Many compounds absorb ultraviolet (UV) or visible (Vis.) light. The diagram below shows a beam the monochromatic radiation of radiant strength P0, directed at a sample solution. Absorb takes place and the beam the radiation leaving the sample has radiant strength P.

You are watching: How to calculate percent transmittance from absorbance


The quantity of radiation absorbed may be measured in a variety of ways: Transmittance, T = ns / P0 % Transmittance, %T = 100 T Absorbance,

A=log10 P0 /P A=log10 1/T A=log10 100/%T A=2-log10 %T

The critical equation, A = 2 - log10 %T , is worth remembering due to the fact that it permits you to easily calculate absorbance from percent transmittance data.

The relationship between absorbance and transmittance is illustrated in the following diagram:


So, if every the light passes v a systems without any absorption, climate absorbance is zero, and also percent transmittance is 100%. If every the irradiate is absorbed, climate percent transmittance is zero, and also absorption is infinite.

The Beer-Lambert Law

Now let us look at the Beer-Lambert law and also explore it"s significance. This is important due to the fact that people who usage the law often don"t understand it - even though the equation representing the regulation is for this reason straightforward:


Where A is absorbance (no units, due to the fact that A = log10 P0 / ns ) e is the molar absorbtivity v units of l mol-1 cm-1 b is the path length of the sample - that is, the path size of the cuvette in i beg your pardon the sample is contained. We will express this measure up in centimetres. c is the concentration the the link in solution, expressed in mol L-1

The reason why we like to to express the law with this equation is since absorbance is straight proportional come the other parameters, as long as the law is obeyed. We room not going to attend to deviations from the law.

Let"s have actually a look at a few questions...

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Question : Why perform we favor to express the Beer-Lambert regulation using absorbance as a measure up of the absorption rather than %T ?

Answer : to begin, let"s think around the equations...


%T = 100 P/P0 = e -ebc

Now, suppose we have a equipment of copper sulphate (which appears blue since it has actually an absorb maximum at 600 nm). We look in ~ the way in i m sorry the intensity of the light (radiant power) transforms as it passes v the solution in a 1 cm cuvette. We will look at the reduction every 0.2 cm as shown in the diagram below. The law says the the portion of the light absorbed by every layer of solution is the same. For our illustration, us will expect that this portion is 0.5 because that each 0.2 cm "layer" and calculate the following data: