## Problem 1.55

$c=\lambda v$

deeksha1I
Posts: 21
Joined: Fri Sep 29, 2017 7:04 am

### Problem 1.55

1.55) If an absorption occurs in the infrared spectrum at 3600 cm^-1, what is the frequency of the radiation that corresponds to that absorption?

I understand that we need to use E = (v/c), and I understand how to the get the right answer once you convert the speed of light to meters. But I'm wondering why the equation won't work in meters instead of cm. Thank you!

Alissa Stanley 3G
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Joined: Fri Sep 29, 2017 7:06 am
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### Re: Problem 1.55

I am also struggling with this problem. How would you know to use the equation E=v/c? And why is an absorption, with units in cm, considered energy?

Gurvardaan Bal1L
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### Re: Problem 1.55

Alissa Stanley 3G wrote:I am also struggling with this problem. How would you know to use the equation E=v/c? And why is an absorption, with units in cm, considered energy?

If you read the problem fully, it tells you that in infrared spectrophotometry energy is expressed in terms of v/c, which is where E = v/c comes from. The units for this is also reciprocal cm, which is where the cm^-1 comes from.

Gurvardaan Bal1L
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Joined: Fri Sep 29, 2017 7:04 am
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### Re: Problem 1.55

deeksha14A wrote:1.55) If an absorption occurs in the infrared spectrum at 3600 cm^-1, what is the frequency of the radiation that corresponds to that absorption?

I understand that we need to use E = (v/c), and I understand how to the get the right answer once you convert the speed of light to meters. But I'm wondering why the equation won't work in meters instead of cm. Thank you!

This equation only works if you're using reciprocal centimeters, as that is the correct unit for this equation. I'm guessing its just because its easier to measure in centimeters.

Matthew Lee 3L
Posts: 51
Joined: Fri Sep 29, 2017 7:07 am

### Re: Problem 1.55

You can do the problem using meters too. To do this, you would need to keep light as 3 x 10^8 m/s and convert 3600 cm^-1 into 360,000 m^-1. Since it is cm^-1 and not cm, you would convert it to a larger number.