## Question

When a surface is irradiated with light of wavelength 4950 Å, a photocurrent appears which vanishes if a potential greater than 0.6 V is applied across the photo-table. When a different source of light is used, it is found that the critical retarding potential is changed to 1.1 V. Find the work function of the emitting surface and the wavelength of the second source. If the photoelectrons (after emission from the surface) are subject to a magnetic field of 10 T, what changes will be observed in the above two retarding potentials?

### Solution

1.9 eV, 4125 Å

The energy (*hv*_{1}) of the light-photon incident on the surface is equal to the work function *W* of the surface plus the kinetic energy *E _{k}* of the emitted photoelectron.

If the retarding potential is V_{1}, then .

= 1.9 eV.

Using the light of another wavelength , we shall have

.

= 4125 Å.

Since the magnetic field does not change the speed of the ejected electrons, there will be no change in the stopping potentials.

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