In an experiemtn measuring the photoelectric effect of a certain metal, it is fo

Question

In an experiemtn measuring the photoelectric effect of a certain metal, it is found that the threshold for photomission (minimum energy needed) is at a certain frequency f0 of violet light.

A.Suppose the incident light is now changed to a frequency in the ultraviolet region without any change in intensity. What if anything will happen to the observed stopping potentil and photo-current. Explain your reasoning.

B. Suppose the incident light is now changed to a frequency in the blue region without any change in intensity. What if anyting will happen to the observed stopping potential and photo-current? Explain your reasoning.

C. Suppose the frequency of the incident light is increased to a value of 1.43 f0. What will be the maximum kinetic energy of emitted electrons? Explain your reasoning.

Explanation / Answer

A)
Stopping potential will increase without any change of photo-current.

Stopping potential is the maximum kinetic energy of the ejected electrons and it increase with increase in the frequency of the incident light. When the frequency is increased to that of uv region, stopping potential is increased. The photo-current depends on the number of photons striking the material, which is a measure of intensity. As the intensity is kept constant, there is no change in the photo-current.

B)
Stopping potential decreases and photo-current remains unchanged.

The same reasoning applies here too. blue region's frequency is less compared to the violet, hence decrease in stopping potential.

C)
In the first case,
KE1max = hfo - w
w is the work function of the material

When fo is increased to 1.43fo,
KE2max = 1.43hfo - w

KE2max - KE1max = 0.43 hfo
Kinetic energy will be increase by 0.43 hfo compared to the first case.

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