- #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? :uhh:
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? :uhh: