Demonstration

Coloured bands of light are produced in thin films because of path differences and interference.

Apparatus and Materials

• Beacker, 400cm ³
• Beacker, 1000cm ³
• Copper wire frame
• Soap solution, fresh
• Glycerine

Health & Safety and Technical Notes

Read our standard health & safety guidance

For 1 below, bend a frame of copper wire, 16 or 18 SWG, as shown. The vertical circle at the top should have a diameter at least 5 cm, preferably 7 cm or more.

Put soap solution in the 400 cm³ beaker. Soap bubble liquid from toy shops does very well. Or make a mixture of detergent and water (preferably the detergent used for washing woollen fabrics).

A dilution of 1 in 100 is probably best. A dilution of detergent of 1 in 10 gives a rather streaky pattern, but the film is strong. 1 in 1000 gives a film with closely spaced fringes but the film is weak.

Only if you need to, add glycerine to make the film stronger — it will make the colours poorer.

Procedure

1. Dip the frame in soap solution to make a film. Use this film as a mirror to reflect light from the sky to students. As the film drains, it thins, and interference bands appear.
2. If you discourage evaporation, the film will last a long time, even when thinned. Place a large beaker, wet inside, over the frame carrying the film as it stands on the bench-top.
3. Blow a soap bubble and catch it on a small piece of carpet made of synthetic fibre. Place a big beaker over the resting bubble.

Teaching Notes

• Apart from providing an opportunity to show the beauty of soap films, this experiment is bound to provoke discussion as the film thins and eventually bursts.
• The coloured bands demonstrate that different colours of light have different wavelengths.
• Light is reflected from the two surfaces of the soap film, and the two waves then interfere. Their path difference is twice the thickness of the film. The thickness of the film is irregular, so the coloured patterns are irregular too. The coloured bands provide a reminder that different colours of light have different wavelengths. This is particularly important if students have only seen Young’s fringes produced by a single wavelength laser.
• Just before the film breaks, the thinnest region becomes invisible, because of the phase change on reflection at the less optically dense, back surface.
• Students generally expect the light-waves from two surfaces very close together to reinforce, not cancel each other. They will be surprised by the black spot. A slight draught makes the experiment more convincing, and dispels any idea that the black region is a hole in the film. The most convincing test is to poke the black region - a good way of breaking any soap film. You will need to judge whether or not it is appropriate to explain the effect.
• More advanced students should understand that light reflected from the top surface undergoes a phase change of pi (180 degrees). This means that a bright fringe is formed whenever the path difference between the two waves is twice the thickness of the film plus a half wavelength.

• Remind students that street lights provide good light sources for viewing interference and diffraction phenomena. These include interference in thin films such as oil films on a wet road, and moisture on a car windscreen. Students can look at the lamps themselves viewed through net curtains or eyelashes.

This experiment was safety-checked in February 2006

Class practical

In this simple experiment, each student holds a pair of glass plates individually.

Apparatus and Materials

• Bunsen burner
• Lamp (behind screen)
• Tissue paper, scraps
• Sodium bicarbonate or common salt
• Red and green colour filter
• Transulcent screen
• Plate-glass plates, pairs
• Iron wire
• Bulldog clips, 2, to hold glass plates

Health & Safety and Technical Notes

Iron wires will need clamps on retort stands to support them. A glass translucent screen is safer than a tracing-paper one when close to flames.

Read our standard health & safety guidance

Although a sodium lamp is the easiest source of monochromatic light for this experiment, a Bunsen burner fed with common salt or, better, sodium bicarbonate, on an iron wire, provides a good simple source.

Arrange the Bunsen burner behind a translucent screen (or in front of white screens) to make an extended source.

The yellow colour in the flame lasts much longer if, instead of using wire, you fold a filter paper along a diagonal and soak it in salt solution. Wrap it around the top of a Bunsen burner with the folded edge 2 or 3 mm above the top.

Clean the pieces of plate glass carefully beforehand. To test them for reasonable flatness, press them together, and examine the fringes by monochromatic light. (N.B. Most microscope slides are not sufficiently flat.)

