Hellwach And Lenards Experiment
Description:
The experimental arrangement for the study of photoelectric effect by Hellwach and Lenard is shown in figure.
Apparatus
It consists of an evacuated glass tube having two metal electrodes – Emitter plate (Plate which emits electrons) and Collector plate (Plate which will collect the emitted electron).
These two electrodes are connected with a supply. The supply is a lavish arrangement that allows us to change the potential difference and also the direction of current.
Rheostat is used to change the potential difference and commutator is used to change the direction of current.
A voltmeter is attached in parallel with the electrodes to measure the potential difference.
An ammeter is fitted in series to measure the photo-current flowing through the circuit.
Terms associated with light
Intensity
Intensity is the strength of light. To change intensity we can change the source of light with another source having different strength of light but colour of light should be same. Example: 100 Watts, 500 Watts, 1000 Watts.
Frequency
Frequency of light in simple terms can be described as colour of light. To change frequency we need to change the colour of light.
Working
A source of light of sufficiently energy enters the tube through quartz window and falls on the emitter plate. The electrons are emitted by the emitter plate and are attracted to the positively charged collector plate. This results in flow of photo-electric current through the circuit. The electric field is created by the battery.
The potential difference between the emitter and collector plates and the photo-electric current in the circuit is measured.
Graph of photo-electric current vs intensity of light
Observations
The moment light is incident of emitter plate, the electrons are emitted and photoelectric current starts to flow.
Photo-electric current and intensity of light are directly proportional to each other i.e. when intensity of light increases, the photo-electric current also increases.