ELECTROMAGNETIC SPECTRUM

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.

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.

VISIBLE COLOURS 8000 angstrom to 4000 angstrom ULTRA VIOLET 4000 angstrom to 0.0001 angstrom INFRA RED 8000 angstrom to 10000000000000 angstrom

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

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.

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

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.

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.