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# Physics AS Waves questions Watch

1. I have a few questions regarding the waves (optics) section:

- Why does single slit diffraction cause fringes to be made?, I understand why it is made for 2 slits and the waves and constructively and destructively interfering, but i dont understand why it happens with single slit?

- is single slit diffraction because of interference?

- there was a question in a book that asked: "What happens to the fringe pattern if one of the double slits it blocked completely?" The answer was that there is no fringe pattern, but after blocking 1 slit, why doesn't it just become single slit diffraction?
2. Why does single slit diffraction cause fringes to be made?

Because each point across the slit acts as a single source from which wave fronts propagate. These can then interfere

is single slit diffraction because of interference?

Yes

"What happens to the fringe pattern if one of the double slits it blocked completely?" The answer was that there is no fringe pattern, but after blocking 1 slit, why doesn't it just become single slit diffraction?

If the slit is down to one wavelength there will be no pattern as the first minimum would be at 90 degrees and hence not occur.
3. (Original post by nerak99)
Why does single slit diffraction cause fringes to be made?

Because each point across the slit acts as a single source from which wave fronts propagate. These can then interfere

is single slit diffraction because of interference?

Yes

"What happens to the fringe pattern if one of the double slits it blocked completely?" The answer was that there is no fringe pattern, but after blocking 1 slit, why doesn't it just become single slit diffraction?

If the slit is down to one wavelength there will be no pattern as the first minimum would be at 90 degrees and hence not occur.
thank you

i dont completely understand the last point you made though
4. Essentially, the wider the slit the more points there are to act as sources to interfere with one another. To get a pattern the path length difference to the screen from the different points has to be different by integral numbers of wavelength for a maximum.

With a narrow slit, the sources are closer and hence the points on the screen which have an integral number of wavelength differences will be further apart. Once you are down to a single wavelength width, the angle from the slit to the screen point where one wavelength difference will occur will be 90 degrees. i.e an infinite distance along the screen and hence no pattern.

Hope that is clearer

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