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- Homework Statement:
- For case where light comes from the glass and reflects off a soap film with vacuum on the other side, what is the formula used to give an interference maximum for reflected light?

- Relevant Equations:
- Thin - film interference

The light comes from glass to boundary of glass and soap film, then there will be light which is reflected and transmitted. The reflected light has no phase shift and the transmitted light will hit the boundary of soap film and vacuum and the reflected light from this boundary will also has no phase shift.

Because the two reflected light has no phase shift, the path difference (2d, where d is the thickness of the soap film) should be mλ (m is integer and λ is the wavelength of the light), so the formula used to give an interference maximum for reflected light will be: 2d = mλ

But the answer key is: 2d = (m + 1/2)λ

Where is my mistake? Thanks

Because the two reflected light has no phase shift, the path difference (2d, where d is the thickness of the soap film) should be mλ (m is integer and λ is the wavelength of the light), so the formula used to give an interference maximum for reflected light will be: 2d = mλ

But the answer key is: 2d = (m + 1/2)λ

Where is my mistake? Thanks