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# Laser: Interference and diffraction

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A helium-neon laser provides coherent red light (wavelength < 1 &#956;m); the dimensions of the apertures or obstacles that we place in the laser beam are correspondingly small.

A white sheet of paper attached with magnets to a screen mounted on one side panel of a lab bench.

Laser set up at the other end of the bench so that it points at the paper.

Laser is switched on.

What would be the size of the laser beam spot (reasonable estimate)?

A piece of silk is placed in the laser beam (close to the laser and perpendicular to the beam).

What would be the pattern of the transmitted light on the screen?

Could this be a shadow pattern of the threads in the cloth?

Silk is rotated about the beam axis- what happens?

Silk stretched slightly in one direction- what happens?

Silk is removed.

Laser beam reflected beam at a glancing angle from the grooves in a piece of a phonograph record. Beam is reflected from a spot where the grooves are running nearly parallel to the beam.

The piece of record is rotated back and forth around the axis of the beam.

Beam is then reflected from a spot where the grooves are running perpendicular to the beam.

What happens as you vary the angle of incidence of the beam until the beam is traveling almost parallel to the surface of the record?

##### Solution Summary

Conceptual questions on interference and diffraction of laser beam are explained.

##### Solution Preview

Laser beam
A helium-neon laser provides coherent red light (wavelength < 1 μm); the dimensions of the apertures or obstacles that we place in the laser beam are correspondingly small.
A white sheet of paper attached with magnets to a screen mounted on one side panel of a lab bench.
Laser set up at the other end of the bench so that it points at the paper.
Laser is switched on.

What would be the size of the laser beam spot (reasonable estimate)?
The diameter of the beam depends on the bore diameter of the gas tube which is generally of the order of 1 mm. A small divergence angle may increase the diameter by 0.01 or 0.02 mm in a distance of the order of a meter.
A piece of silk is placed in the laser beam (close to the laser and perpendicular to the beam).
What would be the pattern of the transmitted light on the screen?
Could this be a shadow pattern of the threads in the cloth?

When monochromatic parallel beam of light falls on a thin edge because of diffraction of light there is light inside the geometric shadow as well as the ...

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