# A2 Physics Diffraction help please

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#1

C is the answer, but I don't get why. We want to minimise diffraction, so we want the wavelength of the electron to not match the wavelength of the atoms in the specimen, right? So by lambda = h/p=h/mv, we need the speed of the electrons to change. That leaves C and D as the options, but why not D?

THANKS
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7 years ago
#2
(Original post by krisshP)
C is the answer, but I don't get why. We want to minimise diffraction, so we want the wavelength of the electron to not match the wavelength of the atoms in the specimen, right? So by lambda = h/p=h/mv, we need the speed of the electrons to change. That leaves C and D as the options, but why not D?

THANKS
0
#3
(Original post by Stonebridge)
0
7 years ago
#4
The smaller the object you want to "see" with a microscope, the shorter must be the wavelength of the waves being used.
I think that's all this question is asking. You need faster electrons if you want to reduce their wavelength.
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#5
(Original post by Stonebridge)
The smaller the object you want to "see" with a microscope, the shorter must be the wavelength of the waves being used.
I think that's all this question is asking. You need faster electrons if you want to reduce their wavelength.
I don't get that bold part. Can you please explain it?

Thanks
0
7 years ago
#6
(Original post by krisshP)

C is the answer, but I don't get why. We want to minimise diffraction, so we want the wavelength of the electron to not match the wavelength of the atoms in the specimen, right? So by lambda = h/p=h/mv, we need the speed of the electrons to change. That leaves C and D as the options, but why not D?

THANKS

Think about this. If you have a wall with an open door in the middle, and one person either side of the door, one person can hear the other. Even if neither can see each other, the sound wave is diffracting so that each person can detect it. If however it was pitch black, and one person shone a laser through the open door, the laser light would not diffract so that the other person could see it by looking where the doorway would be. Why? Because the wavelength of the laser is too small. As it is smaller than that of the doorway, it is not being spread out, so the wave of laser light is not reaching the second person's eyes.

You use an electron microscope because you can achieve much lower wavelengths than you can with standard light microscopes. A high voltage is used to accelerate the electrons (so their velocity is high) and thus their diffraction will be less at a gap of given size.

In my door anaology, consider the laser the fast moving electrons, and the sound waves the slow moving ones. Slower = larger wavelength = more diffraction = lower resolution.
1
#7
(Original post by Steroidsman123)
.
Nice analogy. Makes sense now.

THANKS +REP
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