1. ELECTROMAGNETIC SPECTRUM

2. LIGHT ENERGY IS ELECTROMAGNETIC IN NATURE
IT CAUSES A SENSATION OF VISION
ELECTROMAGNETIC RADIATIONS WHICH CAN EXCITE OUR RETINA HAS WAVELENGTH BETWEEN 8000 ANGSTROM TO 4000 ANGSTROM
ANGSTROM IS THE UNIT TO MEASURE WAVELENGTH.
PURE SPECTRUM: THE BAND OF COLOURS OBTAINED ON A SCREEN WHEN A POLYCHROMATIC LIGHT SPLITS INTO ITS CONSTITUENT COLOURS.
IMPURE SPECTRUM: THE BAND OF COLOURS OBTAINED WHICH HAS NO SHARPLY DEFINED BOUNDARIES
DISPERSION: THE PHENOMENON DUE TO WHICH A POLYCHROMATIC LIGHT SPLITS INTO CMPONENT COLOURS WHEN PASSED THROUGH A PRISM.

3. DEPENDENCE OF DEVIATION ON COLOUR
IF THE LIGHT ENTERING THE PRISM IS A MIXTURE OF DIFFERENT COLOURS, THEN THE EMERGENT BEAM ALSO HAS DIFFERENT COLOURS, BUT ARRANGED IN A DEFINITE ORDER.
THE REASON IS THAT DIFFERENT COLOURS HAVE DIFFERENT SPEEDS.
SPEED REDUCES WHEN THE WAVELENGTH OF LIGHT DECREASES. (i.e) SPEED IS MAXIMUM FOR RED (8000 ANGSTROMS) AND LEAST FOR VIOLET (4000 ANGSTROMS).
THEREFORE VIOLET IS DEVIATED THE MOST AND RED THE LEAST.

4. VISIBLE COLOURS
8000 angstrom to 4000 angstrom
ULTRA VIOLET
4000 angstrom to angstrom
INFRA RED
8000 angstrom to angstrom

5. ULTRA VIOLET RADIATIONS
Sources of ultra violet radiations
any source of light having a temperature of more than 2500
degrees centigrade
properties of ultra violet radiations
they are not affected by electrical or magnetic fields
they produce fluorescence in substances like zinc sulphide,
barium sulphide etc.
uses of ultra violet radiations
used for sterilising surgical instruments and drinking water
used to detect adulteration in ghee

6. INFRA RED RADIATIONS Sources of infra red radiations
any substance which emits heat
for higher intensity, sun, arc lamp etc.
Properties of infra red radiations
not affected by electrical or magnetic fields
least scattered by fog, mist due to their long
wavelength
Uses of infra red radiations
used by doctors for therapeutic purposes
as these radiations have long wavelength, they do
not get scattered. So they can be used for taking
Infra red photographs.

7. ELECTROMAGNETIC SPECTRUM AND ITS BROAD CLASSIFICATION
wavelength V I S B L E
Gamma rays
Infra red
Micro wave
Radio wave
X- rays UV
frequency
R T M I VISIBLE U X G S DECREASING WAVELENGTH
INCREASING FREQUENCY

8. SCATTERING
When sunlight enters the earth’s atmosphere, it gets scattered (spreads in all directions) by the dust particles and air molecules.
SCATTERING IS THE PROCESS OF ABSORPTION AND RE-EMISSION OF LIGHT ENERGY
The intensity of scattered light is inversely proportional to the fourth power of wavelength. (viz) violet has the least wavelength. So it is scattered most. Red has the maximum wavelength. It is scattered least.
Therefore light has lesser intensity of violet, blue, indigo.
It has greater intensity of orange, red.

9. APPLICATIONS OF SCATTERING
RED COLOUR OF SUN AT SUNRISE AND SUNSET
light travels the longest distance to reach the observer. Since red has
the longest wavelength, it is scattered least and reaches our eye.
other colours get scattered. The sky appears red because red colour
scatters closest to the eye.
BLUE COLOUR OF SKY
As light travels through the atmosphere, it gets scattered in different
directions due to the air molecules and dust particles. The blue of
shorter wavelength is scattered more. The sky appears blue because
the particles scattering blue colour are nearest to the eye.