An electromagnetic wave of intensity 200 W/m 2 is incident perpendicularly on a

Question

An electromagnetic wave of intensity 200 W/m2 is incident perpendicularly on a rectangular black card with sides of 20 cm and 30 cm that absorbs all the radiation.

a. Find the force exerted on the card by the radiation.

b. Find the peak value of the electric AND magnetic field of the electromagnetic wave.

c. Which field carries more energy?

d.You send this light (assumed unpolarized) through a polarizer at an angle of 45 degrees with respect to the vertical and then another polarizer at an angle of 90 degrees with respect to the vertical (so, horizontal).

Explanation / Answer

a) radiation pressure on completely obsorbing surface,

P = I/c

= 200/(3*10^8)

= 6.67*10^-7 pa

Force, F = P*A

= 6.67*10^-7*0.2*0.3

= 4.00*10^-8 N

b)

we know,

Emax = sqrt(I*2*c*mue)

= sqrt(200*2*3*10^8*1000*4*pi*10^-7)

= 1.23*10^4 N/c

Bmax = Emax/c

= 1.23*10^4/(3*10^8)

= 4.1*10^-5 T

c) Both carry same amount of energy.

d)

Inetsnity through first polaizer, I1 = Io/2

intensity through second polarizer, I2 = I1*cos^2(45)

If = (Io/2)*cos^2(45)

If = Io/4

If/Io = 0.25

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