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The Human Eye and the Colourful World – Class 10 Science notes, important questions and MCQs

NCERT Class 10 Science, chapter 10. Notes, key terms, important questions with answers and MCQs.

Chapter overview

The eye works like a camera with an adjustable lens. Common vision defects are corrected with lenses, and refraction and scattering of light explain the rainbow, twinkling stars and the blue sky.

The eye

Cornea and lens focus light on the retina; the iris controls pupil size.
Power of accommodation: ciliary muscles change the lens focal length.
Near point of a normal eye is 25 cm; far point is infinity.

Defects

Myopia (short-sightedness): image before the retina; corrected with a concave lens.
Hypermetropia (long-sightedness): image behind the retina; corrected with a convex lens.
Presbyopia (old age): bifocal lenses; cataract: cloudy lens, treated by surgery.

Atmospheric effects

Dispersion: a prism splits white light into VIBGYOR; violet bends most, red least.
Rainbow: dispersion + refraction + internal reflection in raindrops.
Twinkling of stars and advanced sunrise are due to atmospheric refraction; blue sky and red sunset are due to scattering (Tyndall effect).

Key terms

Accommodation: the eye's ability to change focal length for near and far objects
Dispersion: splitting of white light into its colours
Scattering: spreading of light by small particles in its path

Formulas

Least distance of distinct vision = 25 cm
Order of bending: violet most, red least

Important questions with answers

Why does the sky appear blue?2 marks

Air molecules scatter shorter wavelengths (blue) much more than longer ones (red). This scattered blue light reaches our eyes from all directions, so the sky looks blue.

What is myopia? Give two causes and its correction.3 marks

In myopia a person can see near objects clearly but not distant ones; the image forms in front of the retina. Causes: elongated eyeball or excessive curvature of the eye lens. It is corrected with a concave lens of suitable power.

Explain the formation of a rainbow and why stars twinkle but planets do not.5 marks

A rainbow forms when sunlight enters raindrops, which refract it, reflect it internally and refract it again; dispersion splits white light into seven colours, seen opposite the sun. Stars twinkle because starlight passes through layers of air with changing refractive index; as the star is a point source, the amount of light reaching the eye flickers. Planets are much closer and act as extended sources (many points), so the changes average out and they do not twinkle.

Quick revision

Myopia → concave lens; hypermetropia → convex lens.
Sky is blue due to scattering of shorter wavelengths.
Rainbow appears opposite the sun.

Exam tips

Danger lights are red because red is scattered least and travels farthest through fog.
Planets do not twinkle because they are extended sources.

Practice MCQs

The part of the eye on which the image forms is the
1. retina
2. iris
3. cornea
4. pupil
Show answer
Answer: (a) retina. Retina.
Myopia is corrected by a
1. concave lens
2. convex lens
3. bifocal only
4. cylindrical lens
Show answer
Answer: (a) concave lens. Concave lens diverges rays.
The colour of light that bends the least in a prism is
1. red
2. blue
3. green
4. violet
Show answer
Answer: (a) red. Red.
The near point of a normal eye is
1. 25 m
2. 2.5 cm
3. 25 cm
4. infinity
Show answer
Answer: (c) 25 cm. 25 cm.
The sky appears blue because of
1. reflection
2. refraction
3. dispersion only
4. scattering
Show answer
Answer: (d) scattering. Blue light scatters more.
Stars twinkle because of
1. reflection
2. scattering
3. atmospheric refraction
4. dispersion
Show answer
Answer: (c) atmospheric refraction. Changing refraction of starlight.
Why are danger signals red?
1. red is the brightest colour
2. red bends most
3. red is scattered least by fog and smoke
4. red is cheapest
Show answer
Answer: (c) red is scattered least by fog and smoke. Longest visible wavelength scatters least.
Sunrise is seen about 2 minutes early because of
1. scattering
2. dispersion
3. atmospheric refraction
4. Tyndall effect
Show answer
Answer: (c) atmospheric refraction. Light bends through the atmosphere before the sun is above the horizon.

All chapters of Class 10 Science

Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.