Class 12 NEET Physics Dual Nature of Matter and Radiation

For a light source of power P watt, the intensity at a distance d is given by I=

P

4πd2

where we assume light to spread out uniformly in all directions i.e., it is a spherical source.
∴I∝

1

d2

or

I1

I2

=

d22

d12

or,

I1

I2

=(

1

0.5

)2 or,

I1

I2

=4 or, I2=

I1

4

.
In a photoelectric emission, the number of photoelectrons liberated per second from a photosensitive metallic surface is proportional to the intensity of the light. When a intensity of source is reduced by a factor of four, the number of photoelectrons is also reduced by a factor of 4