15 Long-answer questions from the chapter:10 "Human Eye and Colourful World" for Class 10 CBSE Science.

 

Chapter 10 in Physics, "The Human Eye and the Colourful World," follows the previous chapter on light (Chapter 9: Light - Reflection and Refraction). This chapter applies the fundamental principles of light, refraction, and dispersion to explain natural phenomena and the structure of human vision.

 

Weightage in the Board Exam (Approximate)

Chapter 10 belongs to Unit III: Natural Phenomena, which it shares with Chapter 9 (Light – Reflection and Refraction). This unit collectively carries a significant weightage of approximately 12 marks in the 80-mark theory paper.

Individual Weightage: Chapter 10, "The Human Eye and the Colourful World," generally has a lower individual weightage of around 2 to 3 marks compared to the numerical-heavy Chapter 9.

Question Focus: Questions are usually conceptual or diagram-based. Expect:

Questions on the function of eye parts or the correction of a vision defect (often requiring a ray diagram).

Explanation-based questions on atmospheric phenomena (e.g., "Why do stars twinkle?" or "Why is the sky blue?").

Preparation Tip: Focus on mastering the ray diagrams for correcting vision defects and thoroughly understanding the concepts of atmospheric refraction and scattering of light.

 

1. Explain the structure and function of the human eye with a neat diagram.

Answer:

The

human eye

is a natural optical device that enables us to see.

Main parts of the eye:

 

Cornea:

Transparent layer that refracts light.

Iris:

Controls the size of the pupil.

Pupil:

Regulates the amount of light entering the eye.

Lens:

Adjusts focus for near and distant objects.

Retina:

Contains

rods

(for dim light) and

cones

(for

color

vision).

Optic Nerve:

Transmits signals to the brain.

2. What is Myopia? Explain its causes and correction with a ray diagram.

Answer:

Myopia (Short-sightedness):

A defect where a person can see near objects clearly but distant objects appear blurred.

Causes:

 

Elongation of the eyeball.

Excessive curvature of the eye lens.

Correction:

 

Using a

concave lens

(diverging lens) to bring distant objects into focus.

Ray Diagram:

Draw a

concave lens

correcting the image formation before the retina.

3. Explain Hypermetropia. What are its causes and correction?

Answer:

Hypermetropia (Far-sightedness):

A defect where a person can see distant objects clearly but nearby objects appear blurred.

Causes:

 

Shortening of the eyeball.

Decreased curvature of the lens.

Correction:

 

Using a

convex lens

(converging lens) to bring near objects into focus.

Ray Diagram:

Draw a

convex lens

correcting the image formation beyond the retina.

4. What is Presbyopia? How is it different from Myopia and Hypermetropia? How is it corrected?

Answer:

Presbyopia:

An age-related defect where both near and distant vision is affected.

Causes:

 

Weakening of ciliary muscles.

Loss of flexibility in the eye lens.

Correction:

 

Using

bifocal lenses

with

convex (for near)

and

concave (for distance)

sections.

Differences with Myopia & Hypermetropia:

 

Myopia:

Only distant vision is blurry.

Hypermetropia:

Only near vision is blurry.

Presbyopia:

Both near and distant vision are affected.

5. Explain the dispersion of light through a prism. Why does white light split into seven colours?

Answer:

Dispersion:

Splitting of

white light

into seven colours when passing through a prism.

Cause:

Different colours have

different wavelengths

and travel at different speeds in glass.

Order of Colours:

 

Violet, Indigo, Blue, Green, Yellow, Orange, Red (VIBGYOR)

.

Ray Diagram:

Show white light entering a prism and emerging as a spectrum.

 

6. What is atmospheric refraction? Explain how it causes the apparent early sunrise and delayed sunset.

Answer:

Atmospheric refraction

is the bending of light due to changes in the optical density of atmospheric layers.

Apparent Early Sunrise and Delayed Sunset:

 

When the sun is below the horizon, light rays

bend due to atmospheric refraction

, making the sun appear

above the horizon

.

As a result, we see the sun

before it has actually risen

and

after it has actually set

.

Diagram:

Show the bending of light making the sun visible before actual sunrise and after actual sunset.

 

7. Why do stars twinkle but planets do not?

Answer:

Twinkling of Stars:

 

Stars are

very far

from Earth and act as

point sources of light

.

Light from stars passes through layers of air with varying densities,

bending continuously

due to atmospheric refraction.

This makes stars appear

brighter and dimmer intermittently

, causing them to

twinkle

.

Why Planets Do Not Twinkle:

 

Planets are

closer to Earth

and appear as

larger

objects.

The light coming from planets is

not a single point

but a collection of points.

The twinkling effects average out

, so planets do not twinkle.

 

8. Why does the sky appear blue? Why does it appear reddish during sunrise and sunset?

Answer:

Sky Appears Blue:

 

Sunlight consists of

seven colours

(VIBGYOR).

Shorter wavelengths (blue & violet) scatter more

in the atmosphere.

Our eyes are

more sensitive to blue light

than violet, so the sky appears

blue

.

Sunrise and Sunset Appear Red:

 

During sunrise and sunset, sunlight travels

longer distances

through the atmosphere.

Blue and violet light scatter away

, leaving

red and orange light

to reach our eyes.

 

9. Explain the formation of a rainbow.

Answer:

A rainbow is formed by sunlight passing through raindrops, undergoing three phenomena:

 

Refraction:

Light bends when entering a raindrop.

Dispersion:

White light splits into

seven colours

.

Internal Reflection:

Light reflects inside the raindrop before emerging.

Order of Colours (VIBGYOR):

 

Red

appears on the

outer side

, and

violet

on the

inner side

.

Diagram:

Show sunlight entering a raindrop and emerging as different colours.

 

10. Explain Tyndall effect. Give two examples of it.

Answer:

Tyndall Effect:

The scattering of light by particles in a colloidal medium.

Examples:

 

Blue colour of the sky

– Small air molecules scatter blue light more than red light.

Beam of light visible in a dark room

– Dust particles scatter the light.

 

11. Explain why danger signals and traffic lights are red in colour.

Answer:

Red Light is Least Scattered:

 

Red has the

longest wavelength

among visible colours.

It

does not scatter much

and travels the longest distance.

Better Visibility:

 

Due to less scattering, red is visible

even from a far distance

, making it ideal for warning signs.

 

12. What are the main differences between a convex and concave lens? Give their applications.

Answer:

Feature	Convex Lens	Concave Lens
Shape	Thicker at the centre	Thinner at the centre
Nature	Converging lens	Diverging lens
Image Formation	Real and inverted (except when the object is inside F)	Virtual and erect
Uses	Used in magnifying glasses, hypermetropia correction	Used in myopia correction, peepholes

 

13. What is persistence of vision? How is it used in movie projection?

Answer:

Persistence of Vision:

 

The human eye retains an image for

1/16th of a second

after the object is removed.

Movie Projection:

 

In a movie,

frames change quickly

(usually 24 frames per second).

Since the eye retains each image for a short time, the transition appears

smooth and continuous

.

 

14. Why does the bottom of a swimming pool appear shallower than it actually is?

Answer:

Cause:

 

Refraction of light

from water to air.

Explanation:

 

Light coming from the bottom of the pool

bends away from the normal

as it moves to the air.

This makes the bottom appear

higher

than it actually is.

Formula:

 

Apparent Depth = Real Depth / Refractive Index