The Human Eye and the Colourful World Class 10 Notes

The Human Eye and the Colourful World Class 10 explains how your eye focuses light, why some people need glasses, and why we see rainbows, twinkling stars and a blue sky. This guide covers the chapter in order: the parts of the eye, accommodation, vision defects and their correcting lenses, prisms and dispersion, rainbows, atmospheric refraction and scattering, with important questions and model answers at the end.

Parts of the human eye

The eye works like a camera. It is roughly spherical, about 2.3 cm in diameter.

The lens forms a real, inverted image on the retina. Light-sensitive cells turn it into electrical signals, and the brain interprets them so we perceive objects.

Power of accommodation

The ciliary muscles change the curvature of the eye lens.

• Muscles relaxed: lens becomes thin, focal length increases, distant objects are seen clearly.
• Muscles contracted: lens becomes thick, focal length decreases, nearby objects are seen clearly.

This ability to adjust focal length is accommodation. It has limits:

• Near point (least distance of distinct vision): 25 cm for a normal young adult.
• Far point: infinity for a normal eye.

So a normal eye sees clearly anywhere from 25 cm to infinity. In old age, the lens may become milky or cloudy, a condition called cataract, which can be corrected by surgery.

Myopia (near-sightedness)

A myopic person sees near objects clearly but distant ones look blurred. The far point is closer than infinity, and light from a distant object meets in front of the retina.

Causes: too much curvature of the eye lens, or an elongated eyeball. The eye bends light too much.

Correction: a concave (diverging) lens of suitable power. It spreads the rays slightly before they enter the eye so they focus on the retina.

Do not be fooled by the name: near-sighted means you can see near.

Hypermetropia (far-sightedness)

A hypermetropic person sees distant objects clearly but not nearby ones. The near point is farther than 25 cm, and light from an object at 25 cm focuses behind the retina. Such a person holds reading material farther away.

Causes: the focal length of the eye lens is too long, or the eyeball has become too small. The eye does not converge light enough.

Correction: a convex (converging) lens, which adds the focusing power needed.

Example: A person's near point is 1 m. Since 1 m is beyond 25 cm, the defect is hypermetropia.

Presbyopia and bifocal lenses

With age, the ciliary muscles weaken and the lens loses flexibility. The eye gradually loses its power of accommodation and the near point moves farther away. This is presbyopia. It differs from hypermetropia in its cause: ageing muscles and a stiffer lens, rather than eyeball shape or lens focal length.

A person with both myopia and hypermetropia needs bifocal lenses:

• Upper part: concave, for distant vision.
• Lower part: convex, for reading.

Contact lenses and some surgical procedures can also correct refractive errors; an eye-care professional chooses the right correction.

Refraction through a prism

A triangular glass prism has two triangular bases and three rectangular side faces. The angle between the two refracting faces is the angle of the prism (A).

Because the two faces are not parallel, the emergent ray is not parallel to the incident ray. The angle between the incident ray's direction and the emergent ray's direction is the angle of deviation (D).

Dispersion and the spectrum

White light is made of seven colours: VIBGYOR (violet, indigo, blue, green, yellow, orange, red). In a vacuum all colours travel at the same speed, but in glass they travel at different speeds, so each bends by a different angle.

• Red bends the least.
• Violet bends the most.

This splitting of white light into its colours is dispersion, and the band of colours is a spectrum. Isaac Newton placed a second, inverted prism in the path of the spectrum and got white light back. This showed that a prism does not create colours; it only separates colours already present in sunlight.

How a rainbow forms

After rain, tiny water droplets in the air act like small prisms.

1. Refraction and dispersion: sunlight enters a droplet and splits into colours.
2. Internal reflection: light reflects off the back of the droplet.
3. Refraction again: the colours leave the droplet and spread towards the observer.

A rainbow always forms in the direction opposite to the Sun. To see one, keep the Sun behind you and look at a rain-filled sky or a waterfall.

Atmospheric refraction

Hot air is less dense and has a lower refractive index than cool air. Because conditions in the atmosphere keep changing, light passing through it bends along a fluctuating path. This is atmospheric refraction.

• Stars appear higher than they are: starlight bends towards the normal as it enters denser air near the Earth.
• Stars twinkle: a star is a distant point source, so as the air shifts, its apparent position and the amount of light reaching the eye keep changing.
• Planets do not twinkle: they are closer and look like extended sources (many point sources). Their fluctuations average out.
• Advance sunrise and delayed sunset: we see the Sun about 2 minutes before actual sunrise and about 2 minutes after actual sunset, because light from the Sun below the horizon is bent towards us. The day becomes about 2 + 2 = 4 minutes longer.
• Flattened Sun: at sunrise and sunset, light from the top and bottom of the disc bends by different amounts, so the Sun looks slightly oval.

Scattering of light and the Tyndall effect

When light hits tiny particles of smoke, dust or air, it is sent off in many directions. This makes the path of a beam visible, which is the Tyndall effect. You see it when sunlight enters a dusty room through a slit, or when light passes through mist in a forest.

The colour scattered depends on particle size:

• Very fine particles (air molecules) scatter shorter wavelengths, like blue, much more.
• Larger particles (dust, water drops) scatter longer wavelengths, or all wavelengths equally, so the light looks white.

Blue sky: air molecules are smaller than the wavelength of visible light and scatter blue far more than red; this blue light reaches our eyes from all directions.

Red danger signals: red has a longer wavelength and is scattered least by fog or smoke, so it can be seen from far away.

Remember this

• Image on the retina: real and inverted.
• Near point 25 cm; far point infinity.
• Myopia: image in front of retina, concave lens. Hypermetropia: image behind retina, convex lens.
• Red deviates least, violet most.
• Rainbow: refraction, dispersion, internal reflection; Sun behind you.

Important questions with answers

1. Which part controls the amount of light entering the eye? The pupil regulates the light; the iris changes the size of the pupil.

2. What happens to the eye lens when you look at a distant object? The ciliary muscles relax, the lens becomes thin and its focal length increases.

3. Why is a concave lens used for myopia? A myopic eye focuses light in front of the retina. The concave lens diverges the rays first, moving the focus back onto the retina.

4. How is presbyopia different from hypermetropia? Both make near objects hard to see, but presbyopia comes from ageing (weak ciliary muscles, stiff lens), while hypermetropia comes from eyeball size or lens focal length.

5. Define the angle of deviation. The angle between the direction of the incident ray and the emergent ray when light passes through a prism.

6. What did Newton's two-prism experiment show? The inverted second prism recombined the spectrum into white light, showing sunlight already contains the seven colours.

7. Why do planets not twinkle? They are extended sources, so changes from different points on their disc cancel out.

8. By how much does atmospheric refraction lengthen the day? About 4 minutes: 2 minutes at sunrise and 2 minutes at sunset.

9. Why does a clear sky look blue? Air molecules scatter short blue wavelengths much more than long red ones, and this blue light reaches us from all directions.

Common mistakes to avoid

• Thinking a myopic person cannot see near objects.
• Swapping the lenses: concave for myopia, convex for hypermetropia.
• Saying red bends the most; violet bends the most.
• Looking towards the Sun for a rainbow; it forms on the opposite side.

To revise this chapter with guided questions, study this chapter with Joy on Learnijoy.