# Homework Help: Bragg's law and double-slit experiment

1. Feb 22, 2006

### UrbanXrisis

I am asked to construct a double-slit experiment for 5eV electrons. The first minimum of the diffraction pattern needs to occur at 5 degrees, what must the seperation of slits be?

I'm guessing that I should use Bragg's law but I dont know how to calcualte a minumum because bragg's law is for maximums.

$$\lambda=\frac{1240eVnm}{5eV}$$

$$n \lambda=2dsin \theta$$

$$1x240nm=2dsin5$$

i would solve for d but wouldnt that be a maximum?

Also, I am asked to determin how far the slits have to be from the detector plan if the first minima on each side of the central maximum are seperated by 1 cm.

we know that theta is 5 and the height is 1 cm, so... using 1cm/theta(5)=11.4 cm? that doesnt seem right to me.

any ideas as to what i am doing wrong?

2. Feb 25, 2006

### Astronuc

Staff Emeritus
Some references on interference and multiple slits.

http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/mulslidi.html#c2

http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/slits.html

The maxima occur where the difference in distance is an integral multiple of wavelength, so that two peaks constructively interact. The minima occur where a peak and trough occur, or one wave is a half-wavelength out of phase with the other, so that they destructively interfere.

So a minimum should occur approximately halfway between two adjacent maxima.

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