Time of exposure for a photographic print is 10 seconds, 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 s, the value of I (in cd) is

100

$v=\frac{c}{λ}=\frac{3×10^8}{1}=3×10^8Hz$

$v'=\frac{c}{λ'}=\frac{3×10^8}{10}=3×10^7Hz$