Coherent interference of four sources

Grating

A transmission grating is an opaque plate with many closely and regularly-spaced slits on it. It sends light into extremely well-defined directions.

When the incident light is white, the forward direction has a white central fringe.

However, the other fringes show a spectrum of colors.

The first interference maximum (fringe) spreads out according to the wavelength of the colors: blue, green, red. Then there is the secondary fringe with various colors. Higher-order fringes usually overlap.• One sees iridescent of colors from a grating.

Reflection Grating

If there are many regularly spaced reflecting surfaces in an array, we have a Reflection Grating (credit card)

Example of Reflection Grating

“Eye” of a peacock feather, Scales of butterfly wings, Scales of some snakes, Beatles

3-d grating

X-ray scattering.

Interferometers

Devices that make accurate measurements by the use of interference.

Split the beam into two and pass one of them through something that can affect the light and then look at the interference pattern to • Measure the speed of the Earth relative to ether.

• Measure the air temperature variation.

• Measure the gravitational wave…..

Standing Waves---a special example of interference

When two waves with equal frequency and amplitudes travel against each other, they form a standing wave:• Vibration in violin,

• Wave from a rope tied to a wall. The pattern of interference is fixed in

space. The destructive interference happens at the nodes and constructive interference happens at the antinodes.

Lippmann Plate:

Mirror surface is coated with a thick layer of fine-grain photographic emulsion. A reflected wave from the mirror interferes with the incident wave to form an interference pattern which is recorded by the emulsion.

Lippmann plate

Interference filter

Two thin films with highly reflective surfaces.

The light in between the film can form standing waves with certain frequency.

The incident wave is transmitted through the films only at this frequency.