It was observed by Hertz and Lenard around 1880 that when a clean metallic surface is irradiated by monochromatic light of proper frequency, electrons are emitted from it. This phenomenon of ejection of the electrons from metal surface was called as Photoelectric Effect. The electrons thus ejected were called as photoelectrons. For photoemission to take place, energy of incident light photons should be greater than or equal to the work function of the metal.
E ≥ W
hf ≥ W [Where h is plank’s constant]
Study of photoelectric effect
The given set up (as shown in fig.) is used to study the photoelectric effect experimentally.
In an evacuated glass tube, two zinc plates C and D are enclosed. Plates C acts as anode and D acts as photosensitive plate. Two plates are connected to a battery B and ammeter A. If the radiation is incident on the plate D through a quartz window W, electrons are ejected out of plate and current flows in the circuit. The plate C can be maintained at desired potential (+ve or -ve) with respect to plate D.
Laws of Photoelectric Emission
1. For a light of any given frequency, photoelectric current is directly proportional to the intensity of light, provided the frequency is above the threshold frequency.
2. For a given material, there is a certain minimum (energy) frequency, called threshold frequency, below which the emission of photoelectrons stops completely, no matter how high is the intensity of incident light.
3. The maximum kinetic energy of the photoelectrons is found to increase with increase in the frequency of incident light, provided the frequency exceeds the threshold limit. The maximum kinetic energy is independent of the intensity of light.
4. The photo-emission is an instantaneous process. After the radiation strikes the metal surface, it just takes 10–9 s for the ejection of photoelectrons.
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