Diffraction at a narrow obstacle (line) - Babinet's princip-le

Principle

In this experiment the students should come to realize that in the shadow area behind a narrow obstacle regular patterns of brightness will be found which are formed by diffraction at the edges of the obstacle. By comparing the diffraction pattern with that created by diffraction at a slit having the same width as the obstacle, they should gain an understanding of Babinet's Principle and find it confirmed.
Babinet's Principle states that diffraction phenomena behind complementary screens are identical. This diaphragm employed has a slit and a plate (narrow obstacle) of identical size; they act as screens which are complementary to each other, i.e. together they bring about total extinction. Therefore, in the observation plane the amplitudes of the waves which interfere following diffraction at a slit or obstacle, must be identical but phase displaced by 180°.

Benefits

• Multifunctional light box - All-in-one: Can be used for geometric optics on the table, colour mixing and on an optical bench
• Extension with others sets at anytime, no additional light sources needed, recognition value for students

Tasks

What sort of shadow is created behind a narrow obstacle? Direct a narrow pencil of light onto a slender obstacle so that part of the beam grazes the side. Observe the shadow thus created and compare it with the diffraction pattern formed when the light beam strikes a slit of the same width as the obstacle.