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The correct exposure time for a photographic print is 20sec at a distance of 2feet from a 40candela lamp. The exposure time for the same print at a distance of 4 feet from a 20candela lamp is.

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sir,

The correct exposure time for a photographic print is 20sec at a distance of 2feet from a 40candela lamp. The exposure time for the same print at a distance of 4 feet from a 20candela lamp is.

Asked by advguptaneeraj4 4th May 2015, 4:23 PM
Answered by Expert
Answer:

Hi Vishesh,

The exposure time for a particular print from a 40 Cd lamp is 20 sec at 2 feet.

Now, the lamp is changed and the distance is also changed.

The intensity of new lamp is reduced to half and the distance is doubled.

We know that intensity is inversely proportional to square of the distance.

Hence, if intensity is halved and distance is doubled, then we have

begin mathsize 14px style straight I equals fraction numerator begin display style bevelled 1 half end style straight I subscript 0 over denominator open parentheses 2 straight r close parentheses squared end fraction equals 1 over 8 straight I subscript 0 over straight r end style

Hence, the exposure to get the same print has to be 8 times, i.e. 20 × 8 = 160 sec = 2 min 40 secs.

Answered by Expert 4th May 2015, 6:32 PM
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