Students will form an air wedge by spacing the plates apart at one end with a scrap of tissue paper. The fringes will be difficult to see if they are too close together—that is why the tissue must be thin and the bulldog clips must hold the plates tightly together.

Procedure

1. Hold your sandwich of plates and tilt it until you see the yellow sodium light reflected brightly.
2. Press the sheets of flat glass tightly together as shown, so that the two inner reflecting surfaces are very close indeed. Hold the plates as if they were a book you are trying to read by the yellow light. You may see a black spot if you squeeze the plates together tightly.
3. Now open the plates and prop them apart at one end with a scrap of very thin paper, forming an air wedge. Hold them tightly clamped together with a bulldog clip at each end.
4. Look for the zebra stripes. If you knew the wavelength of light, what could you estimate by counting the stripes? Focus your eyes directly on the surface of the glass plates, not on the reflection image of the light source farther behind.

Optional extension: If red and green colour filters are available, use a white light source with each filter in turn to illuminate the sandwich. Make a quick change between red and green, and think about the difference you see. What does that tell you about the wavelengths of those two colours? (N.B. Colour filters heated by flames can be ruined, keep filters away from sodium flames!)

Teaching Notes

• There are four streams of reflected light: two from the inner faces of the sandwich, where the glass meets the thin wedge of air; and two from the outer surfaces of the glass plates. The two streams from the inner surfaces have a small path difference (about twice the thickness of the air wedge at each place}; and you will see the interference bands of bright black and yellow. The streams reflected from the outer surfaces of the glass plates have too great a path difference to show an interference pattern noticeably.
• More advanced students should understand that light reflected from top surface of the lower slide undergoes a phase change of pi (180 degrees). This means that a bright fringe is formed whenever the path difference between the two waves is twice the thickness of the air gap plus a half wavelength.
• Suppose you counted the stripes all the way from one end of the sandwich to the other. Knowing the wavelength of yellow light (about 600 nm), you can estimate the thickness of very thin materials. Newton discovered that, when a thin lens is placed on a flat piece of glass, the circular air film between lens and plate will produce circular fringes. These are known as Newton’s rings.

This experiment was safety-tested in February 2006

Demonstration

This experiment works as a model of what is happening with light in a thin air film, or thin film reflection of microwaves in their own right.

Apparatus and Materials

• Microwave transmitter
• Microwave receiver amplifier
• Glass plates, 2

Health & Safety and Technical Notes

Modern equipment using a solid-state diode transmitter is safe. Older equipment using a klystron tube uses hazardous voltages. The connectors on the leads between the transmitter and the power supply MUST be shielded types to minimize the risk of serious electric shock. The ventilation holes in the power supply may also give access to hazardous voltages, so its use MUST be closely supervised.

Read our standard health & safety guidance

Alternatively, one plate of glass or Perspex and one metal plate can be used.

Glass plates should be 25 cm square.

Procedure

1. First show that microwaves are partially reflected and partially transmitted by a single glass plate.
2. Then set up the transmitter, receiver, and plates as illustrated, and demonstrate the interference between the waves reflected by the first and second glass plates.
3. Show the effect of reducing the thickness of the 'film' (the distance between the plates).

Teaching Notes

This experiment was safety-tested in February 2006

Demonstration

Waste pipe is used to illustrate interference of sound waves coming from a single source by two different paths. By identifying nodes and antinodes, you can find the speed of sound.

Apparatus and Materials

• 1.25 inch waster pipe, 2m
• 1.5 inch waste pipe, 1m
• 90 degree bends, 2 of each size (2 x 1.25inch + 2 x 1.5 inch)
• 1.25inch T-pieces, 2
• Transparent scale of paper scale
• Signal generator and loudspeaker
• Small microphone and oscilloscope )optional)
• Metre rule

Health & Safety and Technical Notes

Read our standard health & safety guidance

Use a frequency range of 1k-10kHz.

