Introduction: The Jamin-interferometer, the Simplest Interferometer in the World
Interferometers play a major role in physics / science. For example, gravitational waves are registered with a giant laser interferometer (LIGO). Also, the ether theory of light could be refuted thanks to the Michelson-Morley interferometer (look at the pictures of my michelson-interferometer made of parts from a DVD-drive). This was the cornerstone of the special theory of relativity. What is the basic principle of an interferometer?
Step 1: Introduction
An interferometer splits monochromatic light with the greatest possible coherence length (lasers are particularly suitable for this purpose) into two or more sub-beams. These will be brought together later. In this combination, there is a constructive or destructive superposition of the individual beams. In case of constructive interference the maximum of the wave hits the maximum of the second wave (phase shift = 0 °) and the superposition of the two waves will increase. In the case of destructive interference, the maximum of one wave hits the minimum of the other wave (phase shift = 180 °) and the superposition of the two waves will be extinguished.
Interferometers are high-precision devices because they already react to shifts in the wavelength of the light. For example, if one path lengthens by half a wavelength, constructive interference changes to destructive interference or vice versa.
Step 2: Parts
Despite this high sensitivity, the so-called Jamin interferometer (wikipedia) can be assembled with very few components. You really only need
- a low power laser pointer (e.g., 1 mW)
- two normal rear glass mirrors with the thickest possible glass layer
- two mirror mounts (for example made of wood cubes)
- double sided tape
- a screen for observing the interference patterns
I got my mirrors at a glass shop for little money. The laser pointer is available on ebay for a few dollars.
Step 3: The Experiment
The two mirrors are glued to the holders by means of adhesive tape. Then set them up as shown in the picture. The laser pointer now aims at an angle of 45 ° to the first mirror. The second mirror is now arranged so that its surface is aligned parallel to the surface of the first mirror. If everything has been done correctly, several light reflections would have to appear on the screen.
If one observes these light reflexes more exactly, one recognizes differences. While the central, brightest light reflection and the weaker rightmost have no structure at all, the other light reflections show an inner stripe structure. This is exactly the interference pattern.
Attention: The arrangement of the light reflections themselves is not the interference pattern, but the strip-like, internal structure of individual light reflections themselves. The pattern with the brighter and darker light reflections is strongly reminiscent of the diffraction pattern using a grating.
But the similarly arranged light reflections here at the Jamin interferometer are not caused by diffraction, but by multiple reflections at the two thick mirrors. The combination of split light rays then leads to interference phenomena!
This Jamin interferometer is ideally suited to show the subject of interference and interferometer in the classroom to the pupils. It is built in 5 minutes by the students themselves.
The following questions can be asked to the students:
- Why do you see several light reflexes in principle?
- Why are the light reflections different in their brightness?
- Does one recognize a different, internal structure of these individual light reflexes?
- How could this happen?
Step 4: More Physics-projects
Participated in the