# Photoelectric effect and stopping potential

1. Jul 27, 2012

### dawn_pingpong

I have a question:

A stopping potential of 0.5 V is required when a phototube is illuminated with monochromatic light of 490nm wavelength. The wavelength of a different monochromatic illumination for which the stopping potential is 1.50V is closest to：

a) 350nm
b) 330 nm
c) 380nm
d) 400 nm
e) 500 nm

I am not very clear with the concept of this whole thing, so it will be nice if anyone can explain the concepts (or rather what are the fornulas) too. Thank you.

2. Jul 27, 2012

### Staff: Mentor

Welcome to the PF.

What can you tell us about the photoelectric effect? How does the energy of the incident photons affect the energy of the emitted electrons? What is the energy of a photon in terms of its wavelength?

There is a reason that we have a Homework Help Template here at the PF. It requires you to list the Relevant Equations...

3. Jul 27, 2012

### dawn_pingpong

I'm sorry I deleted the template... I thought that it was just the prompts...

This is what I know：
Planck's Law
$E=frac(hc/lambda)$, which in this case equals to 4.0567E-19

then KE(max)=eV。=E-work function

(from what I understand V is the stopping potential)

Then, uhhhh. I start to confuse myself. I suppose that e in this case is the energy of an electron. I only have the formulas from powerpoint slides (for lessons that I didn't attend), but they are not very well explained.

Well, Thank you all.

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook