Home
Class 12
PHYSICS
A satisfactory photographic print is obt...

A satisfactory photographic print is obtained when the exposure time is `10 sec` at a distance of `2m` from a `60 cd` lamp. The time of exposure required for the same quality print at a distance of `4m` from a `120 cd` lamp is

A

5 s

B

10 s

C

15 s

D

20 s

Text Solution

Verified by Experts

The correct Answer is:
D
`I_(1) D_(1)^(2) t_(1) = I_(2) D_(2)^(2) t_(2)" " [ D rarr " diameter of aperature" ] `
` I = (L)/(r^(2)) " and " D_(1) = D_(2)`
`therefore " " (L_(1))/(r_(1)^(2)) xx t_(1) = (L_(2))/(r_(2)^(2)) xx t_(2)`
`rArr " " (60)/((2)^(2)) xx 10 = (120)/((4)^(2)) xx t`
`rArr t = 20 ` sec .
Promotional Banner

Similar Questions

Explore conceptually related problems

A good photographic print is obtained by an exposure of two seconds at a distance of 20 cm from the lamp. Calculate the time of exposure required to get an equally good result at a distance of 40 cm.

The proper exposure time for a photographic print is 20 s at a distance of 0.6 m from a 40 candle power lamp. How long will you expose the same print at a distance of 1.2 m from a 20 candle power lamp ?

The proper expsoure time for a photographic print is 20s at a distance of 0.6m froma 40 candles power lamb.How long will you expose the same print at a distance of 1.2m form a 20 candles power lamb?

A photographic print is found satisfactory when the exposure was for 16 s at a distance of 20 m from a 25-watts lamp. At what distance must the same paper be held from a 50-watts lamp in order that an exposure of 25 a may give the same result?