Fresnel double mirror
The Fresnel double mirror , also called Fresnel double mirror, named after Augustin Jean Fresnel , is an optical test setup for the generation of two coherent light bundles from a divergent light source .
Layout and function
The experimental set-up consists of a light source and two mirrors slightly inclined towards each other by a few angular minutes . The two mirrors must hardly be offset from one another at the line of contact, the step height must be less than the coherence length. The expansion of the light source must be less than a millimeter.
If the light from the source falls on the two mirrors, it is reflected . The two reflected waves then appear to originate from two virtual coherent point light sources behind the double mirror for an observer . If the two partial waves are projected onto a screen, interference fringes are visible in the area in which the two reflected waves overlap . This is caused by the path difference between the two waves, which is caused by the slight tilting of the two mirrors. Areas of constructive or destructive interference are located where the path difference is equal to the integer or the odd multiple of half the wavelength . In order for interference fringes to be visible on the screen, the two mirror surfaces must therefore only be inclined to one another for a few arc minutes. Otherwise the path difference will be too great. This can also be understood by slowly reducing the angle. The number of interference fringes increases rapidly for smaller angles and the individual fringes are difficult to separate from one another with the eye. From a certain angle, the path difference between the two partial waves becomes greater than the wavelength of the light and no interference is visible.
The observable interference fringes do not quite correspond to the prediction of the simplified theory. For example, the strips have neither the same width nor the same brightness. This is due to the effects of diffraction at the edge between the two mirrors.
meaning
Similar to the later invented Fresnel biprism , the Fresnel double mirror can be used to generate bright interference figures, since relatively wide parts of the wave fronts are used for the superposition. The relatively simple structure made it possible to carry out first interference tests with natural light as early as the 18th and 19th centuries. Investigations that contributed to gaining new knowledge about the nature of light, cf. Wave-particle dualism .
Web links
- Photograph of the double mirror and the interference figure
- Presentation at student level at LEIFI
- Internship attempt at the University of Oldenburg
- Device description and experiment description
Individual evidence
- ^ A b Encyclopedia of the Mathematical Sciences including its applications . Edited by Arnold Sommerfeld . tape 5 , part 3: physics. Teubner, Leipzig 1898, The Fresnel Mirror , p. 378–379 ( digitized [accessed May 17, 2014]).