# ElectroMagnetic Waves

An electromagnatic wave is traveling in vacuum with frequency 5.7 x 1014 Hz. The wave has average total energy density of 4.6 x 10-6 J/m3.

(e) How much energy does a 1.9 m2 flat surface (perpendicular to the wave propagation direction) receive in 9 s?

E = 23598 J, how do i get that number?

4.6*10^-6 = ATC??

I1 / I0 = .871572

(b) Calculate the electric field amplitude E1 of the light after it has passed through the first polarizer. Express your answer as a fraction of the electric field amplitude E0 of the initial beam.

E1 / E0 = .933

I = c*e0*E2

I cant get the answer either thanks

Hootenanny
Staff Emeritus
Gold Member
Alt+F4 said:
An electromagnatic wave is traveling in vacuum with frequency 5.7 x 1014 Hz. The wave has average total energy density of 4.6 x 10-6 J/m3.

(e) How much energy does a 1.9 m2 flat surface (perpendicular to the wave propagation direction) receive in 9 s?

E = 23598 J, how do i get that number?
How far will the wave have travelled in 9 seconds? What is the volume of the cuboid traced by the surface and the distance travelled by the wave?

Hootenanny said:
How far will the wave have travelled in 9 seconds? What is the volume of the cuboid traced by the surface and the distance travelled by the wave?
the wave would have traveled 17.1 Meters in 9 seconds is that right? (1.9*9) = 17.1

Hootenanny
Staff Emeritus
Gold Member
Alt+F4 said:
the wave would have traveled 17.1 Meters in 9 seconds is that right? (1.9*9) = 17.1
Are you sure about that? How fast does an EM wave travel?

Hootenanny said:
Are you sure about that? How fast does an EM wave travel?
3*10^8 m/s

So 3*10^8 * 1.9 = 5.7E8 meters in a second so 5.13E9 meters in 9 seconds

Ok Got it thanks

Last edited:
Alt+F4 said:
I1 / I0 = .871572

(b) Calculate the electric field amplitude E1 of the light after it has passed through the first polarizer. Express your answer as a fraction of the electric field amplitude E0 of the initial beam.

E1 / E0 = .933

I = c*e0*E2

I cant get the answer either thanks
i have no idea on this one, i cant get the ratio at all

Hootenanny
Staff Emeritus
Gold Member
Alt+F4 said:
3*10^8 m/s

So 3*10^8 * 1.9 = 5.7E8 meters in a second so 5.13E9 meters in 9 seconds

Ok Got it thanks

Careful, your dealing with a volume there not a length, never the less your answer should be correct.

$$\hline$$
For your next question it may be useful to note that;

$$\frac{I}{I_{0}} = \cos^2\theta$$

and

$$E = E_{0}\cos\theta$$

Although it is useful to remember that The ratio of the intensities is equal to the square of the rms of the Electric field.

Thanks alot, one last question

(c) The second polarizer is set at various angles within the range q 2 = 0 to 90°. Calculate the intensity of the light after it has passed through the second polarizer for the following values of q 2. Express each answer as a fraction of I1.

At q 2 = 22°: I2 / I1 =

So i am guessin i need to find final intensity which would be

S = S0 ((cos theta)^2)^2

is that right?

Alt+F4 said:
Thanks alot, one last question

(c) The second polarizer is set at various angles within the range q 2 = 0 to 90°. Calculate the intensity of the light after it has passed through the second polarizer for the following values of q 2. Express each answer as a fraction of I1.

At q 2 = 22°: I2 / I1 =

So i am guessin i need to find final intensity which would be

S = S0 ((cos theta)^2)^2

is that right?
do u have to calculate two diffrent numbers and then add them?

help meeeeeee

Hootenanny
Staff Emeritus
$$\frac{I}{I_{0}} = \cos^2\theta$$