- #1
BobbyBobbyBob
- 5
- 0
We have been doing some experiments using a mercury light source and filters for each wavelenght and a Detector Box (phototube) to measure the output voltage, to investigate planks constant, stopping voltage, intensity's effect... the typical - "energy is quantised" idea. :zzz:
There is one part of it I'm not too clear about and not sure how to explain it correctly.
I know intensity should not affect the output voltage. But as we lower the intensity of the light/wavelength the voltage drops slightly.
Here is the hint my lecturer gave:
"Think of the phototube as a capacitor. The capacitance C is tiny (~pF) but as long as the voltmeter resistance R is very large (in this case ~ 10^13 ohms) the RC time constant will be long enough to ensure that the recorded voltage is accurate. However this may not apply when the incident intensity is low..." WHY?
There is one part of it I'm not too clear about and not sure how to explain it correctly.
I know intensity should not affect the output voltage. But as we lower the intensity of the light/wavelength the voltage drops slightly.
Here is the hint my lecturer gave:
"Think of the phototube as a capacitor. The capacitance C is tiny (~pF) but as long as the voltmeter resistance R is very large (in this case ~ 10^13 ohms) the RC time constant will be long enough to ensure that the recorded voltage is accurate. However this may not apply when the incident intensity is low..." WHY?