Simple Newton's Rings problem

1. Apr 24, 2008

Jason+Strife

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

If the apparatus for showing Newton's rings is illuminated with light at 6250 A (angstroms), what thickness of film underlies each of the first three light rings?

2. Relevant equations

2D (the thickness of the film) equals a multiple of 1/2 of a wavelength
1 angstrom = 1 x 10^-10 wavelength

3. The attempt at a solution

I can't see how the angstroms connect to the fact that the thickness must be a multiple of 1/2.

Last edited: Apr 24, 2008
2. Apr 24, 2008

Staff: Mentor

Generally Newton's Rings are formed when light is reflected off of a hemispherical lens resting on a sheet of glass. So I don't know what you mean by "film"--there is a space between the lens and the glass sheet.

In any case, the phase difference (for constructive interference) must be an odd multiple of 1/2 wavelength, not just 1/2.

3. Apr 24, 2008

Jason+Strife

I apologize for not clarifying wavelength, but I intended to. As for the film, I believe it means some form of glass, but its not really relevant to the solution of my problem.

4. Apr 24, 2008

Staff: Mentor

Do you still have a question?

5. Apr 24, 2008

Jason+Strife

Yes, I dont see a way to connect the 6250 angstroms and the fact that each wavelength must be a multiple of a half. There must be some equation I'm missing, but I just can't figure out the problem.

6. Apr 24, 2008

Staff: Mentor

That's not quite right.

6250 Angstroms is the wavelength; 2D must be an odd multiple of 1/2 wavelength.

An Angstrom is just a unit of length. 1 Angstrom $= 10^{-10}$ meters.

7. Apr 24, 2008

Jason+Strife

OK, but how can I use that information (the angstroms) to figure out the thickness for the first three light rings?

8. Apr 24, 2008

Staff: Mentor

By setting 2D equal to the first three odd integer multiples of 1/2 wavelength. (Solve for D.)

9. Apr 24, 2008

Jason+Strife

Aha, thank you, I appreciate your help.