What is the minimum thickness of a soap bubble for which the first order of brig

What is the minimum thickness of a soap bubble for which the first order of bright fringe is observed in reflected light? (2020)

What is the minimum thickness of a soap bubble for which the first order of bright fringe is observed in reflected light? (2020)

Step 1: Understand that a soap bubble has two surfaces: the outer surface and the inner surface.
Step 2: When light hits the soap bubble, some of it reflects off the outer surface and some off the inner surface.
Step 3: When light reflects off a surface, it can undergo a phase change. For a soap bubble, light reflecting off the outer surface does not change phase, while light reflecting off the inner surface does change phase by 180 degrees (or half a wavelength).
Step 4: For a bright fringe to be observed, the path difference between the two reflected light waves must be equal to a whole number of wavelengths (nλ), where n is an integer (0, 1, 2, ...).
Step 5: The first order of bright fringe corresponds to n = 1, which means the path difference must equal λ.
Step 6: However, because of the phase change of 180 degrees from the inner reflection, we need to adjust our condition. The effective path difference for the first order bright fringe becomes λ/2.
Step 7: The path difference is also related to the thickness of the soap bubble. The light travels down and back up through the bubble, so the effective thickness is 2t, where t is the thickness of the bubble.
Step 8: Set the effective path difference equal to λ/2: 2t = λ/2.
Step 9: Solve for t: t = λ/4. This means the minimum thickness of the soap bubble for the first order of bright fringe is λ/4.

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