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Physics/Photoelectric effect


I am confused by these theoretical questions. Any help would be greatly appreciated. I don't really understand the photoelectric effect.

These following questions are all according to wave theory of light. They also ight use the the image provided.

1. Increasing the intensity of the light source will cause the energy absorbed by the metallic surface to
i. increase
ii. decrease
iii. remain the same.
What concept or equation did you use to answer the above question?

2. Thus as a result of increasing the intensity of the light source the kinetic energy of the electrons will
i. increase
ii. decrease
iii. remain the same,

3. and thus the measured value for the stopping potential will
i. increase
ii. decrease
iii. remain the same

Hello Timmy,

1. Bring up the following site:
The question doesn't tell us if the individual photons have enough energy to liberate an electron, so I guess we have to cover both cases: a) not enough energy and b) enough energy. Notice the explanation of the "all or nothing" principle in the section titled Emission mechanism.
a) So if the photon energy is not sufficient to liberate an electron, the energy is re-emitted. The metallic surface does not hold that energy. Regardless of how intense the light is. The important criteria is the energy of the individual photons.
b) If the photon energy is sufficient to liberate an electron, increasing the intensity of the light increases the number of photons hitting the metal surface, thus exciting more electrons, thus liberating more electrons, but does not increase the energy of the individual electrons that have left. Looking at one representative photon: all the energy of the photon is spent in breaking the electron loose and in whatever kinetic energy it leaves with.

So the answer is remain the same. The principle used in case a) is the "all or nothing" principle. The formula used in case b) is
Kmax = h*f - Phi
which is given in the Mathematical description section of the above wikipedia site.

2. Increasing the intensity of the light increases the number of the photons, not the energy of  the individual photons. So the kinetic energy the electrons leave with is constant if the frequency of the light is constant.

3. Since the electron's kinetic energy is constant even tho the intensity of the light increased, the potential required to prevent the electron from reaching the anode remains the same.

I hope this helps,


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Steve Johnson


I would be delighted to help with questions up through the first year of college Physics. Particularly Electricity, Electronics and Newtonian Mechanics (motion, acceleration etc.). I decline questions on relativity and Atomic Physics. I also could discuss the Space Shuttle and space flight in general.


I have a BS in Physics and an MS in Electrical Engineering. I am retired now. My professional career was in Electrical Engineering with considerable time spent working with accelerometers, gyroscopes and flight dynamics (Physics related topics) while working on the Space Shuttle. I gave formal classroom lessons to technical co-workers periodically over a several year period.

BS Physics, North Dakota State University
MS Electrical Engineering, North Dakota State University

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