# How could one measure degree of coherence with Fresnel birprism?

• BrunoIdeas
In summary, the discussion revolves around the measurement of degree of coherence with a Fresnel biprism. The relationship between coherence and visibility is introduced in a Young Double Slit experiment, but it is also possible to study coherence using a Fresnel biprism. The main question is which two points or regions of space are being referred to in this measurement. Another question is whether changing the width of the source affects the measurement of visibility. The discussion concludes by mentioning the equivalence between a pair of pinholes and a biprism in creating interference between different parts of the same wavefront.
BrunoIdeas
How could one measure degree of coherence with Fresnel biprism?

Hello. I have some questions about the study of partial coherence with a Fresnel biprism.
In common text degree of coherence and its relationship to visibility is introduced considering a Young Double Slit experiment.
At the same time, in basic texts, the Fresnel biprism is introduced as an equivalent to Young's wave-front split interferometer.
HOWEVER, when studying partial coherence one measures visibility of fringes produced by a Young interferometer BECAUSE one is studying to correlation of the fields AT PINHOLES. Therefore giving information about the relationship between to laterally spaced points of the field. I don't see how this could happen with a Fresnel biprism and my question is:
1) Which two points, pinholes, or regions of space am I referring to, if any?

Although I am mainly interested in this first question, I pose another one.
2) In a Young experiment if one varies the width, for example, of the source, the pinhole dimensions stay the same. In Fresnel birprism when the width of the source varies so do the virtual sources. Won't this affect meausres of visibility?

I am seeing this differences and probably I am not considering/understanding something.

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If I understand your question, the pair of pinholes and the biprism are equivalent because both create interference between different parts of the same wavefront.

http://www.pbjacquemin.com/images/scanning_device3.gif
http://www.physics.umd.edu/lecdem/services/demos/demosm5/m5-24.htm

Young's interferometer measures the spatial coherence of a wavefront- how well different spatially-separated portions of a wavefront are correlated. A Michaelson interferometer measured the temporal coherence: how well different temporally-separated portions of a wavefront are correlated.

So, to your question #1, you are sampling the wavefront at the pinholes or the entrance face of the biprism. For #2, I haven't used a biprism so I can't say for sure.

Last edited by a moderator:

## 1. What is a Fresnel biprism?

A Fresnel biprism is a thin, transparent prism with two inclined faces that are used to split a light beam into two coherent beams.

## 2. How does a Fresnel biprism measure degree of coherence?

A Fresnel biprism measures degree of coherence by splitting a light beam into two coherent beams, which are then combined to produce interference fringes. The visibility of these fringes is used to determine the degree of coherence of the original light source.

## 3. What is coherence?

Coherence refers to the property of light waves being in phase or having a constant phase relationship. It is an important factor in determining the interference and diffraction patterns of light.

## 4. How is the degree of coherence quantitatively measured with a Fresnel biprism?

The degree of coherence is quantitatively measured by calculating the visibility of the interference fringes produced by the two coherent beams from the Fresnel biprism. This can be done by measuring the contrast and intensity of the fringes using a photodetector.

## 5. What are the applications of measuring degree of coherence with Fresnel biprism?

Measuring the degree of coherence with Fresnel biprism has various applications in optics, including determining the quality of light sources, characterizing optical components, and studying the coherence properties of lasers. It is also used in interferometry and holography.

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