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Q1. The normal far point of the eye is
The normal far point of the eye is at infinity. This means a normal eye can see objects at any distance (from the near point of 25 cm to infinity) clearly without straining. The eye lens becomes thin and flat enough to focus parallel rays from distant objects exactly on the retina.
Q2. At noon, the Sun appears white because
At noon, the Sun appears white because very little scattering occurs. At this time, the Sun is directly overhead, so sunlight travels the shortest distance through the atmosphere. Since the path is short, very little scattering of blue light occurs, and all colours of sunlight reach our eyes almost equally. The combination of all colours appears white.
Q3. The Sun is visible about how many minutes before actual sunrise?
The Sun is visible about 2 minutes before actual sunrise due to atmospheric refraction. When the Sun is just below the horizon, its light is bent by the atmosphere, making it appear above the horizon. This advance sunrise, combined with delayed sunset (about 2 minutes), increases the apparent duration of daylight by about 4 minutes.
Q4. In a prism, the emergent ray is
In a prism, the emergent ray is deviated from the incident ray (it bends). Unlike a glass slab (where the emergent ray is parallel to the incident ray), a prism’s non-parallel surfaces cause the light to bend at an angle. This deviation is what allows a prism to disperse white light into its component colours.
Q5. White light is a mixture of
White light is a mixture of seven colours: Violet, Indigo, Blue, Green, Yellow, Orange, and Red (VIBGYOR). This was discovered by Sir Isaac Newton using a prism. When white light is dispersed, these seven colours become visible. The combination of all seven colours in the right proportions creates white light.
Q6. Which colour bends the most in a prism?
Violet light bends the most in a prism because it has the shortest wavelength among visible colours. Shorter wavelengths travel slower in glass, so they are refracted more. Red light (longest wavelength) bends the least. The order from most to least bending is: Violet > Indigo > Blue > Green > Yellow > Orange > Red.
Q7. The iris controls
The iris controls the amount of light entering the eye by adjusting the size of the pupil. In bright light, the iris contracts the pupil (makes it smaller) to reduce light entry. In dim light, the iris dilates the pupil (makes it larger) to allow more light in. The iris is the coloured part of the eye.
Q8. Sky appears dark in space because
The sky appears dark in space because there is no air (atmosphere) to scatter sunlight. On Earth, the atmosphere scatters blue light, making the sky appear blue. In space, without an atmosphere, there are no particles to scatter light, so the sky appears black even when the Sun is shining.
Q9. Danger signals are red because red light
Danger signals are red because red light scatters the least by air molecules and particles. Red light has the longest wavelength, so it travels the farthest without being scattered away. This makes red signals visible from a long distance, even in fog, mist, or rain. This is why red is used for stop signs and danger signals.
Q10. Presbyopia occurs due to
Presbyopia occurs due to ageing. As a person ages (usually after age 40), the eye lens becomes harder and less flexible, and the ciliary muscles weaken. This reduces the eye’s power of accommodation, making it difficult to focus on nearby objects. Presbyopia is a natural part of ageing and is corrected using reading glasses.
Q11. Stars twinkle because they are
Stars twinkle because they are very distant point sources of light. Because stars are extremely far away, they appear as just a point of light. Atmospheric refraction affects this point of light, causing it to appear to move and change brightness. Planets are extended sources (small disks), so atmospheric effects average out and they do not twinkle.
Q12. The Sun appears earlier at sunrise due to
The Sun appears earlier at sunrise (advance sunrise) due to atmospheric refraction. When the Sun is just below the horizon, its light passes through the atmosphere and bends (refracts) due to varying air density. This bending makes the Sun appear slightly higher than its actual position, allowing us to see it a few minutes before it actually rises.
Q13. Image formed on retina is
The image formed on the retina is inverted (upside down). The eye lens forms a real and inverted image of an object on the retina. However, our brain interprets this inverted image as upright by processing the signals it receives from the retina. This is similar to how a camera forms an inverted image on film or a sensor.
Q14. A prism has how many refracting surfaces?
A prism has two refracting surfaces (the two inclined faces through which light enters and exits). These two surfaces are non-parallel and meet at an angle called the angle of the prism. Light refracts at both surfaces—first when entering and then when leaving the prism. The third face is the base (non-refracting).
Q15. Newton proved that white light is composed of colours using
Newton proved that white light is composed of colours using a prism. In 1666, he passed a narrow beam of sunlight through a prism and observed the spectrum of colours (VIBGYOR). He also showed that these colours could be recombined to form white light using a second prism, proving that white light is a mixture of seven colours.
Q16. A ray of light bends towards the normal when it travels from
A ray of light bends towards the normal when it travels from a rarer medium (like air) to a denser medium (like glass or water). In this case, the light slows down and bends towards the normal. The angle of refraction is smaller than the angle of incidence. This is a fundamental law of refraction.
Q17. The pupil appears black because
The pupil appears black because no light comes out of it. Light enters the eye through the pupil and is absorbed by the dark choroid layer and retina inside the eye. Since very little light is reflected back out, the pupil looks black. This is similar to why a hole in a box appears black—light goes in but does not come out.
Q18. Hypermetropia can be corrected by
Hypermetropia (far-sightedness) is corrected using a convex lens. In hypermetropia, the image of nearby objects is formed behind the retina. A convex lens converges light before it enters the eye, moving the focus forward onto the retina. Convex lenses have positive power. Myopia is corrected with concave lenses.
Q19. Twinkling of stars is caused by
Twinkling of stars is caused by atmospheric refraction. As starlight passes through different layers of the atmosphere with varying densities, it is continuously refracted (bent). This causes the apparent position and brightness of the star to change rapidly, making it appear to twinkle. Planets do not twinkle because they are extended sources.
Q20. The human eye lens is
The human eye lens is convex (biconvex), meaning it is thicker in the middle and thinner at the edges. A convex lens converges light rays. The eye lens, along with the cornea, focuses light onto the retina. The convex shape allows the lens to change its curvature for accommodation (near and distant vision).
Q21. The sequence VIBGYOR represents
The sequence VIBGYOR represents the colours of the spectrum: Violet, Indigo, Blue, Green, Yellow, Orange, and Red. This is the order of colours from the one that bends the most (Violet) to the one that bends the least (Red). VIBGYOR is a common mnemonic used in schools to remember the colours of the rainbow.
Q22. Which phenomenon explains blue sky and red sunset?
Scattering explains both the blue colour of the sky and the red colour of the Sun at sunset. During the day, blue light is scattered more by air molecules, making the sky blue. At sunset, sunlight travels through more atmosphere, and blue light is scattered away, leaving red light to reach our eyes. Both phenomena are caused by the same scattering process.
Q23. Loss of accommodation occurs mainly due to
Loss of accommodation occurs mainly due to weak ciliary muscles and hardening of the eye lens. With age, the ciliary muscles become weaker and the lens becomes less flexible. This reduces the eye’s ability to change the shape of the lens for focusing on nearby objects. This condition is called presbyopia.
Q24. The eye adjusts focus by changing
The eye adjusts focus by changing the focal length of the lens. This is called accommodation. The ciliary muscles change the shape of the lens—making it thicker (shorter focal length) for near vision and thinner (longer focal length) for distant vision. The retina position and image distance remain fixed.
Q25. Apparent flattening of the Sun at sunrise is due to
The apparent flattening of the Sun’s disc at sunrise is due to atmospheric refraction. When sunlight passes through the thicker layers of the atmosphere near the horizon, the lower part of the Sun is refracted more than the upper part. This causes the Sun’s disc to appear oval or flattened, instead of perfectly round.
Q26. Myopia can be corrected by
Myopia (near-sightedness) is corrected using a concave lens. In myopia, the image of distant objects is formed in front of the retina. A concave lens diverges (spreads out) light rays before they enter the eye, reducing the excessive convergence and moving the focus onto the retina. Concave lenses have negative power.
Q27. Which colour bends the least in a prism?
Red light bends the least in a prism because it has the longest wavelength among visible colours. Longer wavelengths travel faster in glass, so they are refracted less. Violet light (shortest wavelength) bends the most. The order from least to most bending is: Red < Orange < Yellow < Green < Blue < Indigo < Violet.
Q28. A rainbow is formed due to
A rainbow is formed due to dispersion and internal reflection of sunlight inside raindrops. The light enters the raindrop, disperses into colours, reflects off the inner surface, and then emerges. The combination of dispersion (splitting into colours) and total internal reflection creates the beautiful colours of the rainbow.
Q29. The band of colours obtained from white light is called
The band of colours obtained from white light is called a spectrum. A spectrum consists of seven colours: Violet, Indigo, Blue, Green, Yellow, Orange, and Red (VIBGYOR). The spectrum is formed when white light is dispersed into its component colours, like in a prism or rainbow.
Q30. Shorter wavelengths are scattered
Shorter wavelengths (like blue and violet light) are scattered more by air molecules. According to Rayleigh scattering, the amount of scattering is inversely proportional to the fourth power of wavelength. This means shorter wavelengths are scattered much more than longer wavelengths. This is why the sky appears blue—blue light is scattered more.
Q31. Blue colour of the sky is due to scattering of
The blue colour of the sky is due to scattering of blue light by air molecules. Blue light has a shorter wavelength and is scattered more than other colours. This scattered blue light reaches our eyes from all directions, making the sky appear blue. Without scattering, the sky would appear black.
Q32. Refraction of light occurs due to change in
Refraction of light occurs due to a change in the speed of light when it travels from one medium to another. When light enters a different medium, its speed changes, causing it to bend. The frequency remains constant, while the wavelength also changes. This change in speed is the fundamental cause of refraction.
Q33. Longer wavelengths are scattered
Longer wavelengths (like red light) are scattered less by air molecules. According to Rayleigh scattering, longer wavelengths are scattered much less than shorter wavelengths. This is why red light can travel long distances without being scattered, which is why danger signals are red. Red light is scattered the least.
Q34. Dispersion of light occurs because
Dispersion of light occurs because different colours (wavelengths) of light bend differently when passing through a prism. Each colour travels at a different speed in glass, so each is refracted by a different amount. Violet light (shorter wavelength) bends the most, and red light (longer wavelength) bends the least.
Q35. When light travels from air to glass, its speed
When light travels from air to glass, its speed decreases. Glass is a denser medium with a higher refractive index than air. The higher refractive index means light travels slower in glass. This slowing down causes the light to bend towards the normal. The frequency of light remains constant.
Q36. Light travels fastest in
Light travels fastest in a vacuum, where its speed is approximately 3 × 10⁸ m/s. In air, it travels slightly slower (about 3 × 10⁸ m/s, approximately). In water and glass, light travels slower due to their higher refractive indices. Vacuum has the lowest refractive index (1.00), meaning no slowing of light occurs.
Q37. A rainbow is always seen
A rainbow is always seen in a direction opposite to the Sun. This is because the Sun’s light must enter raindrops and be reflected back towards the observer. For this to happen, the observer must be between the Sun and the raindrops, with the Sun behind them and the rainbow in front of them.
Q38. Planets do not twinkle because they appear as
Planets do not twinkle because they appear as extended sources (small disks). Because planets are closer to Earth than stars, they appear as small discs rather than points of light. Light from different parts of the disc undergoes different amounts of atmospheric refraction, and these variations average out, so the planet does not appear to twinkle.
Q39. Tyndall effect is observed in
The Tyndall effect is observed in colloidal solutions. In a colloid, the particles are large enough (between 1 nm and 1000 nm) to scatter light. When a beam of light passes through a colloidal solution, the particles scatter the light, making the path of the beam visible. In a true solution, particles are too small to scatter light.
Q40. Blue colour has
Blue colour has a shorter wavelength (about 400-450 nm) among visible colours. Shorter wavelengths are scattered more by air molecules (Rayleigh scattering), which is why the sky appears blue. In contrast, red light has a longer wavelength (about 700 nm) and is scattered less.
Q41. Scattering is maximum for light having
Scattering is maximum for light having a short wavelength. According to Rayleigh scattering, the amount of scattering is inversely proportional to the fourth power of wavelength. This means shorter wavelengths (like blue and violet) are scattered much more than longer wavelengths (like red and orange).
Q42. Red colour has
Red colour has a long wavelength (about 700 nm) among visible colours. This longer wavelength causes red light to be scattered less by air molecules (Rayleigh scattering), which is why red light can travel long distances and is used for danger signals. Red light is at the opposite end of the spectrum from violet light.
Q43. The angle between the incident ray and emergent ray in a prism is called
The angle between the incident ray (extended) and the emergent ray in a prism is called the angle of deviation. It is the total angle by which light has been bent after passing through the prism. The angle of deviation depends on the prism’s angle, the refractive index of the prism material, and the angle of incidence.
Q44. The least distance of distinct vision is about
The least distance of distinct vision (near point) for a normal adult eye is about 25 cm. This is the closest distance at which the eye can see an object clearly without straining. Objects closer than this appear blurred because the eye cannot accommodate enough to focus them.
Q45. Water droplets act like
In a rainbow, water droplets act like tiny prisms. When sunlight enters a raindrop, it is refracted, dispersed into colours, and undergoes total internal reflection inside the drop. The colours then emerge at different angles, creating a rainbow. Each raindrop acts like a tiny prism, dispersing light into its component colours.
Q46. Retina acts as a
The retina acts as a screen on which the image is formed. It is the light-sensitive layer at the back of the eye that contains photoreceptor cells (rods and cones). The cornea and lens focus light onto the retina, where it is converted into electrical signals. This is similar to how a screen or film captures an image in a camera.
Q47. Scattering of light by colloidal particles is called
Scattering of light by colloidal particles is called the Tyndall effect. When a beam of light passes through a colloidal solution, the particles scatter the light, making the path of the beam visible. This effect is named after the physicist John Tyndall, who studied it. The Tyndall effect is used to distinguish between colloidal solutions and true solutions.
Q48. Which of the following is NOT a defect of vision?
Dispersion is NOT a defect of vision—it is a phenomenon of light (splitting of white light into colours). Presbyopia, hypermetropia, and myopia are all defects of vision. Presbyopia is age-related loss of accommodation, hypermetropia is far-sightedness, and myopia is near-sightedness.
Q49. The angle between the incident ray and the normal is called
The angle between the incident ray and the normal (an imaginary line perpendicular to the surface at the point of incidence) is called the angle of incidence. It is denoted by the letter ‘i’. This is a basic concept in optics and is used in both reflection and refraction.
Q50. The angle between the two refracting surfaces of a prism is called
The angle between the two refracting surfaces of a prism is called the angle of prism (A). It is also known as the refracting angle of the prism. This angle is one of the key factors that determines how much light is deviated when passing through the prism. The larger the angle of the prism, the more deviation occurs.