Question

Time of exposure for a photographic print is 10 s, when a lamp of 50 cd is placed at 1 m from it. Then another lamp of luminous intensity I is used, and is kept at 2 m from it. If the time of exposure now is 20 sec, the value of I is (cd)

• A. 200
• B. 20
• C. 100
• D. 25

Answer 1

Answer: (C)

100

Answer 2

$Using \,I_1\,t_1=I_2,t_2,we\,get \frac{L_1}{d_2^2}t_1=\frac{L_2}{d_2^2}t_2\,i,e.L_2=\frac{L_1}{d^2_1}\times \frac{d^2_2}{t_2}=\frac{50 \times 10 \times 4}{20}=100\,cd$