The experimental setup shows the photoelectric effect. It consists of evacuated glass tube (discharge tube) . It consists two electrodes; andoe and cathode as shown in fig. ‘mA’ is mili ammeter to measure current. V is voltmeter to measure potential difference between two electrodes. The cathode C is coated with alkali metal. When light of suitable frequency incident on the photo metal (alkali metal), electrons are emitted from its surface. The emitted electrons are accelerated toward the anode. As a result mA measures current. This current is known as photocurrent/ photoelectric current and the phenomenon of emission of electron from the photo metal is known as photoelectric effect.
Characteristics of photoelectric effect:
- Photocurrent with exposures time of light:
Photoelectric current increases with the exposure time of photometal upto 10-8 sex.
Afterwards, photocurrent remains stationary and attains a constant value as shown in fig. Photocurrent maintains a constant value even for the low (10-18) intensity of light after the exposure time (10-8 sec).
- Photocurrent with intensity of light:
Photocurrent increases with the increase in intensity of light it is because when the intensity of light is increased no. of photoelectrons emitted can be increased.
- Photocurrent with anode potential:
As the anode potential increases, photo current increases for various intensities of light keeping their frequency constant as shown in fig. If the anode potential is increased in negative current, the value of photocurrent decreases and at a particular potential, photocurrent becomes zero. That particular point is known as stopping potential. Hence stopping potential is the negative potential which stops the emitted electron from the photo-metal. Although the intensity of light is different stopping potential for a particular medium remains same.
- Photocurrent with frequency of light:
If the anode potential is varied for fixed intensity of light but of different frequency the plot obtained is as shown in fig. It indicates That the value of photocurrent corresponding to anode potential increases as the anode potential increases in positive value. Finally, value of photo current remains stable for all frequencies of light as in point 0. IN case magnitude of anode potential is increased in negative order, photo-current decreases. But, stopping potential for different frequency of light becomes different . IN fig, v01, v02 and v03 be the stopping potential for the frequency of light v1, v2 and v3 respectively, where v3>v2>v1. This information shows the relation between anode potential with stopping potential which can be expressed as: