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What happens to the intensity of light f...

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?

Text Solution

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Intensity of light (I ) from the bulb varies with distance ( r) from the bulb as
` I prop (1)/( r^2)`
IF the distance from the bulb becomes twice the intensity of light is reduced to one fourth because the light beam speads on approaching a spherical region of area ` 4 pi r^2` .
Laser beam , on the other hand does not spread Hence , its intensity remains constant with distance the Geometrical characteristics of laser beam whcih are responsible for the constant intensity are :
* Monochromatic
* Unidirectional
* Coherent
Light from a bulb does not possess these propeties .
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