A balloon made of thin rubber becomes larger and rounder when you fill it with air. It is an interesting topic how we can measure the thickness of the thin skin of the balloon. As shown in Fig. 1, a small local area of the balloon surface can be considered as a thin film made of rubber with uniform thickness. When light is incident on the film, the waves reflected from the two surfaces of the film interfere with one another. The thickness of the film can be measured by interference signals.

A tungsten halogen lamp, which is a wide-spectrum source, is used. The incident rays traveling in fiber 1 are normal to the film as in Fig. 1. Since the refraction index of rubber is greater than that of air, Ray 2, which is reflected from the upper surface, undergoes a phase change of 180° with respect...

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