Class 10 Science Chapter 10 The Human Eye and the Colourful World NCERT Solutions 2026 PDF Download

Ans: The ability of the eye lens to adjust its focal length is called accommodation.
- To see distant objects: Ciliary muscles relax, lens becomes thin, focal length increases.
- To see nearby objects: Ciliary muscles contract, lens becomes thick, focal length decreases.

Ans: A person with Myopia needs a Concave Lens (Diverging lens) to correct the vision. The focal length should be equal to the far point of the person ($-1.2 \text{ m}$).

Ans:

• Near Point: 25 cm.
• Far Point: Infinity.

Ans: Stars twinkle due to Atmospheric Refraction. Starlight passes through various layers of the atmosphere with changing densities and temperatures. This causes the path of light to bend continuously. As the physical conditions of the atmosphere are not stationary, the apparent position of the star fluctuates, and the amount of light entering the eye flickers, giving the twinkling effect.

Ans: Planets are much closer to Earth and are seen as extended sources of light (a collection of many point sources). The twinkling effect produced by individual point sources cancels out (some get brighter while others get dimmer), resulting in zero net fluctuation. Hence, they do not twinkle.

Ans: At sunrise/sunset, sunlight travels a longer distance through the thick layers of the atmosphere to reach us. The shorter wavelengths (blue/violet) are scattered away by the particles in the atmosphere. Only the longer wavelengths (red/orange) reach our eyes, making the sun appear reddish.

Ans: The blue colour of the sky is due to the scattering of sunlight by air molecules. In space, there is no atmosphere (no air), so there are no particles to scatter light. Since no scattered light reaches the astronaut's eyes, the sky appears black.

Ans: Defect: The student is suffering from Myopia (Near-sightedness) because he cannot see distant objects (blackboard) clearly but can likely read books nearby.
Correction: It can be corrected using spectacles containing a Concave Lens of suitable power.

Ans: The Pupil controls the amount of light. Its size is regulated by the Iris.

Ans: It is the angle between the direction of the incident ray and the direction of the emergent ray when light passes through a glass prism.

Ans: A rainbow is caused by the Dispersion, Refraction, and Internal Reflection of sunlight by tiny water droplets present in the atmosphere after rain. The droplets act like small prisms.

Ans: Due to Atmospheric Refraction. When the sun is slightly below the horizon, its light rays bend as they enter the Earth's atmosphere (moving from vacuum to air), making the sun appear visible above the horizon before it actually is.

Ans: The retina acts as a screen where the image is formed. It contains light-sensitive cells called Rods (dim light) and Cones (colour/bright light) that convert light signals into electrical signals sent to the brain.

Ans: Hypermetropia (Far-sightedness): A person can see distant objects clearly but nearby objects appear blurry.
Cause: Eye lens focal length is too long OR Eyeball is too small. Image forms behind the retina.
Correction: A Convex Lens is used to converge rays so the image forms on the retina.

Ans:

• Glass Slab: The refracting surfaces are parallel. The emergent ray is parallel to the incident ray (Lateral Displacement). No dispersion occurs.
• Glass Prism: The refracting surfaces are at an angle. The emergent ray is not parallel (Angle of Deviation). Dispersion occurs (Spectrum formed).

Ans: (Newton's Prism Experiment)
1. Place a glass prism in the path of a narrow beam of sunlight.
2. Place a white screen on the other side.
3. A band of seven colours (VIBGYOR) is observed on the screen.
4. If an inverted identical prism is placed after the first one, the colours recombine to form white light again. This proves white light consists of seven colours.

Ans: According to Rayleigh's law of scattering, the intensity of scattered light is inversely proportional to the wavelength ($\frac{1}{\lambda^4}$). Red has the longest wavelength in the visible spectrum. Therefore, it is scattered the least by fog or smoke and can travel long distances to reach the eye, making it effective for danger signals.

Ans: Air molecules and fine particles in the atmosphere are smaller than the wavelength of visible light. They are more effective at scattering light of shorter wavelengths (blue/violet) than longer wavelengths (red). This scattered blue light enters our eyes, making the sky look blue.

Ans: It is an age-related defect where the power of accommodation of the eye decreases. The ciliary muscles weaken, and the lens becomes less flexible. The person finds it difficult to see nearby objects. It is corrected using bifocal lenses.

Ans: 1. Wider Field of View: One eye gives ~150°, two eyes give ~180°.
2. Depth Perception: Two eyes provide 3D vision, allowing us to judge distances accurately.

Ans: The phenomenon of scattering of light by colloidal particles gives rise to the Tyndall effect. This makes the path of a light beam visible (e.g., sunlight entering a dusty room through a small hole).

Ans: Violet < Indigo < Blue < Green < Yellow < Orange < Red.
(Red has the maximum wavelength; Violet has the minimum).

Ans: The pupil dilates (expands). In bright light, the pupil is small to restrict light. When entering the dark, the iris expands the pupil to allow more light to enter so we can see clearly.