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Electromagnetic Waves and Spectrum 1. Angstrom unit ( ) : The unit used to measure wavelength of electromagnetic spectrum is called angstrom unit. One Angstrom (1 ) = 10 8 cm = l0 10 m. 2. Monochromatic light : A light which consists of one colour (or one wavelength) only is called monochromatic light. For example, red light, blue light, etc. 3. Polychromatic light : A light which is a mixture of several colours (wave bands) is called polychromatic light. For example, sunlight (white light), candle light, etc., are polychromatic lights. 4. Colour : The sensation produced in the brain due to the excitation of retina by an electromagnetic wave of some particular wavelength is called colour. 5. White colour : It is the sensation produced in the brain when retina is excited for all the colours of visible light. 6. Black colour is the sensation when retina is not excited at all as no visible light falls on it. 7. Dispersion : The phenomenon due to which a polychromatic light (white light) splits into component colours (seven colours) when passed through a prism is called dispersion. Class-X 1 Key points 8. Spectrum : A band of colours obtained on screen when polychromatic light splits into component colours is called spectrum. 9. Solar spectrum : A band of seven colours (VIBGYOR) formed on the screen when white light splits into component colours is called solar spectrum. 10. Impure spectrum : A spectrum in which various bands of colours do not have sharp boundaries but merge in each other is called impure spectrum. 11. Pure spectrum : A spectrum in which various bands of colours have sharp boundaries and not merge in each other is called pure spectrum. 12. Refrangibility : The angle through which light of a particular wavelength can deviate while passing through prism is called refrangibility. 13. (a) Refrangibility of red light is least and maximum for violet light. (b) Refractive index of a material for red light is least and maximum for violet light. (c) Seven colours of dispersed light can be recombined by using either another inverted equilateral prism or by using Newton s colour disc. 14. Invisible spectrum : The regions of spectrum which do not excite retina and hence not visible are collectively called invisible spectrum. 15. Infra-red spectrum : The invisible spectrum beyond red end of visible spectrum is called infra-red spectrum. Its range is between 8000 to 400,000 . 16. Infra-red radiations : The electromagnetic radiations, beyond red end of visible spectrum are called infra-red radiations. Class-X 2 Key points 17. Infrared radiations (i) Travel with the velocity of light, i.e., 3 l08 ms 1, (ii) obey laws of reflection and refraction, (iii) produce heating effect on being absorbed by material objects, (iv) do not scatter easily in fog. 18. Infra-red radiations are used : (i) by doctors for therapeutic purposes, (ii) in infra-red photography, (iii) night vision devices, (iv) in heat seeking missiles. 19. Ultra-violet spectrum : The region of spectrum, which extends beyond violet end of visible spectrum is called ultraviolet spectrum. 20. Ultra-violet radiations : The electromagnetic radiations found beyond violet end of visible spectrum are called ultra-violet radiations. Their range is from 4000 to 100 . 21. Ultraviolet radiations : (i) Travel with the velocity of light, (ii) cause fluorescence in zinc sulphide, (iii) bring about chemical reactions in silver salts, (iv) obey laws of reflection and refraction. 22. Ultra-violet radiations are used : (i) To stimulate production of vitamin D in human body, (ii) in detecting fake currency and diamonds, (iii) in detecting adulteration in ghee, (iv) in killing bacteria, in drinking water. Class-X 3 Key points
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