BR Diffraction problem~

[hellocubed]

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A typical double-slit is actually two single slits placed near one another. The equations describing the location of the bright and dark fringes that result from two-slit interference do not take into account the diffractive effects of each single slit. This is because we assume that:

A.) single slit effects are masked by the double-slit pattern.
B.) d is much greater than wavelength
C.) d+a is much greater than wavelength
D.) a is comparable to or smaller than wavelength

The answer is D (highlight)


This question doesn't make any sense. When a is comparable or smaller than wavelength, the effect from diffraction is GREATER. So... question is saying that we ignore diffractive effects because they are significant?

Anyone care to explain?

 

[Dasypus]

What are the 'd' and 'a' in the question?

 

[hellocubed]

It is not noted anywhere in the question.

But a= width of the slit
d is the distance between the slits.

 

[Morsetlis]

We assume that slit width (a) is small compared to wavelength of light, so that the slits => point sources of light and produce 1 single wavefront (instead of 2+).

If slit width were bigger than wavelength, MORE than 1 wave of light could hit the slit, which will produce interference within that 1 single slit.

 

[chaser0]

Why does that mean that we ignore diffractive effects?

Considering Mcat diffraction Is always only one wavelength.
That only says that diffraction occurs.

 

[MedPR]

There was a huge thread about this a month or two ago and nobody could explain the answer. A bunch of people made pretty good arguments for a typo in the answer as well.

 

[gettheleadout]

There was a huge thread about this a month or two ago and nobody could explain the answer. A bunch of people made pretty good arguments for a typo in the answer as well.

Great.

 

[milski]

There is no typo, Morsetlis is right on it.

The simple way to derive a formula for the intensity from the interference of two slits is to treat each slit as point source. Then you need to be concerned only with the path difference between each each slit but assume that there is a single ray through the slit. The result is a patters with equally spaced peaks with equal amplitude. For the assumption about a point source to hold, the width of the slit has to be less than the wavelength of the light.

Normally slits are much wider than that. In that case, to have the correct formula, you have to account that there is a path difference between all slits but there is also a path difference between the rays going through the same slit. That leads to the pattern more typically observed in real life: equally spaced peaks with amplitudes decreasing away from the center.

 

[MedPR]

Great.

There is no typo, Morsetlis is right on it.

The simple way to derive a formula for the intensity from the interference of two slits is to treat each slit as point source. Then you need to be concerned only with the path difference between each each slit but assume that there is a single ray through the slit. The result is a patters with equally spaced peaks with equal amplitude. For the assumption about a point source to hold, the width of the slit has to be less than the wavelength of the light.

Normally slits are much wider than that. In that case, to have the correct formula, you have to account that there is a path difference between all slits but there is also a path difference between the rays going through the same slit. That leads to the pattern more typically observed in real life: equally spaced peaks with amplitudes decreasing away from the center.

Sorry, I thought I was in this thread: http://forums.studentdoctor.net/showthread.php?t=910293

I think the problem with TBR's explanation had to do with the way the slides were bending around the hair.. Not quite sure. Perhaps we were all just on the completely wrong wavelength back then though.

 

[milski]

Sorry, I thought I was in this thread: http://forums.studentdoctor.net/showthread.php?t=910293

I think the problem with TBR's explanation had to do with the way the slides were bending around the hair.. Not quite sure. Perhaps we were all just on the completely wrong wavelength back then though.

Right, I remember that discussion too. I think BR was also correct there - I remember talking to someone else a week or two later and their answer did make sense. MrNeuro, if I remember correctly.

The only where we did not get anywhere was the one about number of base pair/nucleotides but I don't want to go back to that. 😎

 

[gettheleadout]

Right, I remember that discussion too. I think BR was also correct there - I remember talking to someone else a week or two later and their answer did make sense. MrNeuro, if I remember correctly.

The only where we did not get anywhere was the one about number of base pair/nucleotides but I don't want to go back to that. 😎

Oh god haha I remember that one... I think in the end the only possibility was that BR was wrong.

 

[milski]

Oh god haha I remember that one... I think in the end the only possibility was that BR was wrong.

It was almost worth it to see all the "frequent flyers" here show up, explain quickly why the right answer is the right answer than do the "Oh, wait, no, that can't be right" routine. 😀

 

[MedPR]

Right, I remember that discussion too. I think BR was also correct there - I remember talking to someone else a week or two later and their answer did make sense. MrNeuro, if I remember correctly.

The only where we did not get anywhere was the one about number of base pair/nucleotides but I don't want to go back to that. 😎

I thought MrNeuro was the main one who proved BR wrong 😕

 

[milski]

I thought MrNeuro was the main one who proved BR wrong 😕

For the two slides with the hair? I think I had him converted at the end.

 

[MedPR]

For the two slides with the hair? I think I had him converted at the end.

Oh. I stopped reading that thread when I couldn't prove him wrong 🙄 Glad I won't ever have to do another physics problem come noon on Wednesday.

 

[milski]

Oh. I stopped reading that thread when I couldn't prove him wrong 🙄 Glad I won't ever have to do another physics problem come noon on Wednesday.

I tried to avoid it since it was already long the first time I looked there. But curiosity, as usual, got the best of me and I had to dig deeper. On the bright side, I have much better idea of what was going on in that experiment. How useful is that is a whole different story...