CBSE Class 10 Science (2026–27)
Chapter 10: Human Eye and the Colourful World
20 Important Questions and Answers
1. What is the function of the human eye? Explain the role of the retina.
Answer:
The human eye is an important sense organ that enables us to see objects around us. It works like a camera by collecting light and forming images. Light enters through the cornea and pupil, passes through the eye lens, and forms an image on the retina. The retina is a light-sensitive layer located at the back of the eye. It contains rod and cone cells that detect light and colours. These cells convert light signals into electrical impulses, which are carried to the brain by the optic nerve. The brain interprets these signals and allows us to see and recognize objects. Thus, the retina plays a vital role in vision.
2. What is accommodation of the eye?
Answer:
Accommodation is the ability of the eye lens to adjust its focal length so that objects at different distances can be seen clearly. The eye lens is flexible and is attached to ciliary muscles. When a person looks at a nearby object, the ciliary muscles contract, making the lens thicker and increasing its converging power. When viewing distant objects, the muscles relax, making the lens thinner and decreasing its converging power. This adjustment ensures that images are formed exactly on the retina. Accommodation helps us focus on both near and far objects without difficulty. It is one of the unique features of the human eye that provides clear vision.
3. What is the least distance of distinct vision?
Answer:
The least distance of distinct vision is the minimum distance from the eye at which an object can be seen clearly without strain. For a normal adult eye, this distance is about 25 cm. If an object is brought closer than 25 cm, the ciliary muscles cannot increase the curvature of the lens sufficiently to focus the image on the retina. As a result, the object appears blurred. This distance is also called the near point of the eye. The concept is important because it determines the comfortable reading distance. Good lighting and proper posture help maintain healthy vision while reading or working.
4. What is the far point of a normal eye?
Answer:
The far point of a normal eye is the farthest point up to which an object can be seen clearly without any strain. For a normal eye, the far point is at infinity. This means a person with normal vision can clearly see distant objects such as mountains, stars, or buildings without adjusting the eye excessively. When viewing distant objects, the ciliary muscles relax, and the eye lens becomes thinner, reducing its converging power. This allows the image to form directly on the retina. The concept of the far point helps in understanding vision defects such as myopia and hypermetropia.
5. What is myopia? How is it corrected?
Answer:
Myopia, or short-sightedness, is a defect of vision in which a person can see nearby objects clearly but cannot see distant objects distinctly. In a myopic eye, the image of a distant object is formed in front of the retina rather than on it. This defect may occur due to excessive curvature of the eye lens or elongation of the eyeball. Myopia is corrected using a concave lens of suitable power. The concave lens diverges incoming light rays so that the image is formed correctly on the retina. Proper correction enables a person to see distant objects clearly and comfortably.
6. What is hypermetropia? How is it corrected?
Answer:
Hypermetropia, or long-sightedness, is a vision defect in which distant objects are seen clearly, but nearby objects appear blurred. This occurs when the image of a nearby object is formed behind the retina. The defect may be caused by a shorter eyeball or insufficient converging power of the eye lens. A hypermetropic person experiences difficulty while reading or doing close work. This defect is corrected using a convex lens of suitable power. The convex lens converges light rays before they enter the eye, helping the image form on the retina. As a result, near objects become visible clearly.
7. What is presbyopia? Explain its causes.
Answer:
Presbyopia is an age-related defect of vision that usually occurs in elderly people. In this condition, a person finds it difficult to see nearby objects clearly. It occurs because the eye lens gradually loses its flexibility and the ciliary muscles become weak with age. As a result, the eye cannot accommodate effectively for near vision. Presbyopia generally appears after the age of 40 years. People suffering from this defect often need reading glasses. Depending on the condition, convex lenses or bifocal lenses may be prescribed. Presbyopia is a natural part of aging and cannot be completely prevented.
8. What are bifocal lenses? Why are they used?
Answer:
Bifocal lenses are special lenses that contain two different optical powers in the same lens. They are commonly used by people suffering from presbyopia along with myopia or hypermetropia. The upper part of the lens is usually designed for distant vision, while the lower part helps in viewing nearby objects such as books and mobile screens. Bifocal lenses eliminate the need for switching between two separate pairs of glasses. They provide convenience and clear vision for both near and distant objects. Such lenses are widely prescribed for older individuals who experience multiple vision-related problems.
9. What is scattering of light?
Answer:
Scattering of light is the phenomenon in which light changes its direction when it strikes small particles present in a medium. The amount of scattering depends on the size of the particles and the wavelength of light. Shorter wavelengths such as blue and violet scatter more than longer wavelengths like red. Scattering is responsible for several natural phenomena, including the blue colour of the sky and the reddish appearance of the sun during sunrise and sunset. Dust particles, water droplets, and molecules in the atmosphere cause scattering. This phenomenon helps explain many optical effects observed in everyday life.
10. Why is the sky blue?
Answer:
The sky appears blue due to the scattering of sunlight by tiny molecules and particles present in the Earth’s atmosphere. Sunlight consists of seven colours, each having different wavelengths. Blue light has a shorter wavelength than most other colours and is scattered more effectively by atmospheric particles. As sunlight passes through the atmosphere, blue light gets scattered in all directions and reaches our eyes from every part of the sky. Therefore, the sky appears blue during the day. Violet light is scattered even more, but our eyes are more sensitive to blue light, making blue the dominant colour.
