Modern Physics 4 Question 12

12. Electrons with de-Broglie wavelength λ fall on the target in an X-ray tube. The cut-off wavelength of the emitted X-rays is

(2007, 3M)

(a) λ0=2mcλ2h

(b) λ0=2hmc

(c) λ0=2m2c2λ3h2

(d) λ0=λ

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Solution:

  1. Momentum of striking electrons

p=hλ

Kinetic energy of striking electrons

K=p22m=h22mλ2

This is also, maximum energy of X-ray photons.

Therefore, hcλ0=h22mλ2 or λ0=2mλ2ch



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