Answer:

Step-by-step explanation:
For a totally reflecting surface, the radiation pressure is related to the intensity of radiation by
(1)
where
I is the intensity of radiation
c is the speed of light
The radiation pressure is given by

where in this case:
is the force exerted by the beam
A is the area on which the force is exerted
The beam has a diameter of d = 1.30 mm, so its area is

Now we can write eq(1) as

From which we find the intensity of radiation, I:

Now we can find the amplitude of the electric field using the equation

where
is the vacuum permittivity
E is the amplitude of the electric field
And solving for E,
