Question

A small helium-neon laser emits red visible light with a power of 3.80 mW in a...

A small helium-neon laser emits red visible light with a power of 3.80 mW in a beam that has a diameter of 2.70 mm.

(a) What are the amplitudes of the electric and magnetic fields of the light?

Emax =  V/m
Bmax =  T


(b) What are the average energy densities associated with the electric field and with the magnetic field?

uE,av =  J/m3
uB,av =  J/m3


(c) What is the total energy contained in a 1.00-m length of the beam?
J

Homework Answers

Answer #1

Given Data:

P = 3.80 mw = 3.80 x 10-3 W

D = 2.70mm = 2.70 x 10-3 m

Part A:

laser intensity I = P/A

I = 4P/D2

I = (4*3.80 x 10-3)/[3.14*(2.70 x 10-3)2]

I = 664.028 W/m2

and we also know that I = ½*o*c*E2

E = sqrt(2I/oc)

E = sqrt(2*664.028/8.854 x 10-12*3 x 108)

E = sqrt(499983.43)

Emax = 707.1 V/m

and

B = E/c

B = (707.1)/(3 x 108)

Bmax = 2.357 x 10-6 T

Part 2:

uBav = uEav = ¼*e*E2max

uBav = ¼*8.854 x 10-12­*707.12

uBav = 1.1 x 10-6 J/m3

Part 3:

Etot = utot*V

Etot = 2*uE*0.25**D2*L

Etot = 2*1.1 x 10-6*0.25*3.1415*0.00272*1

Etot = 1.2589 x 10-11 J

Etot = 1.26 x 10-11 J

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