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Topic: Finding the maximum wavelength that will eject an electron  (Read 2391 times)

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Offline eglaud

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Hey everybody i've been going over some of my practice exams and I am baffled by how I answer this one question. It provides the table
___________________________
Frequency (s^(-1)   | Kinetic Energy|
6.977 * 10^14        | ---
1.145 * 10^15        | 0
1.266 * 10^15        | .50


and asks my to determine the maximum wavelength in nm that will eject an electron from an unknown sample.


Looking at my work, I took the equation: frequency = speed of light/wavelength. I changed this so it was wavelength = speed of light/frequency, and found myself dividing (3*10^8) by (1.145*10^15) to find my wavelength. However, the answer I got on my test, 26.182 m, is not the answer I get when I divide this now (I get 2.62 * 10^(-7)). What am I doing wrong? Why is the answer im getting now smaller by a factor of 10^8?. In nanometers, my final answer on the test was 2.6 * 10^10 nm, which is the correct answer. Thanks in advance.


Offline eglaud

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Re: Finding the maximum wavelength that will eject an electron
« Reply #1 on: May 08, 2016, 08:20:18 PM »
The more I look at it the more i'm thinking the answer that was marked correct on the test is actually not, and the correct answer is roughly 262 nm. Can someone back me up?

Offline mjc123

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Re: Finding the maximum wavelength that will eject an electron
« Reply #2 on: May 09, 2016, 04:43:33 AM »
Obviously the first time you mistakenly divided by 1.145*10^-15. The second answer looks right: 2.62*10^-7 m = 262 nm, which is the right order of magnitude. 2.6*10^10 nm is nonsense, where did you get that from? Oh, it's the same as 26 m , isn't it? Maybe the given answer is just wrong. That happens sometimes.

Offline Enthalpy

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Re: Finding the maximum wavelength that will eject an electron
« Reply #3 on: May 09, 2016, 05:14:22 AM »
It is necessary to know some orders of magnitude. Checking them in every computation is the main way to avoid mistakes.

"Eject an electron" needs UV or visible light in normal life, and visible light extends from 400nm to 800nm. Also, 1m wavelength corresponds to 300MHz in vacuum.

Offline eglaud

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Re: Finding the maximum wavelength that will eject an electron
« Reply #4 on: May 09, 2016, 02:44:24 PM »
Thanks everyone, I guess my exam graders just made a huge error? I got a point off for improper sig figs, so haha who knows what happened. 262 nm makes sense, thanks everyone!

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