What happens to the intensity of light from a bulb if the distance from the bulb is doubled? As a laser beam travels across the length of a room, its intensity essentially remains constant. What geometrical characteristics of the LASER beam is responsible for the constant intensity which is missing in the case of light from the bulb?
Bulb spreads its light in all ground spherically and symmetrically. So, if the distance from the bulb is doubled, the surface area covered by radiations change from 4πr2 to 4π(2r)2ie., 22 or four times decreased in straight laser. But in a bulb it decreased by 4(4π) ie., 16π decreased. So, the intensity becomes one-fourth the initial value in straight line. Since (I∝r21) and for spherical source (I∝4πr21) In case of laser, it does not spread in all directions. It passes only along a straight line. So, its intensity remains same almost. Geometrical characteristics of LASER beam which is responsible for the constant intensity are: (i) Monochromatic (ii) Coherent (iii) Highly collimated from all around source, (iv) Unidirectional these characteristics are missing in the case of light from the bulb.
