Page 1 of 1

### De Broglie Wavelength on the Midterm

Posted: Sun May 20, 2018 1:57 pm
I was wondering about a question on the midterm. It asked for the wavelength of an ejected electron when you're given the velocity of an electron. This is all in the context of the photoelectric effect.

Apparently, the information that was given was to be used in the De Broglie wavelength equation to solve for the wavelength. Apparently I did not use the De Broglie equation, but used the KE= .5MV^2 and the speed of light= (frequency)*(wavelength). My question is, why wouldn't this have worked, to use another equation? Or, how was I supposed to know that I was to use De Broglie if it doesn't explicitly say?

### Re: De Broglie Wavelength on the Midterm

Posted: Sun May 20, 2018 2:25 pm
The way I think through these problems is to first write down what I'm given and what is unknown. You're given the velocity of an electron and the mass of an electron is constant. What is unknown is the wavelength of that moving electron. Next, I look for equations that involve my given and unknowns. You can't use c = wavelength * frequency because the velocity the electron is moving at is not c, the speed of light. Since you know mass and velocity of the electron, you can determine the p (momentum of it) as p = mass * velocity. That takes care of the given, now you need to find the unknown. The only equation that uses both momentum and wavelength is wavelength = h / p. h is a constant, so this is the right equation to use!

I write out given and unknown in every problem I do, and that helps a lot in finding the right equations to use, so that's my best advice to you.

### Re: De Broglie Wavelength on the Midterm

Posted: Sun May 20, 2018 3:30 pm
The information provided was for the deBroglie wavelength. C=(Lambda)(Nu) is also used for things moving at the speed of light.

### Re: De Broglie Wavelength on the Midterm

Posted: Sun May 20, 2018 3:47 pm
Hi since they said that you were finding the wavelength of an ejected electron and we know that an electron has a mass that is why we know to use the de Broglie wavelength equation. Hope that helps!

### Re: De Broglie Wavelength on the Midterm

Posted: Sun May 20, 2018 11:06 pm
Hello, I believe I had a similar question regarding the midterm as well. I finally realized that if you see what the question is asking, we can tell that we must use the De Broglies wavelength Equation. Really hope this helps you :)