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CHAPTER -11
THE HUMAN EYE AND THE
COLOURFUL WORLD

4) Refraction of lightthrough a glass prism:

https/youtu.be/cQho 88edW xU

Glass prism -
Angle of deviation
Normal
- Angle of emergence

Incident ray Emergent ray

Air Glass Glass Air

Refracted ray

When a ray of light passes glass prism, it gets

through a
and glass- air intertace.
air- glass Intertace
bent twce at the ray angle to the incident
The emergent isdeviated by andeviation.
ray.This angle is called the angle of
 H

N M

C
PE- Incident ray i - Angle of incidence
EF Refracted ray Lr- Angle of refraction
FS-Emergent ray Le-Angle of emergence
ZA - Angle of the prism Angle of deviation
D-

Figure 11.4 Refraction ofight through a triangular glass prism

5a) Disper sion of white light by a glass prism :

Spectrum
Beam of white light

Glass prism
When a beam of white light is passed through a glass
prism, it is split up into a band of colours called spectrum.
This is called dispersion of white light. The spectrum of
white has the colours violet, indigo, blue, green, yellow,
orange and red The red
(VIBGYOR). light bends the least
and the violet light bends the most.
 6bc1e8ee-a4ed-42a0-b925.
Ureen

Blue
White Light Indigo
Glass Prism Violet

b) Recombination of thespectrumofwhite light

produceswhite light:-
White light

R R

White light

Glass prisms

When a beam of white light is passed through glass

a

into its component colours. When these

prism, it is split up it
colours allowed to fall on an inverted glass prism
are
recombines to produce white light.
 Red
Orange
Yellow
Green
Blue
White Light Indigo
Glass Prism Violet

b) Recombination of the spectr um of white light

produces white light
White light
R
R R

White light V

Glass prisms

When beam of white light is passed

a
through a glass
prism, it is split up into its component colours. When these
colours are allowed to fall on an inverted
recombines to produce white light.
glass prism it

c) Rainbow for mation-

Raindrop
Sunlight
Refraction and dispersion
Internal reflection

Refraction
Red 7/14

O
c) Rainbow for mation Raindrop

Sunlight

Refraction and dispersion

Internal reflection

Refraction
Violet
Red

Observer

A rainbow is a natural spectrum appearing in the sky

after rain shower. It is caused by the dispersion of
a
The
sunlight by water droplets present in the atmosphere.
small prisms. They refract and
water droplets act likethen
disperse the sunlight reflect it internally and finally
to
again when it comes out of the rain drops. Due
refract it
the dispersion of sunlight and internal reflection by the
water droplets we see the rainbow Colours.
i) Advance sunrise and delayed sunset
Apparent position
Apparent position
Atmosphere w

Horizon Horizon sunrise

Sunset Observer

Earth wwww
Real position
Real position

2 minutes before sunrise

The sun is visible to us about
sunset due to atmospheric
and about two minutes after
refraction.
and
The apparent flattening of
the sun's disc at sunrise
refraction.
to atmospheric
at sunset is also due

) Scattering of light:
i) Tyndall effect the path
a beam of light passes through
a colloidal solution,colloid
When due to the scattering of light by the
of light becomes visible
as Tyndall effect.
particles. This is known water droplets, dust,
contains air molecules,
The earth's atmosphere the path of
smoke etc. When sunlight passes
through the atmosphere
these
becomes visible due to
the scattering of light by
the light
particles. the size of the
scattered light depends upon
The colour of the Larger
fine particles scatter blue light.
scattering particles. Very colours of
scatter different light.
particles
i)Why isthecolour of the dear sky blue ?
The fine particles in the atmosphere have
size smaller
than the wave length of visible light. They can scatter blue
light which has a shorter wave length than red light which
has a longer wave length. when sunlight passes through
the atmosphere, the fine particles in the atmosphere scatter
the blue more strongly than the red and so the sky
colour
appears blue.
If the earth had no atmosphere there would not be any
scattering of light and the sky would appear dark. The sky
appears dark at very high altitudes.

Cloud droplets scatter all

linbt oenatinn ihe aauR wavelengths of visibile
 Cloud droplets scatter all wavelengths of visible
g n t creatng the appearance of white clouds.

ii) Colour ofthe sky at sunrise and sunset

w
Sun overhead

Blue light scattered away

Observer Atmosphere
Sun appears reddish
Sun near horizon Earth

At sunrise and at súnset the sun is near the horizon and

he light from the sun travels through the thicker layers of
the atmosphere aridlenger distance throughthe
atmosphere. Near the horizon most of the blue light and
shorter wave lengths are scattered away by the particles of
he air and the red light and longer wave lengths reaches
ur eyes. So the sun appears reddish at sunrise and sunset

Question 12
Why doesthe sun appear reddish early in the morning?
Answer:
The light comingfromthe sun passesthrough variousdenser layers of air in the earth's
atmosphere before reaching our eyes near the horizon.
Most of the part of blue light and light of small wavelength gets scattered by dust
partides near the horion. So, the light reaching our eyesis of
large wavelength. Due to thisthe sun appearsreddish at the time of sunrise and sunset.
Question 13
Why doesthe sky appear dark instead of blue to an astronaut ??
Answer:
Asan astronaut moves away from the atmosphere of earth, the atmosphere becomes
thin. Due to the absence of molecules(or dust partides) in
air, the scattering of light doesnot take place. Thus, sky appears dark in the absence of
scattering