# Homework Help: Standing waves and refractive index question. help needed

1. Dec 2, 2012

### ppy

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

Two optically flat glass plates, 16.0 cm long, are in contact at one end and separated by 0.0200 mm at the other end. The space between the plates is occupied by oil with index of refraction 1.45. The index of refraction of the glass plates is 1.55. The plates are illuminated at normal incidence with monochromatic light, and fringes are observed. If the dark fringes are spaced 2.00 mm apart, what is the wavelength of the monochromatic light?

2. Relevant equations

n=c/v where n=index of refraction v= velocity in medium

maybe y=(p * wavelength*L)/a y=position of dark fringe p=0,1,2,....

a=slit width L= length from slit to screen

This is the equation for single slit diffraction.

3. The attempt at a solution

I am not sure. Is this question to do with single slit diffraction. If so how do i use the refactive index of the glass and oil in the question. alltogether very confused.

Help appreciated thanks.

Also is the light source shining on the side of the glass plate?

2. Dec 3, 2012

### ehild

It is thin-film interference on the oil wedge.

ehild

3. Dec 3, 2012

### ppy

How do i attempt the question though? What do i us etc?

4. Dec 3, 2012

5. Dec 3, 2012

### ppy

I have tried that link and found this equation
2nt=(m+1/2)* wavelength

but is n the refractive index of the glass or the oil. Also what is t in response to my question? Is it the thickness of the wedge and if so what is the thickness of the wedge in the question?

Really do not understand

6. Dec 3, 2012

### ehild

n is the refractive index of the material between the plates, and t is the thickness of the wedge at the place where destructive interference occurs. The thickness of the wedge changes, and m, the range of interference changes by 1 between a fringe and the next one. You know that the fringes are 2 mm apart. How much does the thickness change between the fringes then? You know both the length and the height of the wedge.

ehild