# Thin Film Interference- Transmission

1. Dec 15, 2011

### jayjay713

1. The problem statement, all variables and given/known data

A thin layer of water (n=1.33) is surrounded by air what wavelength of light will not be transmitted through the water at the point where the water is 113 nm thick?

3. The attempt at a solution

- In phase
- Path difference of 2T.
- Constructive Interference

I just don't know how to set up the formula to find the wavelength. I know to find the wavelength in water I divide wavelenght in air by the index of refraction of water.

Any help appreciated, cheers.

2. Dec 15, 2011

### technician

Are you certain the question asks for light not TRANSMITTED. ?

3. Dec 15, 2011

### jayjay713

yes! The answer is 601 nm.

If you take the wavelength and times it by the index of refraction of water (1.33), you get 150.29. Multiply that by 4 and get ~601 nm. Don't know how that fits into the formula of 2T = wavelength / something (in example its over 4 or 2, don't know how to get that value)

4. Dec 15, 2011

### technician

there is some confusion here.
The wavelength of light in water is shorter than in air =λair/1.33

5. Dec 15, 2011

### ehild

For the transmittance of a thin layer surrounded by air, the condition for destructive interference is 2Tn=(m+1/2)lambda. The transmittance is minimum at λ=601 nm but it is not zero. The reflectance can be zero because of destructive interference, the transmittance cannot.

ehild

6. Dec 15, 2011

### technician

It is not a weird total internal reflection question...... is it??????
I agree with ehild .....there will always be some transmission..... mostly transmission.
It is possible to get no reflection as a result of interference but this requires some reflection from the air/water and water/air boundaries. This is never 100% (unless it involves total internal reflection)