# Single Slit Diffraction in Double Slit Experiment

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#### drXanthine

5+ Year Member
Quick question from TBR light/optics Review questions, question #15.

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 lambda
C. d + a is comparable to lambda
D. a is comparable to or smaller than lambda

a is the slit width, d is the distance between the slits, and lambda is the wavelength of light.

TBR says that the answer is D. I thought the answer was A. Here's my reasoning as to why D is wrong: In any double-slit experiment, the slit widths are assumed to be small. In fact, I believe that they are assumed to be infinitely small. This is why no single slit interference occurs. This is not what D says, though. In order for single-slit interference to occur, doesn't D have to be true as well? If this wasn't the case, we wouldn't get circular wavelets extending past the slit. Diffraction is an essential part of both experiments, and in order for diffraction to occur at all, the slit width has to be on the order of the wavelength of light. Therefore, D is true of both single and double slit interference, so it can't be the correct answer.

I chose A because I thought B through D were wrong. Am I misunderstanding something basic about diffraction and interference here?

OP
D

#### drXanthine

5+ Year Member
I think I have it figured out if anyone else is curious: I found a good animation of what is going on here. Look at the gif's to the right of the page.

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