens is required and what is its power?

Solution: The person has myopia. Far point = 50 cm = 0.50 m.

f = −0.50 m

P = 1/f = 1/−0.50 = −2 D

Answer: A concave lens of power −2 D is required.

Q2. A hypermetropic person has near point at 1 m. Find the focal length of the lens needed to read at 25 cm.

Solution:

u = −25 cm, v = −100 cm

1/f = 1/v − 1/u

1/f = −1/100 + 1/25 = 3/100

f = 100/3 = 33.3 cm

Answer: A convex lens of focal length approximately 33.3 cm is required.

Q3. If blue light scatters more than red light, why is red used in signals?

Answer: Red light scatters less than blue light, so it travels farther through fog, smoke and dust without losing intensity. Therefore, red signals are visible from a long distance.

Q4. A beam of white light passes through a prism and then through an identical inverted prism. What will be obtained?

Answer: The first prism disperses white light into seven colours. The second inverted prism recombines these colours to form white light again.

Q5. Why is the apparent position of a star higher than its actual position?

Answer: The atmosphere has layers of different optical densities. Light from a star bends towards the normal as it passes through these layers. Due to this atmospheric refraction, the star appears higher than its actual position.

Q6. Why does a normal eye not see clearly below 25 cm?

Answer: Below 25 cm, the eye lens cannot decrease its focal length enough to form a clear image on the retina. Therefore, the image becomes blurred.

Q7. A person needs a lens of power +3 D. Is the lens converging or diverging? What is its focal length?

Solution:

Positive power means the lens is convex and converging.

f = 1/P = 1/3 m = 0.333 m = 33.3 cm

Answer: The lens is converging and its focal length is 33.3 cm.

Q8. If the atmosphere suddenly disappeared, name two optical phenomena that would not occur.

Answer: Twinkling of stars and blue colour of the sky would not occur. Advanced sunrise and delayed sunset would also not occur.

Q1. Assertion: The human eye lens changes its focal length. Reason: Ciliary muscles change the curvature of the eye lens.

Answer: A. Both Assertion and Reason are true and Reason is the correct explanation.

Q2. Assertion: Myopia is corrected using a convex lens. Reason: A convex lens converges light rays.

Answer: D. Assertion is false but Reason is true. Myopia is corrected using a concave lens.

Q3. Assertion: White light splits into seven colours through a prism. Reason: Different colours bend by different amounts in glass.

Answer: A. Both Assertion and Reason are true and Reason is the correct explanation.

Q4. Assertion: Stars twinkle due to atmospheric refraction. Reason: The refractive index of atmosphere remains perfectly constant.

Answer: C. Assertion is true but Reason is false. The refractive index of the atmosphere changes continuously.

Q5. Assertion: The sky appears blue. Reason: Blue light is scattered more than red light by small particles in the atmosphere.

Answer: A. Both Assertion and Reason are true and Reason is the correct explanation.

Q6. Assertion: Red light is used in danger signals. Reason: Red light is scattered the least among visible colours.

Answer: A. Both Assertion and Reason are true and Reason is the correct explanation.