11. Why does the Sun appear reddish at sunrise and sunset?
Answer:
At sunrise and sunset, sunlight has to travel a much longer distance through the Earth’s atmosphere before reaching our eyes. During this long journey, shorter wavelengths such as blue and violet are scattered away by atmospheric particles. As a result, mainly longer wavelengths like red and orange reach the observer. Since red light is scattered the least, it dominates the sunlight reaching our eyes. Therefore, the Sun appears reddish during sunrise and sunset. This phenomenon is a direct consequence of atmospheric scattering and demonstrates how the wavelength of light affects the scattering process.
12. What is the Tyndall effect?
Answer:
The Tyndall effect is the scattering of light by colloidal particles present in a medium. It occurs when the particles are large enough to scatter light but small enough to remain suspended. This effect makes the path of a beam of light visible. Examples include sunlight entering a dusty room, light passing through fog, and headlights visible in mist. The Tyndall effect helps demonstrate the phenomenon of scattering. It is named after the scientist John Tyndall. The effect is commonly observed in everyday life and plays an important role in understanding atmospheric optical phenomena.
13. Why is danger signal light red?
Answer:
Danger signals, traffic stop lights, and warning indicators are generally red because red light has the longest wavelength among visible colours. Due to its longer wavelength, red light undergoes the least scattering in the atmosphere. As a result, it can travel greater distances and remain visible even in foggy, smoky, or dusty conditions. This makes red an ideal colour for warning and safety signals. The high visibility of red light helps drivers and pedestrians recognize danger from a distance. Therefore, red is universally used as the colour of caution and emergency signals.
14. Explain the structure of the human eye.
Answer:
The human eye consists of several important parts that work together to produce vision. The transparent cornea allows light to enter the eye. The iris controls the size of the pupil, which regulates the amount of light entering. Behind the pupil lies the crystalline lens, which focuses light onto the retina. The retina contains light-sensitive cells called rods and cones. The ciliary muscles adjust the shape of the lens for accommodation. The optic nerve carries visual information from the retina to the brain. Together, these structures help us perceive shapes, colours, and movements accurately in our surroundings.
15. What are rods and cones?
Answer:
Rods and cones are specialized photoreceptor cells present in the retina of the human eye. Rod cells are highly sensitive to low-intensity light and help us see in dim conditions. However, they do not detect colours. Cone cells function best in bright light and are responsible for colour vision. They enable us to distinguish between different colours and see fine details clearly. There are three types of cone cells sensitive to red, green, and blue light. The combined activity of rods and cones allows us to see objects under various lighting conditions and recognize colours effectively.
16. Why do stars twinkle?
Answer:
Stars appear to twinkle because of atmospheric refraction. As starlight travels through the Earth’s atmosphere, it passes through layers of air with varying densities and temperatures. These layers continuously change the direction of light. Since stars are very far away, they appear as point sources of light. The rapid and irregular refraction causes the apparent position and brightness of stars to change continuously. This creates the twinkling effect observed at night. Planets generally do not twinkle because they appear as extended sources and the fluctuations in light average out before reaching our eyes.
17. Why do planets not twinkle?
Answer:
Planets generally do not twinkle because they are much closer to Earth than stars and appear as extended sources of light rather than point sources. Although atmospheric refraction affects the light coming from planets, light rays from different parts of the planet’s image reach our eyes simultaneously. The fluctuations caused by refraction in different rays tend to cancel each other out. As a result, the brightness and position of planets remain relatively stable. Therefore, planets shine steadily in the night sky, unlike stars, which show noticeable twinkling due to their point-like appearance.
18. Explain atmospheric refraction.
Answer:
Atmospheric refraction is the bending of light as it passes through different layers of the Earth’s atmosphere. The atmosphere consists of layers with varying densities and temperatures, causing changes in the refractive index. As light moves through these layers, its path bends continuously. Atmospheric refraction is responsible for several natural phenomena, such as the twinkling of stars, advanced sunrise, and delayed sunset. It also affects the apparent position of celestial objects. The phenomenon demonstrates how variations in air density can influence the travel of light and alter what we observe in the sky.
19. What is advanced sunrise and delayed sunset?
Answer:
Advanced sunrise and delayed sunset occur due to atmospheric refraction. When the Sun is slightly below the horizon, its light bends as it passes through the Earth’s atmosphere. This bending makes the Sun appear higher than its actual position. As a result, we can see the Sun about two minutes before it actually rises and about two minutes after it has set. Therefore, the duration of the day is slightly increased. This phenomenon is known as advanced sunrise and delayed sunset. It is a practical example of atmospheric refraction affecting our observation of celestial objects.
20. Differentiate between myopia and hypermetropia.
Answer:
Myopia and hypermetropia are common defects of vision, but they affect sight differently. In myopia, a person can see nearby objects clearly but has difficulty seeing distant objects. The image forms in front of the retina and is corrected using a concave lens. In hypermetropia, distant objects are visible clearly, but nearby objects appear blurred. Here, the image forms behind the retina and is corrected using a convex lens. Myopia may result from an elongated eyeball, whereas hypermetropia may occur due to a shorter eyeball or reduced lens power. Both defects can be effectively corrected using suitable lenses.