# Young's double slit experiment question

Hi I am studying Young's double slit experiment. For the graph of Intensity against position, I have seen two graphs for the double slits - one is where the intensity is same for all the fringes in the double slit experiment.

But the other graph I saw for the Double slit experiment is where the brightest fringe is at the centre and then subsequent maximas are less intense. I am confused as to which one is correct since I thought that it was the same intensity for Young's double slit.

I completely understand that for the single slit diffraction pattern, the brightest fringe is the central maxima and then subsequent maximas decrease in intensity.

(edited 2 years ago)
Original post by integration4ever
Hi I am studying Young's double slit experiment. For the graph of Intensity against position, I have seen two graphs for the double slits - one is where the intensity is same for all the fringes in the double slit experiment.

But the other graph I saw for the Double slit experiment is where the brightest fringe is at the centre and then subsequent maximas are less intense. I am confused as to which one is correct since I thought that it was the same intensity for Young's double slit.

I completely understand that for the single slit diffraction pattern, the brightest fringe is the central maxima and then subsequent maximas decrease in intensity.

Double slit diffraction patterns have two elements multiplied together, one is the (infinite) set of interference fringes you get just from the light from the two slits going in and out of phase as a function of position. The second is the diffraction pattern from a single slit that modifies the overall distribution so it dies away that’s your final picture.

The maths to show this formally turns out to be neatly described by the Fourier transform, but you don’t get to that until university.
Original post by Mr Wednesday
Double slit diffraction patterns have two elements multiplied together, one is the (infinite) set of interference fringes you get just from the light from the two slits going in and out of phase as a function of position. The second is the diffraction pattern from a single slit that modifies the overall distribution so it dies away that’s your final picture.

The maths to show this formally turns out to be neatly described by the Fourier transform, but you don’t get to that until university.

Thanks.

Isn't the second picture the diffraction pattern of a double slit.

And does Young's double slit experiment have a central bright fringe and then subsidiary maximas with lower intensity?

i.e. why is the young's double slit pattern like the SOLID RED line below and not the light purple one?
(edited 2 years ago)
And I think I've confused myself as I am not sure the difference between young's double slit and diffraction:

One way to think about this is that a single slit generates a sinc^2 intensity diffraction pattern. 2 or more slits of the same width as the single slit then generate the same sinc^2 pattern, but “decorate” or modify it with additional interference terms as the light from different sources go in and out of phase.
Original post by integration4ever
Hi I am studying Young's double slit experiment. For the graph of Intensity against position, I have seen two graphs for the double slits - one is where the intensity is same for all the fringes in the double slit experiment.

But the other graph I saw for the Double slit experiment is where the brightest fringe is at the centre and then subsequent maximas are less intense. I am confused as to which one is correct since I thought that it was the same intensity for Young's double slit.

I completely understand that for the single slit diffraction pattern, the brightest fringe is the central maxima and then subsequent maximas decrease in intensity.

Original post by integration4ever
Thanks.

Isn't the second picture the diffraction pattern of a double slit.

And does Young's double slit experiment have a central bright fringe and then subsidiary maximas with lower intensity?

i.e. why is the young's double slit pattern like the SOLID RED line below and not the light purple one?

I think I can understand your confusion. Here is a way of seeing the 2 seemings “different” results.

Usually, you would study or learn Young's Double slits experiment first and your teacher would derive some maths equations and you will be shown a picture with dark and bright fringes that are of uniform intensity. This is illustrated in the picture posted in post #3 (the purple curve which the legend states interference).
You can think of this as the “ideal” result which is derived by considering the “interference pattern from two ideal light sources” instead of from 2 slits of certain width.
Original post by integration4ever
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After this, you would encounter the diffraction pattern from a single slit of certain width. This diffraction pattern result is again shown in the picture posted in post #3 (the blue curve which the legend states diffraction). After learning the diffraction pattern from a single slit, then you can deal with the interference pattern from the Young Double slits experiment because the 2 slits are of certain width NOT really 2-point sources of light.
Original post by integration4ever
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In the Young Double slits experiment, we need to consider diffraction patterns (the blue curve) due to the individual slits and the interference patterns (the purple curve) due to the waves coming from the 2 slits, these combined patterns are shown in the bold red curve in the picture posted in post #3.
Original post by integration4ever
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These combined patterns are also shown in the first picture in post #1.
Original post by integration4ever
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Hope this “explanation” makes sense to you.