Question

The time of exposure is 4 s to print a photograph from a negative when source is 1 m away. If the source is placed 0.5 m away the time of exposure will be

• A. $4s$
• B. $2s$
• C. $1s$
• D. $0.5s$

Answer 1

The correct option is C $1s$

$\frac{I_1}{r_1^2}{t}_1=\frac{I_2{t}_2}{r_2^2}$

$t_2=\frac{t_1{r}_2^2}{r_2^2}$

$=\frac{{10.51}^2\times 4}{I^2}=1s$