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An electric bulb illuminates a plane sur...

An electric bulb illuminates a plane surface. The intensity of illumination on the surface at a point 2m away from the bulb is `5 xx 10^(-4)` phot. The line joining the bulb to the point makes an angle of `60^(@)` with the normal to the surface. calculate the intensity of the bulb in candela ? (1 phot `= 1m//cm^(2))`.

A

`40sqrt3`

B

`40`

C

`20`

D

`40 xx 10 ^(-4)`

Text Solution

Verified by Experts

The correct Answer is:
B
The intensity of illumination at a surface is given
`E = (I cos theta )/( r ^(2))`
`E = 5 xx 10 ^(-4) "lumen/cm"^(2) = 5 xx 10 ^(-4) xx 10 ^(4) "lumen/m"^(2)`
`or I = ( Er ^(2))/( cos theta ) = ( 5 xx (2) ^(2))/( cos 60 ^(@))= 40` candela
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