The intensity of sound from a radio at a distance of 2 metres from its speaker is `1xx0^(-2) mu W//m^(2)`. The intensity at a distance of 10 meters would be

Options

A. `0.2 xx 10^(-2) muW//m^(2)`
B. `1 xx 10^(-2) muW//m^(2)`
C. `4 xx 10^(-2) muW//m^(2)`
D. `5 xx 10^(-2) muW//m^(2)`

Answer

C

Explanation

For an isotropic source we use
`I infty (1)/(r^(2))`
`I_(2)/(I_(1)) (r_(1)^(2))/(r_(2)^(2))`
implies ` (I_(2))/(1 xx 10^(-2)) = (2^(2))/(10^(2)) = (4)/(100)`
implies `I_(2) = (4xx10^(-2))/(100) = 4 xx 10^(-4) mu W//m^(2)`.