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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
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