# Homework Help: Interference in Air Wedges exercise

1. Dec 18, 2011

### Supergabson

I have this question for my physics assignment.
1. The problem statement, all variables and given/known data
Two plane glass plates 10.0 cm long, touching at one end , are separated at the other end by strip of paper 1.5x10$^{-3}$ mm thick. When the plates are illuminated by monochromatic light, the avreage distance between constructive dark fringes is 0.20 cm calculate the wave length of the light.

I used the equation 2t=$\frac{λ}{2}$ but it gives me the wavelength of light in glass and I am looking for wavelength in air. please help

My physics book doesn't say how to do this type of question :(

2. Relevant equations
how to solve this question?

2. Dec 18, 2011

### technician

The interference occurs as a result of the path in the air wedge so you are correct.... you need the λ of the light in air.
One other consideration is that a phase shift of ∏ occurs at the reflection from the air/glass surface.

3. Dec 18, 2011

### technician

Just realised !!!!
The question wants you to calculate the wavelength of light !!!
You have information about the separation of dark fringes, the dimensions of the glass and the separation of the glass plates

4. Dec 18, 2011

### Supergabson

What formula should I use that has the dimensions of the glass? I have no idea how the dimensions of the glass can lead me to the right answer

5. Dec 18, 2011

### ehild

You need the dimension of the glass to get the thickness at distance d from the edge. The interference happens between the wave reflected from the front air wedge-glass interface and the one reflected from the back interface: the blue and green rays in the figure. "t " means the thickness at the spot of a dark strip along the wedge. As the green wave changes phase at the back air-glass interface, destructive interference occurs when 2t n(air) = mλ. The thickness varies with the distance d from the edge, how? For the consecutive dark strips, separated by 0.2 cm,
the order m differs by 1. What is the difference of the thickness between those spots?

ehild

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