Q.

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]

1 λ0=2mcλ2h  
2 λ0=2hmc  
3 λ0=2m2c2λ3h2  
4 λ0=λ  

Ans.

(1)

The cut-off wavelength of the emitted X-ray,

λ0=hceV                                                  ⋯(i)

According to de Broglie equation,

λ=hp=h2meV

or   λ2=h22meV⇒V=h22meλ2              ⋯(ii)

From eq. (i) and (ii),

λ0=hc×2meλ2eh2=2mcλ2h