Explian how a spectrophotometer is use to measure the absorptin of light?

Question

it is machian used in lab and i want to know how does work .
How a spectrophotometer is used to meausuew the absorption of light?

Answer 1

There are many different designs. What I'm describing is the one that is easiest to understand. Different versions are used for different jobs.

First you split the light into different wavelengths. The part that does this is called a "monochromator", and may be a prism or a grating.

Then you split the beam into two parts. You send one part through your sample, and the other part through a reference cell. For instance, if you want to know how strong the something in solution absorbs light, your reference will be the pure solvent.

Finally, you measure the intensity of the light that came through your sample, compared with the intensity of the light that came through your reference.

That gives you information about how strongly your sample absorbs light of that particular wavelength.

Answer 2

A spectrometer measures the light of absorption of a sample two times for each wavelength or frequency used. First it measures the amount of light that passes through a blank solution or solvent. Then you place a sample with the substance to be measured and the machine or you will record the amount of light at the same frequency as the blank . If there is something in the solution which absorbs light there will be less light passing through.

Some machines are double beam instruments and that means that the simultaneously measure the bank and then measure the sample and print out the Absorption

In the visible and UV portion of the spectrum Beers Law obtains and it is A = elc

Where A = the absorption (Intensity of sample /intensity of blank) at a given wavelength

c = the concentration of the sample in the solution given in g/ml (or cm cubed)
l = the path length of the light that goes through the cell (Usually the diameter of the tube
e= the extinction coefficient ( this is usally given with a known substance and can be found in hand books) ) Or if you know A, l, and c you can determine it)


If you know or measure A and you know l and e you can determine the concentration of the substrate in the solution