The colours seen in soap bubbles are due to interference between light reflected from the two surfaces of the bubble. A similar interference effect occurs in thin layers of oil on water. A practical use of this kind of interference is found in the coating of high quality lenses for cameras, telescopes etc.

When light enters a lens some of it is reflected from the lens surface.

The amount of light entering the optical instrument is reduced by this reflection. The next diagram shows a magnified view of the front surface of a coated (or “bloomed”) lens in which two reflections occur.

Both these reflections are external reflections (light going from less to a more dense medium: phase change of rads). Therefore, if the thickness of the film is equal to the two reflections will interfere destructively. The destructive interference means that the net amount of light reflected is reduced and the amount of light transmitted into the instruments is greater than without the thin film. This might sound paradoxical and a (slightly) more satisfying explanation can be found using the particle model of light. However, it works: a “bloomed” lens transmits more light than one with no coating!

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