The Human Eye and the Colourful World – Class 10 Physics Chapter 10 NCERT Notes

Chapter 10: The Human Eye and the Colourful World

10.1 The Human Eye

The human eye is a natural optical instrument that uses a convex lens (the eye lens) to form a real, inverted, and diminished image of objects on the retina.

Structure of the Eye

• Cornea: Transparent front surface that refracts most of the light entering the eye.
• Iris: Coloured diaphragm behind the cornea; controls the size of the pupil.
• Pupil: Central opening in the iris; regulates the amount of light entering the eye (dilates in dim light, constricts in bright light).
• Eye Lens: A transparent, flexible, biconvex lens that fine-focuses the light on the retina. Its curvature can be adjusted by the ciliary muscles.
• Ciliary Muscles: Hold the eye lens and change its shape (curvature) to focus on objects at different distances.
• Retina: Light-sensitive screen at the back of the eye. Contains rod cells (detect light intensity) and cone cells (detect colour). The image is formed here.
• Optic Nerve: Carries electrical signals from the retina to the brain for interpretation.
• Blind Spot: The point on the retina where the optic nerve exits — no photoreceptor cells are present here, so no image is formed at this point.

Near Point and Far Point

• Near Point (Least Distance of Distinct Vision): The closest distance at which the eye can see an object clearly = 25 cm for a normal eye (denoted by D).
• Far Point: The farthest distance at which the eye can see clearly = infinity (∞) for a normal eye.

10.2 Defects of Vision and Their Correction

1. Myopia (Short-sightedness / Near-sightedness)

• Problem: Distant objects appear blurry; near objects are seen clearly.
• Cause: Eyeball is too long OR the eye lens is too curved (too converging), so the image forms in front of the retina.
• Correction: A concave lens (diverging lens) of appropriate power is used. It diverges the incoming rays before they enter the eye, so the image forms exactly on the retina.

2. Hypermetropia (Long-sightedness / Far-sightedness)

• Problem: Near objects appear blurry; distant objects are seen clearly.
• Cause: Eyeball is too short OR the eye lens is too flat (low converging power), so the image forms behind the retina.
• Correction: A convex lens (converging lens) of appropriate power is used.

3. Presbyopia

• Problem: Difficulty seeing both near and distant objects clearly (common in old age).
• Cause: Gradual weakening of ciliary muscles and decreased flexibility of the eye lens.
• Correction: Bifocal lenses — upper portion is concave (for distant vision), lower portion is convex (for near vision).

10.3 Refraction Through a Glass Prism

When white light enters a glass prism, it bends (refracts) at both surfaces and gets separated into its component colours. This splitting of white light into its spectrum of colours is called dispersion.

10.4 Atmospheric Refraction

Earth's atmosphere has layers of air with varying densities and temperatures. Light bends as it passes through these layers.

Examples:

• Twinkling of Stars: Starlight passes through changing atmospheric layers, causing fluctuations in the apparent position and brightness of stars. Planets don't twinkle because they are much closer and appear as extended sources (not point sources).
• Advance Sunrise and Delayed Sunset: The Sun is visible about 2 minutes before actual sunrise and 2 minutes after actual sunset because atmospheric refraction bends sunlight upward when the Sun is below the horizon.

10.5 Scattering of Light

When light encounters particles much smaller than its wavelength, it gets scattered. The amount of scattering depends on the size of the particles and the wavelength of light.

Phenomena Explained by Scattering

• Blue Colour of the Sky: Blue light (short wavelength) is scattered much more by atmospheric molecules. This scattered blue light reaches our eyes from all directions, making the sky appear blue.
• White Colour of Clouds: Water droplets in clouds are much larger than air molecules. They scatter all wavelengths of light equally, so clouds appear white.
• Red Colour of Sun at Sunrise/Sunset: At sunrise/sunset, sunlight travels through a much thicker layer of atmosphere. Most of the blue light gets scattered away, and only red/orange light (long wavelengths) reaches our eyes.
• Tyndall Effect: Scattering of light by colloidal particles. Example: a beam of sunlight entering a dark room through a small gap appears visible because dust particles scatter the light.

10.6 Rainbow Formation

A rainbow is formed by the dispersion, internal reflection, and refraction of sunlight by tiny water droplets in the atmosphere after rain. The observer must stand with the Sun behind them and the rain/water droplets in front to see a rainbow.