Effects of Intensity and Frequency on Photocurrent

The current produced due to the flow of photoelectrons in an external circuit is called photocurrent.

The maximum photocurrent obtained when all emitted photoelectrons are collected by the anode is called the saturation current.

The minimum negative potential applied to the collector plate that stops even the fastest photoelectrons is called the stopping potential.

Basic Setup

A photosensitive plate is illuminated by light. The emitted electrons are attracted toward another plate, and the resulting current is measured.

Experimental Variable

• Intensity of light.
• Potential difference between emitter and collector.
• Frequency of incident radiation.

Observation

• Change in photocurrent.
• Change in saturation current.
• Change in stopping potential.

Main Observation

When the frequency of incident light is kept constant and its intensity is increased, the photocurrent increases.

Reason

Higher intensity means a greater number of photons fall on the surface per second. As a result, more electrons are emitted per second, increasing the current.

• Photocurrent is directly proportional to the intensity of incident light, provided the frequency is above the threshold frequency.
• Saturation current also increases with intensity.
• Stopping potential does not increase with intensity.

Main Observation

When the collector plate is made more positive, photocurrent increases and finally reaches a maximum value called the saturation current.

Cause of Saturation

At sufficiently high positive potential, all emitted photoelectrons are collected. Beyond this point, increasing potential further does not increase current.

Negative Potential Case

When the collector plate is made negative, it repels photoelectrons. Therefore, photocurrent decreases.

When the negative potential becomes large enough, even the fastest photoelectrons cannot reach the collector, and the current becomes zero.

That potential is called the stopping potential or cut-off potential.

Main Observation

For a given photosensitive material, the stopping potential increases when the frequency of the incident radiation increases.

Meaning

Higher frequency radiation gives greater maximum kinetic energy to emitted photoelectrons. Therefore, a larger negative potential is required to stop them.

Key Conclusion

• Maximum kinetic energy depends on frequency.
• Maximum kinetic energy does not depend on intensity.
• Stopping potential is therefore dependent on frequency, not on intensity.

Threshold Condition

If the frequency of light is below the threshold frequency, no photoelectric emission occurs, no matter how high the intensity is.

Photoelectric emission starts almost immediately when radiation of frequency greater than the threshold frequency falls on the metal surface.

The time lag between illumination and emission is extremely small, of the order of 10⁻⁹ s or less.