Spectrum

Dependence of deviation on colour of light

The refractive index of the material of prism increases with the decrease in the wavelength of
light. Hence, the deviation caused by the prism also increases with the decrease in the wavelength
of light.
In visible light, violet (wavelength = 4000 Å) is deviated the most & red (wavelength λ = 8000
Å) is deviated the least
Dispersion of white light by a glass prism – Newton’s experiment

Dispersion:
The phenomenon of splitting of white light by a prism into its constituent colours is known as
dispersion.
Rainbow is the natural phenomenon in which dispersion takes place. The cause of dispersion is
that sunlight consists of seven constituents (colours namely violet, indigo, blue, green, yellow,
orange and red popularly referred to as VIBGYOR) that have a different refractive index with
respect to a medium. The dispersion of white light occurs because colours of white light travel at
different speeds through the glass prism. The wavelength of each colour is different and this
causes the difference in velocity of the corresponding light when passing from one medium to
another.
Spectrum:
The band of colours obtained on a screen on passing white light through a prism is called spectrum.
The prism has split the incident white light into a band of colours. The various colours seen are
Violet, Indigo, Blue, Green, Yellow, Orange and Red. The band of the coloured components of a
light beam is called its spectrum.
Cause of dispersion:
 The dispersion of white light into its constituent colours takes place at the first surface of
a prism.
 The cause of dispersion is the change in speed of light with a wavelength (or frequency).
 On the second surface of the prism, the only refraction takes place.
 Prism itself produces no colours.
 The colours of a spectrum of white light combine to reproduce white light.
Ascending order with respect to their frequency and wavelength
Ascending order of their frequency,
 Red < Orange < Yellow < Green < Blue < Indigo < Violet.
 Opposite order of VIBGYOR.
Ascending order of their wavelength,
 Violet < Indigo < Blue < Green < Yellow < Orange < Red.
 Same as VIBGYOR.

The arrangement of electromagnetic waves in accordance with their increasing or decreasing order
of frequency or wavelength is called the electromagnetic spectrum.
The electromagnetic spectrum broadly contains seven different kinds of electromagnetic waves. In
the increasing order of wavelength, the waves are:
 Gamma rays,
 X-rays,
 Ultraviolet rays,
 Visible light,
 Infrared waves,
 Microwaves, and
 Radio waves
Invisible spectrum
Electromagnetic waves can be classified and arranged according to their various
wavelengths/frequencies; this classification is known as the electromagnetic spectrum.
Parts of an Electromagnetic spectrum:
 The part of spectrum except red & violet ends is called the invisible spectrum
 The portion of the spectrum beyond the red is called the infrared spectrum
 The portion of the spectrum before violet is called the ultraviolet spectrum
Properties common to all the electromagnetic waves
 They do not require any material medium for propagation.
 They all travel with the same speed in vacuum = speed of light = 3 x108 m s-1.
 They exhibit the properties of reflection and refraction.
 They are not affected by the electric and magnetic fields.
 They are the transverse waves.

More about electromagnetic spectrum:

Ranges of
Name of Frequenc Discover Method of
wavelengt Source
the wave y in Hz y detection
h in

From
Gamma Above Less than Becquerel By their
radioactive
rays 3 x 1019 0.01 Å & Curie large
substances
 penetrating
power

When
highly
energetic
By the
electrons
fluorescenc
are stopped
3 x 1019 – 0.01 Å– e produced
X-rays Roentgen by a heavy
3 x 1016 100 Å on a zinc
metal
sulphide
target of
screen
high
melting
point

By their
Sunlight,
3 x 1016 – 100 Å – chemical
Ultraviolet Ritter arc-lamp
7.5 x 1014 4000 Å activity on
or spark
dyes

Sunlight,
light from
Other
7.5 electric
Visible 4000 Å – objects can
x 1014 – Newton bulb,
light 8000 Å be seen in
3.75 x 1014 flame,
its presence
white hot
bodies

Lamp with
3.75 thoriated Heating
Infrared 8000 Å –
x 1014 – 3 Hershell filament, effect is
waves 107 Å
x 1011 red hot more
bodies

Electronic
devices Oscillatory
Microwave 3 x 1011 – 107 Å–
Hertz like electrical
s 3 x 107 1011 Å
klystron circuit
tube
 TV and
Aerials of
Radio Below 3 Above radio
Marconi radio and
waves x 107 1011 Å transmitter
TV receiver
s

Uses of different radiations of electromagnetic spectrum:

Name of the wave Uses

· Used in medical science to kill cancer cells

Gamma rays
· In industry to check welding

· For diagnostic purposes like CT scan in medical science

X-rays
· For studying atomic arrangement in crystals

· For sterilizing purposes

Ultraviolet · For detecting the purity of gems, eggs, ghee etc.
· In producing vitamin D in food of plants and animals

· In photography, photosynthesis
Visible light
· To see objects around us

· Used for therapeutic purposes

· Used in photography at night
Infrared waves
· Used as signals during war
· Used in remote control of television and other gadgets

· Used for satellite communication

· For analysis of atomic and molecular structure
Microwaves
· For cooking in microwave ovens
· In radar communication
 · In radar communication
Radio waves
· In radio and television transmission

Scattering of light:
Scattering of light means to throw light in various random directions. Light is scattered when it
falls on various types of suspended particles in its path. Depending on the size of particles,
the scattering can be of white sunlight or of the component colours of sunlight.
Applications:
1. The sky appears blue during a clear cloudless day because the molecules in the air scatter
blue light from the sun more than they scatter red light.
2. During sunrise and sunset, the sky appears red and orange because the blue light has been
scattered out and away from the line of sight.

Why is the colour of the clear sky blue?

The sky appears blue during a clear cloudless day because the molecules in the air scatter blue
light from the sun more than they scatter red light. Scattering of light causes the blue colour of
the sky.
These are more effective in scattering light of shorter wavelengths at the blue end than the light
of longer wavelengths at the red end. The scattered blue light enters our eyes.
If the earth had no atmosphere, there would not have been any scattering. Then, the sky would
have looked dark. The sky appears dark to passengers flying at very high altitudes, as scattering
is not prominent at such heights.

Colour of the sun at sunrise and sunset

The scattering of light causes the reddish appearance of the Sun at the sunrise or the sunset.
Light from the sun near the horizon passes through thicker layers of air and larger distance in the
earth’s atmosphere before reaching our eyes. Near the horizon, most of the blue light and shorter
wavelengths are scattered away by the particles.
Therefore, the light that reaches our eyes is of longer wavelengths. This gives rise to the reddish
appearance of the Sun.