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The mean free path of the electrons in a...

The mean free path of the electrons in a discharge tube is 20 cm. The tube itself is 10cm long. What is the length of the Crookes dark space?

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The mean free path of the electrons is much longer than the length of the tube. Thus, the electrons, in general, do not collide in between, no ionization takes place and hence no light is emitted. The crookes dark space fills the entire tube and hence is 10cm long.
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