Q8 of 38 Page 454

A photographic plate placed at a distance of 5 cm from a weak point source is exposed for 3 s. If the plate is kept at a distance of 10 cm from the source, the time needed for the same exposure is

Given:


Distance,


Distance,


Calculation:


Consider the incident illuminance is E0, illuminance at distance 3 cm and 10 cm are E1 and E2 respectively.


Illuminance at 3 cm is given by,



Time is given by,





Thus, time required is,




So, option B is correct.

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