# Homework Help: Light intensity in matter

1. Apr 22, 2015

### sunmaggot

1. The problem statement, all variables and given/known data
An electromagnetic wave with frequency 65.0Hz travels in an insulating magnetic material that has dielectric constant 3.64 and relative permeability 5.18 at this frequency. The electric field has amplitude 7.20×10−3V/m.

What is the intensity of the wave?

2. Relevant equations
I = S average = 1/2 ε0 c Emax2

3. The attempt at a solution
the above equation is what I only have from my textbook, and it is for vacuum case. However, in the question, it has dielectric constant and relative permeability. I tried changing c into the light speed in the medium, not correct. I tried using equation from wiki, that is, times refractive index in the above equation, doesn't work. I tried simply ignore the refractive index problem, and doesn't work.

2. Apr 23, 2015

### Simon Bridge

How do you know "it didn't work"?

You book should have shown you a derivation for the formula it gave you... check that you are accounting for all the assumptions correctly.
Your book should include some text about how EM fields are different in a medium.
Have a go deriving the formula for intensity in a magnetic medium.

3. Sep 20, 2017

### forkinsocket

Can someone help? I have literally the exact same question, which I already gave up on and have the answer for, but still don't understand.

My book says that the required modifications for the intensity in a medium are "simple." Replace $\epsilon_0$ with the permittivity $\epsilon$, replace $\mu_0$ with the permeability $\mu$, and replace $c$ with the speed $v$.

I replaced $\mu_0$ with the permeability given, and replaced $c$ with the speed found in a previous problem, and still got the problem completely wrong (by many orders of magnitude). I did:
$$(Emax * Bmax)/(2*5.18)$$ and $$(Emax ^2)/(2*5.18*v)$$ [all with the correct substitutions as found in previous problems] and both ended up with the same result, which was wrong.