Intensity of the electromagnetic radiation is given as
I=
Energy
Area × Time
=
Power
Area
=Uavg C=
1
2
Cε0E02
Here, Uavg = average energy density, C= speed of radiation in air (or vacuum), ε0= permittivity of the free space and E0= peak value of the electric field. ⇒P∝E02⇒
P1
P2
=
E12
E22
⇒
E1
E2
=√
P1
P2
Substituting the values in the above equation, we get ⇒