Demonstration

This can be shown to a sixth form class as a fun demonstration of light interference.

Apparatus and Materials

• Laser or different colour lasers if available
• Small rectangular glass box as used for optics expts. (e.g. use a 6 cm hollow cube)
• Small right-angled 45 degree glass prism to fit inside the box with the right angle in one corner
• Bottle of (cheap) brandy - works better than ethanol
• Screen

Health & Safety and Technical Notes

The laser should be positioned so the beam cannot fall onto the eyes either directly or indirectly.

Be careful of laser reflections from the face of the glass box.

Don't drink the brandy!!

Read our standard health & safety guidance

The brandy can be re-used although eventually the alcohol content becomes too low.

It is possible to use a brandy glass and brandy rather than the apparatus used here but it is quite tricky to get working well.

The glass box and prism must be clean and dry.

Procedure

1. Arrange the box and prism on the bench with the laser set to shine in through the side and into the prism, passing perpendicularly through the prism face.
2. The light will internally reflect from the back face and emerge from the third side of the prism; then pass onto the screen, which should be set around two metres from the glass box.
3. Pour some brandy into the box until the level is about 1 cm below the point on the prism face where the laser beam is entering.
4. After a while brandy tears will creep up the prism face.
5. Adjust the laser so that it hits one of the tears (be patient).
6. A striking interference pattern will be seen on the screen and will be continually changing. It is best seen in a darkened room.
7. If lasers of different wavelengths are available, the different fringe separations are obvious.

Teaching Notes

• Without the brandy the laser totally internally reflects from the prism's back face.

• Brandy in contact with the face changes the critical angle at that point (because the change in the speed of light between glass and brandy is less than the speed change between glass and air) with the result that the light now enters the brandy tear, reflects from the back surface (a complex shape) and continues back through the prism to the screen, interfering with itself and producing the fringes on the screen.
• Alcohol continually evaporates from the brandy tear, changing its concentration and so the surface tension, and hence the angle of contact with the glass. As a result, the liquid in the tear is flowing up and down the glass all the time, changing the shape and thickness of the tear and so causing the interference patterns to shift in a fascinating way. The effect is rather beautiful and very soothing.

This experiment was submitted by Rod Smith from Cranbrook School in Kent.

This experiment was safety-tested in February 2008

Demonstration

To show that a laser beam can easily be modulated to carry information.

Apparatus and Materials

• Laser (e.g. He Ne or diode type)
• Strobe wheel
• Photodiode light detector circuit
• Audio frequency (AF) generator with amplification stage, or audio amplifier
• Loudspeaker
• Second version
• Small mirror mounted on any stiff surface e.g. a sheet of metal

Health & Safety and Technical Notes

Check that the laser is labelled 'Class 2' and warn students not to stare into the beam.

Read our standard health & safety guidance

Alignment of the beam on the photodiode is very important; if the beam is too strong the photodiode will saturate and not work well. You may need to adjust the beam so it falls just on the edge of the photodiode. You may also need to turn the lights off in the room: fluorescent tubes will cause the detector to hum at 100 Hz.

A photodiode detector circuit may be found in most electronics texts. A diagram of a very simple one is shown. It uses a 9 V battery, a 100 kΩ resistor and 0.1 μF capacitor.

Procedure

1. Align the laser so that the beam falls on the photodiode. The photodiode can be connected to the amplification stage of the AF generator and the output of the generator goes to a speaker. Note the AF generator is used only as an amplifier - an ordinary audio amplifier would also do.
2. Place the hand strobe wheel so it intercepts the laser beam. Spinning the wheel interrupts the beam many times a second, causing the audio output to click or hum.
3. If the beam is first reflected off a small mirror mounted on a surface, then a click will be heard when the surface is tapped, e.g. with a pencil.

Teaching Notes

• The photodiode is crudely converting the presence or absence of light into a click on the speaker. Interrupting or modulating (i.e. changing the intensity) the laser beam, therefore, changes the resistance of the diode. This demonstrates how digital data can be carried by a laser beam.
• The reflection experiment shows how laser detectors can be used to pick up conversations inside a room, by analyzing the vibrations of the glass windows.

