# Diffraction gratings and young's double slit experiment

• physiks
In summary: Exactly!In summary, diffraction is the phenomenon where a single wave passes a slit or obstacle and spreads out, producing a diffraction pattern. Interference is when we have more than one source of coherent waves which combine together at a given point, and their phase difference determines the intensity at that point. With double slit interference, we have multiple wave sources which still produce interference. Finally, diffraction is the name given to the various phenomena that result from interference.
physiks
Just a question about the naming of optical phenomena.

I've taken these explanations from a book I'm using:

Diffraction is when a single wave passes a slit or obstacle and spreads out. The single wave produces a diffraction pattern.

Interference is when we have more than one source of coherent waves which combine together at a given point. Their phase difference determines the intensity at that point.

These definitions make sense in terms of the interference pattern formed in Young's double slit experiment, and the diffraction pattern seen from single slit diffraction. However, what about a diffraction grating - the above seem to imply it is an interference phenomenon producing the lines.

Can anybody please explain the idea behind this naming - or is it a good idea to not worry too much about the names? Thanks.

physiks said:
Just a question about the naming of optical phenomena.

I've taken these explanations from a book I'm using:

Diffraction is when a single wave passes a slit or obstacle and spreads out. The single wave produces a diffraction pattern.
Not quite - it is possible for a wave to pass through a wide slit with no (discernable) diffraction pattern forming - like a water wave passing into a harbour.
You also get diffraction from multiple sources and no slits.

Diffraction occurs as a result of interference from multiple wave sources.

Interference is when we have more than one source of coherent waves which combine together at a given point. Their phase difference determines the intensity at that point.
Almost - parts of a single wave can interfere with other parts - i.e. if it passes through a slit or slits or is reflected.

These definitions make sense in terms of the interference pattern formed in Young's double slit experiment, and the diffraction pattern seen from single slit diffraction. However, what about a diffraction grating - the above seem to imply it is an interference phenomenon producing the lines.
They all produce interference - diffraction can occur from multiple sources.

Can anybody please explain the idea behind this naming - or is it a good idea to not worry too much about the names? Thanks.
The names are mired in history - it is a good idea not to worry about the names: concentrate on the phenomena.

diffraction: from "to break apart".

interference: from "interfere" - in the sense of subtraction or difference.

Simon Bridge said:
Not quite - it is possible for a wave to pass through a wide slit with no (discernable) diffraction pattern forming - like a water wave passing into a harbour.
You also get diffraction from multiple sources and no slits.

Diffraction occurs as a result of interference from multiple wave sources.

Almost - parts of a single wave can interfere with other parts - i.e. if it passes through a slit or slits or is reflected.

They all produce interference - diffraction can occur from multiple sources.

The names are mired in history - it is a good idea not to worry about the names: concentrate on the phenomena.

diffraction: from "to break apart".

interference: from "interfere" - in the sense of subtraction or difference.

So in the three scenarios I mentioned (single slit diffraction, Young's double slits and a diffraction grating), we have interference going on.

You said 'Diffraction occurs as a result of interference from multiple wave sources' which explains the two first and third above. In the first case the Huygen's wavelets make up the multiple sources, and in the third we have many slits making up the sources.

In terms of double slit interference however, don't we still have multiple wave sources - why then is this not classed as diffraction?

physiks said:

So in the three scenarios I mentioned (single slit diffraction, Young's double slits and a diffraction grating), we have interference going on.
Yes.

You said 'Diffraction occurs as a result of interference from multiple wave sources' which explains the two first and third above. In the first case the Huygen's wavelets make up the multiple sources, and in the third we have many slits making up the sources.
That's right.

At this stage just be aware that this is the wave model for diffraction and interference - there is also a particle model. Worry about the details later - get these ideas down first. Just understand that what you are learning now is not the last word.

In terms of double slit interference however, don't we still have multiple wave sources - why then is this not classed as diffraction?
It is classed as [an example of] diffraction.

Compare the pics below - starting with one slit (top) adding a second one makes the pattern brighter but with bits "missing".

This "removal" is what was originally meant by "interference".
... it is as if adding a second slit interfered with the normal 1-slit pattern.

You are still worrying too much about the exact names.
The use of words changes with time so we get stuck with names for things that don't always make a lot of sense.

The above is called "Young's Interference" because of history - it is equally acurate to call it "double-slit diffraction" to show it is an example of the class of phenomena "diffraction at slits".

Last edited:
Simon Bridge said:
Yes.

That's right.

At this stage just be aware that this is the wave model for diffraction and interference - there is also a particle model. Worry about the details later - get these ideas down first. Just understand that what you are learning now is not the last word.

It is classed as [an example of] diffraction.

Compare the pics below - starting with one slit (top) adding a second one makes the pattern brighter but with bits "missing".

This "removal" is what was originally meant by "interference".
... it is as if adding a second slit interfered with the normal 1-slit pattern.

You are still worrying too much about the exact names.
The use of words changes with time so we get stuck with names for things that don't always make a lot of sense.

The above is called "Young's Interference" because of history - it is equally acurate to call it "double-slit diffraction" to show it is an example of the class of phenomena "diffraction at slits".

Ok, I got it, thanks! :)

No worries...
And don't sweat the terminology ;)

## 1. What is a diffraction grating and how does it work?

A diffraction grating is a device that consists of a large number of parallel, evenly spaced slits that are used to separate light into its component wavelengths. When light passes through a diffraction grating, it undergoes diffraction, or bending, which results in the splitting of the light into its various colors. This is due to the interference pattern created by the slits, where the peaks and troughs of the waves align or cancel out, creating the pattern of colors.

## 2. What is the Young's double slit experiment and what does it demonstrate?

The Young's double slit experiment is a classic experiment that demonstrates the wave nature of light. It involves passing light through two parallel slits and observing the resulting interference pattern on a screen. This pattern shows the constructive and destructive interference of the light waves, proving that light behaves like a wave and can undergo diffraction.

## 3. How does the spacing between the slits affect the interference pattern in the double slit experiment?

The spacing between the slits in the double slit experiment plays a crucial role in determining the interference pattern. The wider the spacing, the narrower the central bright fringe will be, and the more distinct the other fringes will appear. Conversely, if the spacing is smaller, the central fringe will be wider and the other fringes will be less distinct.

## 4. What is the difference between a diffraction grating and a single slit in terms of the resulting interference pattern?

A single slit produces a diffraction pattern with a central bright band and alternating dark and bright fringes on either side. In contrast, a diffraction grating produces multiple bright bands, with the central band being the brightest, and the other bands gradually getting dimmer. This is because a diffraction grating has many parallel slits, which results in more interference patterns and a more distinct pattern overall.

## 5. How is the wavelength of light determined using a diffraction grating?

The wavelength of light can be determined using a diffraction grating by measuring the distance between the slits and the resulting interference pattern on a screen. By using the equation d sinθ = mλ, where d is the spacing between the slits, θ is the angle at which the bands appear, m is the order of the bright band, and λ is the wavelength of light, we can calculate the wavelength of light based on the observed interference pattern.

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