Question

Photoelectric effect is the phenomenon of the ejection of electrons when the radiation of suitable frequency is made to fall on the surface of a metal. When light of a suitable wavelength falls on the emitter C given in the diagram, the photoelectrons are emitted. These photoelectrons are drawn to the collector A. The photoelectric current of the order of a few microamperes can be normally obtained from the device given in the figure. The device given converts a change in intensity of illumination into a change in photocurrent. This current can be used to operate control systems and in light measuring devices. The devices are made up of metals with low ionization enthalpies, for example, platinum, whose work function is 6.35 eV.

1. If infrared radiation of 3 × 10¹¹ Hz is used as incident radiation, determine the reading of microammeter? Justify mathematically.  (2)
2. In the given diagram, if terminal B is shifted towards the left, then how will it affect the reading of the microammeter?  (1)
3. Plot a graph showing this variation in the reading of the micrometer on shifting the terminal B towards the right.  (1)

Answer 1

I. If infrared radiation is used as incident radiation, determine the reading W_o = hν_o

Threshold frequency, v₀ = `W_0/h`

= `(6.35 xx 1.6 xx 10^-19)/(6.63 xx 10^-34)`

= 1.5 × 10¹⁵ hz

Frequency of infrared radiation < threshold frequency (ν_o); hence, no emission of photoelectrons will take place; therefore, the reading of the microammeter is 0.

II. Photoelectric current decreases with a decrease in potential. At some stage, for a certain potential of plate A, all the emitted electrons are stopped by plate A, and the photoelectric current becomes zero.

III.