156.PHOTOCHEMISTRY(5) – LAWS OF PHOTOCHEMISTRY(2).

August Beer

August Beer was a German Chemist, physicist and mathematician. He was studying absorption of red light by various solutions, when he formulated a law called Beer’s law.

The quantity of light absorbed by a solute (dissolved in a non absorbing solvent) is directly proportional to the concentration of the solute.
or
The absorptive capacity of a dissolved substance is directly proportional to its concentration in a solution.

Beer’s Law

The Lambert and Beer’s law put together give us the Lambert – Beer law which can be stated as follows –

The quantity of light absorbed by a solution is directly proportional to the concentration of the solute and the path length of the light through the solution.

Beer – Lamberts’ law /Lambert – Beer law.

We studied in the previous post , that ,according to Lambert’s law, absorbance (A) is directly proportional to the path length (L) –

A ‌∝ L

In the above figure, the cell containing the absorbing solution in case 2 has a greater path length and so more light is absorbed, than in case 1( the concentration of both solutions is same). This means that each successive layer of the medium absorbs equal amount of light and thus as the path length increases, more and more light gets absorbed. Thus, the intensity of the transmitted beam reduces.

According to Beer’s law, absorbance (A) is directly proportional to concentration (c)

A ‌∝ c

As is obvious from the figure above, the more concentrated solution in case 2 will absorb more light than the less concentrated solution in case 1(even though the path lengths are same).

‌Combining the two laws , we get – 

A ‌∝ c L

Introducing the constant of proportionality(ε) ,mathematically, the Beer- Lambert’s law can be written as – 

A = εLc

where,
A = Absorbance of the solution ( No units )
L = path length ( cm )
c= concentration of the solution i.e amount of solute in the solution ( mol / L)
ε = Molar extinction co-efficient ( L mol-1 cm-1) .

If we calculate the absorbance of the solution experimentally, then we can easily calculate the concentration of the absorbing species in the solution. Thus, this law helps us to determine concentration of solutes in solution.

Intensity of light

Before we proceed any further, it is important that we understand the concept of intensity of light. What does intensity mean? In the afternoon we say ‘the sun is very intense‘ because the intensity of light is high. During sunset though, that intensity reduces and thus, we can look at the sun directly.

Thus, intensity is quantity of radiation/ no. of photons emitted per unit time .The everyday term for it is ‘brightness’. The more the brightness of light, more is it’s intensity and vice versa.

In the next post we shall study what absorbance and transmittance mean and further talk about the laws of photochemistry. Till then,

Be a perpetual student of life and keep learning….

Good day !

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