 So your next lab involves using the spectrophotometer and the spectrophotometer is actually a machine that can measure light that is absorbed at different wavelengths. And if you can measure what is absorbed by a particular substance, like a food dye for example, you can use that absorbent value to determine the concentration of the food dye that you may have in your particular product. So that is why sometimes scientists use spectrophotometer to determine the concentration of something like protein. How much protein do you have in your sample? So today I'm going to briefly go over the key parts to the spectrophotometer that allows it to do what it does to measure absorbance and also percentage transmission. And if you notice here I have a diagram written on the board and one important component that the spectrophotometer has is a light source. Now this light source can emit light at all different areas or range of the white light spectrum. But what is important here is that it also has a centrifuge, the spectrophotometer also has what is called a monochromometer. This monochromometer is able to filter the different spectra of this light and just allow one wavelength to pass through. So one wavelength of light to pass through to hit your sample of interest. So this emits light at many different wavelengths. This monochromometer sorry will allow just light of one wavelength to pass to hit your sample here. So if you want to determine the absorbance of your sample at one wavelength the spectrophotometer allows you to do that. So this represents either a quvet or a test tube. So in your lab you're probably going to be using a test tube and the test tube is going to have your sample of interest. So the light is going to pass and hit your sample and the sample if it contains a food dye it will absorb some of the light that is transmitted to it and some of it it will emit. And it depends on the wavelength that you're exposing your sample to at a particular wavelength your sample may absorb light at another wavelength it may not. So we're going to actually examine food dye to determine at what wavelength it absorbs the most light. So your sample absorbs light but it gives off what is not absorbed and this is called the transmitted intensity. This is the original intensity of light that's hitting the mixture the light of a particular wavelength this is whatever is left what didn't get absorbed and is left and is going to be transmitted. Now this transmitted intensity is going to be received by this photo multiplier and what it does it receives light and it converts it to an electronic or electrical signal which is going to be picked up by a meter which will convert this electrical signal into a measured value. So that is how you can determine the you can obtain your percentage transmission and from that you can determine the absorbance of your dye at a particular wavelength. So light water for example is a liquid that doesn't have any dye in it so basically it is not going to absorb any of the light at a particular spectrum of light and because of this it's going to emit most if not all of the light. So water for example would have 100% trans close to 100% transmittance and very very low if any absorbance of light at a particular wavelength and what is also important is when you are measuring your particular dye you need to have what is called a blank which will subtract whatever residual light is going to be absorbed by the solution that your dye is in because you just want the absorbance of your dye itself you don't want this absorbance of your dye plus the solution it is in. So you have to use a blank and get the transmittance of that blank and then subtract that transmittance from what you get for your dye itself and then you'd be able to get just the absorbance and the percentage transmittance of your dye alone. So that is what we're going to be doing. So I'm going to go through these different steps that we are you would be required to do in the lab and tell you an example that I'm going to show you that will give you an idea of how you can do your experiment well.