Screen S is illuminated by two point sources A and B. Another source C sends a parallel beam of light towards point P on the screen (see figure). Line AP is normal to the screen and the lines AP, BP and CP are in one plane. The distance AP, BP and CP are 3m, 1.5m and 1.5m respectively. The radiant powers of sources A and B are `90 watts` and `180 watts` respectively. The beam from C is of intensity `(20watts)/(m2)`. Calculate the intensity at P on the screen. `

Answer

A, C

Explanation

The total intensity at point P will be `=I_(A)+I_(B)+I_(C)`
`I_(A)=((illumination power)xxcostheta)/(4pir^(2))=(90xxcostheta)/(4pixx3^(2))=(10)/(4pi)` `watts/m^(2)`
`I_(B)=(180xxcos60degree)/(4pixx(1.5)^(2))=(10)/(pi)``watt/m^(2)`
`I_(C)=20cos60degree=10`
`:'``I_(p)=(10)/(4pi)+(10)/(pi)+10=13.9 W/m^(2)`