Monochromatic light vs Laser - Young's double slit

[BOAS]

Hello,

I have a quick question about the distinguishing features between a 'monochromatic light source' and a laser.

In my notes on wave optics, I have a diagram of monochromatic light being diffracted through a single slit, to produce a coherent wave formation that then hits a double slit. Since the waves 'created' by the double slit originated from the same wave, we can say they are in phase with one another.

In my lab script, it appears that the single slit is unnecessary when using a laser. It says that the nature of the light produced by the laser means that the two beams leaving both slits will be in phase.

What is the difference between these light sources that make the experimental set up different?

BOAS

 

[Cthugha]

What is the difference between these light sources that make the experimental set up different?

The difference is spatial coherence (which is also what a double slit measures). The more point-like a light source is, the higher its spatial coherence will be.

Imagine comparing a nice laser and floodlight sent through a narrow spectral filter to make it more or less monochromatic. The floodlight has a huge surface area and light is coming from all of these points. This also means that the path difference between all of these points and the double slit differs strongly, which gives you a large spread in the relative phase of the light fields at the two slits of the double slit. In that case you will not be able to see a good interference pattern. The solution in that case is a narrow pinhole or slit that gives you an effective point-like source.

 

[BOAS]

The difference is spatial coherence (which is also what a double slit measures). The more point-like a light source is, the higher its spatial coherence will be.

Imagine comparing a nice laser and floodlight sent through a narrow spectral filter to make it more or less monochromatic. The floodlight has a huge surface area and light is coming from all of these points. This also means that the path difference between all of these points and the double slit differs strongly, which gives you a large spread in the relative phase of the light fields at the two slits of the double slit. In that case you will not be able to see a good interference pattern. The solution in that case is a narrow pinhole or slit that gives you an effective point-like source.

Thanks for your answer

With regards to the monochromatic light source, I have written "The single slit produces a coherent wave, whose pattern does not shift in time or space, relative to a regular periodic motion" Is this a description of 'high spatial coherence'? (i'm not familiar with that term).

 

[Cthugha]

[...]"The single slit produces a coherent wave, whose pattern does not shift in time or space, relative to a regular periodic motion" Is this a description of 'high spatial coherence'? (i'm not familiar with that term).

The "in space" is the spatial coherence part. The "in time" would give you temporal coherence. A slit gives you spatial coherence.

In a nutshell you can interpret spatial coherence as a measure of "If I know the phase of my light field at this point, how well can I predict the phase of the field at some other point at a distance".

Temporal coherence then does the same for two times.

 

[sardar]

,

Thank you for your question. The main difference between monochromatic light and a laser is the coherence of the light waves. Monochromatic light refers to light of a single wavelength or color, while a laser produces highly coherent light waves of a specific wavelength.

In terms of Young's double slit experiment, a monochromatic light source will produce a coherent wave pattern when diffracted through a single slit, but the waves passing through the double slit may not be in phase with each other. This is because the light waves from a monochromatic source are not perfectly in sync with each other.

On the other hand, a laser produces coherent light waves that are all in phase with each other. This means that the waves passing through the double slit will interfere constructively, creating a distinct pattern of light and dark fringes. The use of a single slit is not necessary in this case because the laser already produces a coherent beam.

Overall, the difference between a monochromatic light source and a laser lies in the coherence of the light waves they produce. This has important implications for experiments like Young's double slit, where the interference of light waves is a crucial factor. I hope this helps clarify the difference between these two light sources.