Subject: Applied Physics 2

Topic: Interference And Diffraction Of Light

Difficulty: Medium

Thin film is the film whose thickness is of the order of wavelength of the light which is used to expose it.

When a thin film is exposed to white light from an extended source, it shows beautiful colours in the reflected system. Light is reflected from the top and bottom surfaces of a thin film and the reflected rays interfere.

The path difference between the interfering rays depends on the thickness of the film and the angle of refraction r and hence on the inclination of the incident ray.

White light consists of a continuous range of wavelengths. At a particular point of the film and for a particular position of the eye (i.e.t and r constant) those wavelengths of incident light that satisfy the condition for the constructive interference in the reflected system will be seen in reflected light.

The colouration will vary with the thickness of the film and inclination of the rays (i.e. With the position of the eye with respect to the film).Hence if the same point of the film is observed with an eye in different positions or different points of the film are observed with the eye in the same position, a different set of colours is observed each time.