1)

Light  of wavelength λph falls on a  cathode plate inside a vacuum tube as shown in the figure. The work function of the cathode surface is Φ and the anode is a wire mesh of conducting material kept at a distance d from the cathode. A potential difference V is maintained between the electrodes. If the minimum de-Broglie wavelength of the electrons passing through  the anode is λe . which of the following statement (s) is (are) true ?1122021206_b17.JPG


A) λe increases at the same rate as λph for λph < hcϕ

B) λe is approximately halved, if d is doubled

C) λe decreases with increase in ϕ and o λph

D) For large potential difference (Vϕ/e),λe is approximately halved if V is made four times

Answer:

Option D

Explanation:

kmax=hcλphϕ

Kinetic energy of electron reaching the anode will be

K=hcλphϕ+eV

Now λe=h2mK

  =h2m(hCλphϕ+eV)

If eV>>Φ

     λe=h2m(hCλph+eV)

If  Vf=4Vi,(λe)f(λe)i