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