Class demonstration

This demonstration shows that a beam of light is diffracted as it passes around a wire, highlighting the wave nature of light.

Apparatus and Materials

• Laser source
• Thin, straight wire, approx 25 cm
• Stand with 2 clamps
• Screen

Health & Safety and Technical Notes

Read our standard health & safety guidance

You will probably need to work in a darkened room.

Care should be taken to ensure that the laser beam does not shine directly into students’ eyes. This can be avoided by fixing it firmly in a clamp directed away from the students and towards the screen. Ensure that there are no shiny, reflective objects close to the path of the beam.

Procedure

1. Mount the laser pointer horizontally in a clamp
2. Mount the wire vertically between two clamps.
3. Direct the laser light onto the screen. You will see a bright dot.
4. As suggested in the film, ask your students to predict what they will se when the wire partially blocks the laser beam.
5. Move the wire into the beam. You should see a diffraction pattern of light and dark ‘fringes’ on the screen.

Teaching Notes

• We may talk casually about ‘light waves’, but students need to be convinced that light travels as a wave. This demonstration shows it.
• Students will need to be familiar with two ideas: that waves diffract as they pass around an obstacle, and that waves interfere constructively and destructively when they overlap. These ideas can be shown using a ripple tank.
• You can show diffraction and interference of light using single, double or multiple slits. However, students may find these difficult to appreciate. Diffraction by a simple wire is a more straightforward situation to explain. Students can also be asked to predict what will be seen on the screen when the wire is placed in the path of the light beam. They will probably expect to see a vertical shadow. The appearance of a diffraction pattern spread across the screen is a surprise worth exploring.
• A laser is used because it is a convenient source of a narrow beam of light. It has the added advantage that it produces light of a single wavelength; white light would produce a similar effect but the diffraction pattern would not be as wide as different wavelengths (colours) would interfere at different points.
• It is worth emphasising the extent to which light is diffracted as it passes around the wire. The diffraction pattern may be 50 cm wide when the diffracting wire is one metre from the screen. So light is being diffracted (bent) through an appreciable angle – perhaps 20 degrees.
• You could investigate the effect of rotating the wire; can students predict what will happen? (A vertical wire produces a horizontal diffraction pattern; a horizontal wire will produce a vertical pattern.)
• A sequence of experiments to show the diffraction of light and how this can be used to determine the wavelength of light:

  diffraction of light collection

• The video shows how to demonstrate the diffraction of light using a laser pointer and a wire:

• This video can be used with your students in the classroom in place of the actual demonstration:

Demonstration

A laser is used to illustrate how crystal geometry can be inferred from X-ray diffraction patterns.

Apparatus and Materials

• Laser, preferably mounted and mains powered
• Sieve or fine mesh (nylon stocking)
• Screen

Health & Safety and Technical Notes

Check that the laser is labelled 'class 2'.

Take care with reflections from the laser beam so that there is no danger of them striking a student's eye.

Read our standard health & safety guidance

Use sieves with the collar facing the laser.

Sieves are commonly used in geology and biology. You should use the 3 finest or mesh such as from a nylon stocking, stretched over a small embroidery frame.

There should be only a low level of light in the room.

Procedure

1. Start with laser beam showing a single spot on the screen.
2. Introduce the coarsest sieve and ask students to observe the pattern.
3. Remove and introduce finer mesh sieves showing that the pattern will broaden.
4. Using the finest sieve, move the sieve around but keep the same orientation. There will be no effect on pattern.
5. Twist the sieve to about 20 degrees from the normal to the beam. The beam expands in one direction.
6. Return to normal incidence.
7. Rotate the sieve. The pattern rotates.

Teaching Notes

Hopefully, the students will see that the diffraction pattern is a result of the angle of the mesh to the beam and the geometry of the mesh. If it doesn't work, the meshes available are too coarse. The same principles can be applied to X-ray crystallography or electron beam diffraction in metal films.

This experiment was submitted by David Ferguson, the physics technician at Uppingham School