Interference with air wedge

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