Laser Light Explorations

Photons are Bosons

Introduction

Photons are bosons. In a laser, the photons which are produced by stimulated emission are in the same state: That is, they have the same:
direction,
wavelength, frequency and therefore color,
polarization
and
phase.
In this activity we will look for proof that photons produced by a laser have all of these properties in common.

Material

Needed for all 4 explorations:

• a pointer laser (any laser will do)
• a magnetic optical bench or other mounting system to hold the laser.
• a white screen larger than 25 cm square with supports e.g. large binder clips.

Etc.

The universe of particles is divided into two types of particles: fermions and bosons. Electrons are fermions. No two fermions can have the same quantum mechanical state. Photons are bosons, many photons can be in the same state. Lasers produce a beam of photons all in the same state.

Direction

Material

• a meter stick
• a piece of paper
• binder clips

Assembly

Mount the laser in the magnetic optical bench holder and point it at the screen.

Cover the screen with a piece of paper, use the binder clips to attach the paper to the screen.

To Do and Notice

Find the expansion of the laser beam as it travels away from the laser.

Place the screen a centimeter from the laser, use a pencil to mark the edge of the spot of laser light at this distance.

Repeat your measurements at 10 cm, 1 m, 5 m, 10 m
add other distances you think might be illuminating.

Notice that drawing a boundary around the laser beam becomes more difficult at greater distances as the beam shape becomes more complex.

What’s Going On?

A perfect laser beam would spread a little due to diffraction. However inexpensive laser pointers often include a poor lens which focuses their beams a meter or more in front of the laser. The beam then spreads faster than it would by diffraction alone.

Math Root

The spread of the laser beam is measured as an angle, A, usually in radians or milliradians.

The angular spread of the beam is simply the diameter of the beam, d, divided by the distance to the screen, D.

A = d/D

When you divide the diameter of your beam by the distance at 5 m, then at 10 m is the angle nearly the same?

The smalest angle of spread of a beam is caused by diffraction. Theoretically the angle of spread of the laser beam, in radians, would be its narrowest diameter divided by the wavelength of light.

A = L/d

If the beam were 1 mm in diameter then its angular spread would be:

A = L/d = 6 x 10-7 m/10-3 m = 6 x 10-4 radians or 60 milliradians.

This means that from the equation above at 10 m your beam would have a diameter of

d = A x D = 6 x 10 -4 x 10 = 6 x 10 -3 m = 6 mm.

It doesn't have such a small diameter for two reasons:

There is a lens in the system which focuses the beam a few meters in front of the laser and then causes it to diverge.

The smallest diameter of the beam is as it emerges from the laser diode where it is less than 0.1 mm in diameter. This would make its angular divergence 10 times large or 6 cm at 10 m

Wavelength

Material

• a diffraction grating
• optional a compact disk

Assembly

Shine the laser beam at the screen a meter or more away.

To Do and Notice

Hold a good holographic diffraction grating in front of and near the laser.

Notice that there is a central red spot flanked by two red spots.

The diffraction grating will spread out different wavelengths of light to the side of the central spot. The dots which are bent to the side are about the same width as the original dot. This only happens if the light has one, or nearly one, wavelength.

What’s Going On?

When an atom is stimulated to emit light it has a very narrow range of frequencies. The laser light has one color.

Optional method

Bounce the laser off a music CD or a CD-ROM. Notice that the reflected beam has three dots. The central dot is the same one that would appear if the laser were reflected off a mirror, the two dots to the side are produced by the diffraction grating of the CD. The dots are red, they show no mix of color.

Polarization

Material

a polarizer (polarized sunglasses will work)

Assembly

Shine the laser beam at a screen a meter or more away.

To Do and Notice

Hold a polarizer in front of the laser.

Slowly rotate the polarizer.

Notice that the beam gets brighter and dimmer as the polarizer is rotated.

What’s Going On?

the laser beam is polarized. A polarizer will allow light polarized parallel to it to get through, light polarized perpendicular to the polarizer will be blocked.
As some laser warm up, particularly cheap gas lasers, their direction of polarization changes over a time scale of minutes. With these lasers you can simple mount the polarizer in front of the laser and watch the beam get brighter and dimmer as the direction of polarization of the beam changes.

Phase

Material

• a short focal length lens like a magnifying glass

Assembly

Mount the laser in a magnetic optical bench or other mount.
You cannot hold the laser in your hand for this activity.
Shine the laser beam at the screen a meter or more away.
Place a lens in the beam so that the beam is diverged to make a spot of light on the screen. The spot should be a centimeter or more in diameter.

To Do and Notice

Notice the speckled pattern on the screen. The pattern may appear to swim across the dot of light as you move your head.

What’s Going On?

To create an interference pattern of light and dark regions the laser light must be in-phase all across the beam.

This is why no other light sources produce the speckle pattern.

Only the light in a laser beam has the same phase all across the beam.

You can also explore the phase in the laser beam by projecting an interference pattern, see the interference exploration.