Dear Arul,

you can define transmittance T as the ratio between the intensity of a beam transmitted through the material, I, vs. the intensity of the incident beam, I0. These quantities are related by the Lambert-Beer law, i.e.

I=I0 exp(-mu*d)

where mu is the absorption coefficient (length^-1) (it is a non-monotonous function of the wavelength of the incident beam), d is the sample thickness. In other words, for each wavelength, the less the material is absorbing (lower mu), the greater will be the transmittance.

When you are dealing with a liquid, you can express mu as the product of a molar absorption coefficient and the concentration of your sample:

mu=epsilon*c

'epsilon' quantifies how much 1 mol of your sample is willing to absorb EM radiation at each specific wavelength, i.e. it is also a non-monotonous function of the wavelength. It is also often referred to as 'molar extinction coefficient'.

So, eventually, transmittance T can be expressed as:

T = (I/I0) = exp(-epsilon*c*d)

Note also that the term 'extinction coefficient' is also used in the field of X-ray diffractometry, but in that case it has to do with a very different physical phenomenon, namely the ability of highly perfect crystals to attenuate the diffraction response due to dynamic scattering effects. So, in this latter context, the term 'extinction coefficient' indicates a different quantity, with a very different physical meaning.

Hope this helps.

Pls go through wood ward fieser rule....Arul

To see the relationship between the extinction coefficient and the transmittance we should depart from the relation saying that T+R+A=1, so the intensity of light entering the material is splited into two; one part is reflected with a reflectance R and another part has penetrated into the material, the part which penetrated into the material will follow one of the two aspects, an amount of it will be absorbed by the material with the Absorbance A and the other will be transmitted with the transmittance T, with a normal and simple lab instrument we can measure R and T but not A, so we use a numerical model that relate T to A therefore to the extinction coefficient k, and we deduce this k with curve fitting method between the model and the real transmittance data

the relation is done by the equation attached (you may find one more or less complicated)

"avec" in french means "with"