Short Questions Answer
1. What is the function of the retina in the human eye?
Answer: The retina is a light-sensitive membrane that receives images and sends electrical signals to the brain through the optic nerve.
2. What is the least distance of distinct vision for a normal human eye?
Answer: The least distance of distinct vision is 25 cm.
3. What is the far point of a normal human eye?
Answer: The far point of a normal human eye is infinity.
4. What is meant by the power of accommodation of the eye?
Answer: The ability of the eye lens to adjust its focal length to see distant and nearby objects clearly is called power of accommodation.
5. Which type of lens is used to correct myopia?
Answer: Concave lens is used to correct myopia.
6. Which type of lens is used to correct hypermetropia?
Answer: Convex lens is used to correct hypermetropia.
7. What causes presbyopia?
Answer: Presbyopia is caused due to weakening of ciliary muscles and loss of flexibility of the eye lens with age.
8. What is dispersion of light?
Answer: The splitting of white light into its seven colours (VIBGYOR) is called dispersion.
9. Who first obtained the spectrum of sunlight using a prism?
Answer: Isaac Newton first obtained the spectrum of sunlight using a glass prism.
10. What causes the formation of a rainbow?
Answer: A rainbow is formed due to dispersion, internal reflection, and refraction of sunlight by water droplets.
11. What causes stars to twinkle?
Answer: Atmospheric refraction causes starlight to bend irregularly, making stars appear to twinkle.
12. Why don’t planets twinkle?
Answer: Planets appear as extended sources of light, and the variations in light average out, so they do not twinkle.
13. What causes advance sunrise and delayed sunset?
Answer: Atmospheric refraction makes the Sun appear above the horizon even when it is below it.
14. What is the Tyndall effect?
Answer: The scattering of light by colloidal particles that makes the light beam visible is called the Tyndall effect.
15. Why is the colour of the sky blue?
Answer: Because small particles in the atmosphere scatter blue light more effectively than red light.
Long Questions Answer
1. Explain the structure and working of the human eye.
Answer: The human eye is spherical, about 2.3 cm in diameter. Light enters through the cornea, passes through the pupil (controlled by the iris), and then through the eye lens, which focuses the image on the retina. The retina converts light into electrical signals sent to the brain via the optic nerve, allowing us to perceive images.
2. What is accommodation of the eye? How does it work?
Answer: The eye lens changes its curvature to focus light from objects at different distances.
- When looking at distant objects, ciliary muscles relax, making the lens thin (longer focal length).
- For nearby objects, muscles contract, making the lens thick (shorter focal length).
This ability is called accommodation.
3. Explain myopia and its correction.
Answer: Myopia (near-sightedness) occurs when a person can see nearby objects clearly but not distant ones. The image forms in front of the retina due to an elongated eyeball or high curvature of the lens. It is corrected by using a concave lens, which diverges light rays so that the image forms on the retina.
4. Explain hypermetropia and its correction.
Answer: Hypermetropia (far-sightedness) occurs when a person can see distant objects clearly but not nearby ones. The image forms behind the retina due to a small eyeball or long focal length of the lens. It is corrected using a convex lens, which converges the light rays before they enter the eye.
5. What is presbyopia? How can it be corrected?
Answer: Presbyopia occurs with age, as the ciliary muscles weaken and the lens becomes less flexible, reducing accommodation. It is corrected using bi-focal lenses, which have concave (for distant) and convex (for near) portions.
6. Describe the process of refraction and dispersion of light through a prism.
Answer: When white light passes through a prism, it refracts twice—once entering and once leaving. Each colour bends differently because of different refractive indices. Violet bends most and red least, forming a spectrum of seven colours (VIBGYOR).
7. Explain the formation of a rainbow.
Answer: A rainbow forms due to dispersion, internal reflection, and refraction of sunlight by water droplets in the air. Each droplet acts as a prism, splitting sunlight into colours. The observer sees the colours in the order VIBGYOR, with red on top and violet at the bottom.
8. Why do stars twinkle but planets do not?
Answer: Starlight passes through various atmospheric layers with different densities, causing fluctuating refraction and brightness — this makes stars twinkle. Planets, being closer, act as extended sources, so the fluctuations average out, and they don’t twinkle.
9. Explain the phenomenon of advance sunrise and delayed sunset.
Answer: Due to atmospheric refraction, sunlight bends towards the normal as it enters denser air layers. As a result, the Sun appears above the horizon even before it actually rises and remains visible for about 2 minutes after actual sunset.
10. Explain the Tyndall effect and its role in the colour of the sky.
Answer: The Tyndall effect is the scattering of light by small particles in the atmosphere. Shorter wavelengths (blue) are scattered more than longer wavelengths (red), making the sky appear blue. At sunrise and sunset, light travels longer distances, so red light predominates, making the Sun appear red.
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