hey everybody professor Davis here from chem survival comm and the YouTube channel chem survival and today I'm going to give you a brief introduction to UV visible spectroscopy and the beer-lambert law to begin our discussion I've drawn a schematic on the bottom half of this screen depicting a fairly typical setup for a UV visible spectrophotometer now there are many different ways to build a spectrophotometer but this is one of the simpler designs so I thought we'd start there it consists of a source lamp which is something as simple as the headlamp from a motor scooter or more complicated like a deuterium lamp or xenon arc lamp the next device in line is what's known as a monochromator which is two slits which separate or are separated by a prism or a diffraction grating the next element of our spectrophotometer is what's known as a beam splitter which divides a beam of light into two equal parallel beams of light next is the sample compartment which contains cells for both a reference and a sample and finally detectors which are devices that convert the impact of photons into electrical current that can be monitored by a computer so now that we've defined each of the smaller pieces within our spectrophotometer let's turn it on and see what happens I'll ignite the source lamp creating a variety of wavelengths of different light this light passes through the first slit of the monochromator ensuring that all of those light photons are traveling along parallel pathways so that when they strike the prism they are refracted into a rainbow of colors so each wavelength of light is moving to a different place in space so only one wavelength of light in this situation is going to make it through the second slit in my monochromator striking the beam splitter and becoming two beams of equal intensity these two beams of equal intensity will traverse a cell a different one for each of course one being the reference and one being the sample cell as the beam exits these cells that strikes the detectors which are firing away creating an electrical current and you'll notice at the moment that the intensity of the light exiting both the reference and sample cells is identical therefore the current generated by each detector is identical so in this case the intensity detected by each of my two detectors are the same so if I were to consider the ratio of the intensity leaving the sample cell to that leaving the reference cell I see that the transmittance of my sample cell is 100% that of the reference so I'm going to plot that here at zero concentration 100 percent transmittance now let's add a little sample to that cell something that might absorb this light now notice that when I do this the intensity of the light exiting the sample cell has decreased and therefore the current generated by its detector has also decreased so now the ratio of intensities is no longer 100% in this case let's say that it's down to 50% at a given concentration which I'm going to call X if I add another equivalent of my sample to the sample cell that decreases the intensity even more not only that it decreases the intensity at the sample cell by a known amount exactly one half for each equivalent of sample that I add so when I compare my new intensities at the sample and reference cells I see that I'm now at 25 percent transmittance and similarly an incremental increase in the concentration once more in my sample cell leads to another reduction by 50 percent and therefore a percent transmittance of 12 and a half now we have enough data points to see something very interesting here the relationship between the percent transmittance and the concentration of a sample is not linear instead it's exponential and while it's very useful to have this information scientists and spectroscopy prefer if they can to discuss linear relationships within their data because this makes for a much simpler discussion and it's much easier to predict how things will behave if we have a simple linear plot to compare this is where the beer-lambert law comes in you'll notice that right now I have an exponential relationship between my percent transmittance and my concentration we can see that in the equation here where the concentration is a term up here in the exponent but what August beer did was to convert that percent transmittance into a new unit called absorbance and he did so by taking the logarithm or in really the negative logarithm of the transmittance taking the logarithm of an exponential function creates a linear function and so the data when plotted as absorbance rather than transmittance is actually considerably easier to look at not to mention it's much easier to predict the exact absorbance by either extrapolating or interpolating within the data set that I've collected this is the utility of the beer-lambert law and the reason why we convert percent transmittance into absorbance so often when conducting UV visible spectroscopy that's all for now everyone I'm professor Davis from chem survival comm and the YouTube channel chem Survival I'll see you on my next video I'd like to thank everyone for making chem survival calm and my youtube channel chem survival such a success and I'd also like to invite you to check out a new project I've been working on coming in October 2014 it's a 36 part organic chemistry course developed in collaboration with the great courses to get more information about my course go to WWE em survival com that's wwm survival com thanks again for watching everyone and as always I'll see you in my next video Oh