Question

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 legth of room, its intesity essentially ramain constant.
What geonetrical characteristric of LASER beam is responsible for the constnat intensity which is missing in the case of light from the bulb?

Answer 1

As the distance is doubled , the area of spherical region `(4pi r^(2))` will become four times so, the intensity becomes one fourth the initial value `(":. " I prop (1)/(r^(2)))` but in case of laser it does not spread so its intensity remain same.
Geometrical characteristic of LASER beam which is responsible for the constant intensity are as following
`"(i) Unidirection "" ""(ii) Monochromatic"`
`"(iii) Coherent light"" ""(iv) Highly collimated"`
these characteristic are missing in the case of light from the bulb.