The shearing interferometer is an extremely simple means to observe interference. The laser source should have a coherence length which is longer than the thickness of the shear plate so that the basic condition for interference is fulfilled. The phenomenon can be used to test collimation of light beams.

The shear plate made from high quality optical glass (like NBK-7, Fused silica) has extremely flat optical surfaces and a slight angle between them. When a plane wave is incident at an angle of 45° (which gives maximum sensitivity) both the surfaces reflect light as shown in the figure. The two reflections are laterally separated due to the finite thickness of the plate and by the wedge. This separation is referred to as the shear and has given the instrument its name.

Shear plate is made from N-BK7 optical glass. It is fixed on a rotary mount and kept in a closed chamber with two optical ports for the protection of optics. Mounts and enclosures are made of aluminium alloy and finished by black anodizing to avoid reflections from the mounting.