# Absorption vs. Transmission

1. Mar 11, 2006

### Pengwuino

I'm a little confused, we have tested chemical concentrations based on how much light it can absorb. What i don't understand is why something can have a reading of 60% transmission yet also have an absorption of 22.2% via the equation A = LOG(1/T), T being tranmission, A being absorption. Seems like it should be a 40% absorption. Then i realized i might not quite understand what it means when a spectrometer is reading "Absorption and Transmission". Can someone explain?

2. Mar 12, 2006

### inha

The rest goes to scattering processes I guess. One can split the mass-energy absorption factor to elastic and inelastic scattering and photoabsorption.

3. Mar 12, 2006

### Ich

You may have got something wrong with your equation. Maybe you should consult the manual again.

4. Mar 12, 2006

### Pengwuino

No that's how the math works. The meter has a transmission and an absorption scale on the same dial going opposite ways but the Absorption is logarithmic.

5. Mar 12, 2006

### Dr Transport

Reflection accounts for some of the difference....

$$R + T + A = 1$$ for all processes.

6. Mar 12, 2006

### Ich

Hey, when T<0.1 -> A>1. That surely does NOT work. You must have forgot something crucial.

7. Mar 12, 2006

### KingNothing

Ich is right, not only does that allow for absorption to be more than 100%, but A+T > 100% whenever the transmission is less than 13.7%.

8. Mar 12, 2006

### Dr Transport

This is the key to the argument....... start consulting.

9. Mar 12, 2006

### Pengwuino

It was a lab for our chem course, we didn't have manuals to consult, we were just told "here, use this equation" and did as we were told haha.

10. Mar 12, 2006

### KingNothing

Ich is right that reflection does come into play, but the percent of light absorbed, transmitted, and reflected depends on what compound you are talking about. Are you sure it wasn't a sort of "introductory thought experiment" to get to used to using equations to model light A&R&T?

11. Mar 13, 2006

### Pengwuino

It was probably the only time we used the spectrometer this semester haha. It was one of those "hey look, you can determine concentration via light absorption, how bout that" experiments.

12. Mar 13, 2006

### Ich

Got it now - You´re measuring http://en.wikipedia.org/wiki/Absorbance" [Broken], not absorption.

Last edited by a moderator: May 2, 2017
13. May 12, 2008

### Greggor

It's less complicated than that...

Your reading of 22.2 is about right. If transmission = T is 60%, or 0.60, then per your formula absorption = A is log(1/T). Well, 1/.60 is 1.66, and the log of that is .2218, or about 22.2%. So your work stands.