**Absorbance **

In the previous post we introduced the term absorbance. ** The term absorbance quantifies the amount of light absorbed by a solution.** We know ,from the previous post –

We stated that absorbance and transmittance are two sides of a coin. How are they related?

Absorbance is commonly expressed in terms of I_{o} and I_{t} , as follows –

**A = log **_{10} (I_{o} /I_{t})

_{10}(I

_{o}/I

_{t})

The following table gives the values for both A and T –

In mathematics , *logarithm means opposite of an exponential.*

In simple terms, **log _{b} x = y means b^{y} = x**

Thus,

**log**

_{10}x = y means 10^{y}= xe.g. –

**log**

_{10}1000 = 3 means 10^{3 }= 100Also , **– log (x) = log (1/x)**

It’s not important, but just for you information , we can derive this expression as follows –

Graphically the relationship between A and T can be shown as –

How to understand this graph ?

The blue line indicates that as the absorbance(A) decreases , the %T value increases. This is because both those terms are inversely proportional. However, this inverse relationship is* NOT linear* but a

**one –**

*logarithmic**e.g*– If 50% of the photons of monochromatic light are transmitted by a sample, and 50% of the photons are absorbed, T = 0.5, but A is not 0.5, A is 0.3 (

*inverse logarithmic relationship between T and A*).

The above graph also corroborates the relationship –

**A = log _{10} (1/T)**

The difference between the graphs of expressions y= log _{10} (x) and y= log _{10} (1/x) is shown below –

Transmittance and absorbance have no units .The term * optical density* is an older term for absorbance and is no more used by the scientific world.

After studying the terms absorbance and transmittance and the Beer – Lamberts’s law , we shall proceed to discuss the next law of photochemistry in the next post.Till then,

Be a perpetual student of life and keep learning….

Good day !

References and further reading –

1.Physical Chemistry by W J Moore, first edition

2.http://stevegallik.org/cellbiologyolm_spectrophotometry.html