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# Photoelectric Effect - Maximum Wavelength Watch

1. Hi, I am currently revising photoelectric effect, and i have this question:

A metal surface at zero potential emits electrons from its surface if light of wavelength of 450 nm is directed at it but not if light of 650nm is used. Explain why photoelectric emission happens with light of wavelength 450nm but not with light of wavelength 650nm

All i know is that the wavelength must be below a certain maximum value equal to the speed of light divided by the threshold frequency.

I dont know why this is, can someone explain why? Thanks.

This is a question in the AQA AS Physics book on page 31. But i dont have the answers in the back of my book to check with.
2. Your answer is fine. They don't expect you to know the mechanics behind it, you just need to know that there is a maximum wavelength of photon below which photoelectrons are emitted from a given metal's surface. Then you simply state that the 650nm is greater than this threshold wavelength, and the 450nm is less than it.
3. Wouldn't it be sensible to mention Einstein's Photoelectric equation as this is what's behind the effect.
The maximum wavelength comes out of this formula by changing frequency to wavelength and putting max ke of electron = to zero.

If it isn't an exam question with a mark scheme and a clear indication of what's expected, you need to look at what information is presented in the relevant chapter of the book, and what it says there about the "minimum frequency" and how this comes from that equation.
4. Thanks for the replies guys. The only part in the chapter of the book the question is in is what i said in my original question, so perhaps its correct like you said cheemaftw.

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