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