Procedure

1. Connect up the pipes as shown in the photo. Fix the metre rule parallel to the sliding tube.
2. Put a small speaker operating at about 2 kHz under the bottom T-piece, and a detector (your ear or a microphone and CRO) near the top T-piece.
3. Starting with the slider fully closed, carefully move it out until you find the first position of minimum intensity (node). Record the position of the slider.
4. Continue moving the slider out, until you reach the next nodal position. Again, record the position of the slider.
5. Repeat step 4, recording the position of the slider at each node you find.
6. Plot a graph of slide position against node number. The gradient of this graph is the distance between nodes = 0.5 wavelength.
7. To calculate the speed of sound, multiply wavelength by frequency.

Teaching Notes

• This is known as the Quincke's tube method for finding the speed of sound. Students are interested by the unusual use of waste pipes and the clear pattern of nodes and antinodes.
• The sound from the speaker has two possible paths to the detector. One path (pipe A) has a fixed length and so at the detector the phase of the wave travelling through pipe A does not change.
• The path along the side with the slider (pipe B) changes length as you move the slider, and so at the detector the phase of the wave travelling through pipe B does change. Nodal positions occur when waves reaching the detector by the two different pipes are in anti-phase (compression from one path meets rarefaction from the other).
• As you extend pipe B, the distance between positions of that wave in the same phase is one wavelength. This is twice the distance the pipe is extended, because both sides of the pipe extend at the same time.
• Note: there is not a standing wave pattern in the pipes.
• You could repeat the whole procedure for other values of frequency.

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

Demonstration

This simple demonstration shows how the interference of light can be used to determine the thickness of a thin film.

Apparatus and Materials

• Travelling microscope
• Plano convex lens
• Plain glass plate (optically flat)
• Reflector
• Sodium light source
• A thin film whose thickness is to be measured (may be a strip of paper)

Health & Safety and Technical Notes

Read our standard health & safety guidance

The thin film must be inserted carefully between the plain glass plate and plano convex lens without disturbing the fringes.

NB A high-pressure sodium street lamp as used in colour studies will not give sharp fringes in this experiment. If the centre fringe is not dark, try polishing the lens and flat with a spectacle cleaning cloth.

Procedure

1. Turn on the sodium lamp.
2. Place the plano convex lens on the plain glass plate with curved surface in contact with the glass plate and place this system inside a reflector located under a travelling microscope.
3. Use a reflector to direct light onto the optical system. Adjust the inclination of the reflector to get maximum brightness (or the height of the lamp).
4. Focus the travelling microscope to see the bright and dark circular fringes (Newton's rings).
5. Carefully insert the thin film (say a paper strip), of thickness t , between the plano convex lens and the plane glass plate until the paper stops moving (see the diagram).
6. Now look through the microscope. Start from the central dark spot and count the number of dark fringes (or bright fringes) to the fringe that is adjacent to the thin film. Generally for ordinary paper samples, the number of fringes are of the order of 175 to 230\. Patience is needed!
For dark fringes...
7. Destructive interference occurs when the path difference, 2t = nλ , where λ = wavelength and n is an integer.
8. This simplifies to t = (nλ)/2
For bright fringes
9. Constructive interference occurs when the path difference, 2t = (n + 1/2)λ, which simplifies to t = (2n - 1)λ/4.

Teaching Notes

• When sodium light is incident on the plano convex lens system, light rays reflect from upper and lower layers of the air present between lens and the glass plate. The sodium light source is almost monochromatic.
• There is no phase change at the lens-air surface, because the wave if going from a higher to a lower refraction index medium. At the air-plate surface, however, there is a phase shift of π with the reflection from a medium of higher refractive index.
• Waves reflected from these two surfaces interfere, forming bright bands where the path length in air produces two waves in phase and dark bands where the waves are in antiphase.
• The centre of the pattern is black. There is no reflection because there is no air gap here.
• The thickness of the thin film is equal to the thickness of the air film adjacent to it.
• The fringes are circular as the locus of points of equal thickness of air is a circle.

This experiment was submitted by K.H. Raveesha, Head of Physics at the CMR Institute of Technology in India.

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