Question 11 Mark
A sphere of radius 1.00cm is placed in the path of a parallel beam of light of large aperture. The intensity of the light is $0.5W/cm^{-2}$. If the sphere completely absorbs the radiation falling on it, find the force exerted by the light beam on the sphere.
Answer
View full question & answer→We know,If a perfectly reflecting solid sphere of radius ‘r’ is kept in the path of a parallel beam of light of large aperture if intensity is I,
$\text{Force}=\frac{\pi\text{r}^2\text{I}}{\text{C}}$
$\text{I}=0.5\text{W/m}^2.\text{r}=1\text{cm},\text{C}=3\times10^8\text{m/s}$
$\text{Force}=\frac{\pi(1)^2\times0.5}{3\times10^8}=\frac{3.14\times0.5}{3\times10^8}$
$=0.523\times10^{-8}=5.2\times10^{-9}\text{N.}$
$\text{Force}=\frac{\pi\text{r}^2\text{I}}{\text{C}}$
$\text{I}=0.5\text{W/m}^2.\text{r}=1\text{cm},\text{C}=3\times10^8\text{m/s}$
$\text{Force}=\frac{\pi(1)^2\times0.5}{3\times10^8}=\frac{3.14\times0.5}{3\times10^8}$
$=0.523\times10^{-8}=5.2\times10^{-9}\text{N.}$