# Photoelectric emission

## Main Question or Discussion Point

Hello
This is a question regarding photoelectric emission.
When frequency of the radiation increases at a constant intensity, the number of electrons emitted per unit time decreases. Why is that so?

p.s. I need an answer ample for Alevel purposes. Not too complex that is.

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Hello
This is a question regarding photoelectric emission.
When frequency of the radiation increases at a constant intensity, the number of electrons emitted per unit time decreases. Why is that so?

p.s. I need an answer ample for Alevel purposes. Not too complex that is.
1. what does it mean constant intensity?
2. which is the energy of a single photon?

I mean no. of electrons per unit time is affected by changing intensity, which has been kept constant. And frequnecy usually affects max Ek,not the current?
So why is it so?

1) Intensity is no. of photons arriving per unit area per unit time. So if intensity is constant, the no of electrons emitted per unit time should be const.
2) Energy of a single photone is hf. So if frequency is increased, Ek should be affected, but not the no of electrons / time.

???????????

jtbell
Mentor
Intensity is no. of photons arriving per unit area per unit time.
Usually, "intensity" of light means the amount of energy (e.g. joules) arriving per unit area per unit time.

now im confused....someone with a comprehensive explanation to the original question please?

Hootenanny
Staff Emeritus
Gold Member
now im confused....someone with a comprehensive explanation to the original question please?
Referring to jt's comment:

Intensity is a measure of the energy incident on a surface per unit area per unit time. And as you correctly say, the frequency is directly proportional to the energy per photon.

Therefore, if the frequency of the light is increased, this means that the energy that each photon carries is increased. However, since the intensity or total energy per unit time and area remains constant, it follows that the number of photons incident on the surface must decrease since each photon carries more energy. Ergo, the number of photo-electrons liberated per unit time (current) will also decrease.

I hope that clears things up.

All you have to realize is that intensity is not the same as the number of photons. The number of electrons liberated depends on the number of photons, but if the intensity is the same while increasing the frequency, that means fewer photons and therefore fewer liberated electrons.

right....
thankyou everyone...