# Ask an Expert: After the experiment.

**Moderators:** kgudger, Moderators

### After the experiment.

I have finished conducting my experiment (Cellphone Spectrophotometer). I do not feel that there is enough to talk about in the conclusion of my paper. I have generated the graphs and now what? What should I say was the point of it? Would it be possible for me to talk about the Beer-Lambert Law in regards to my experiment given that I only have three weeks?

Link to project: http://www.sciencebuddies.org/science-f ... ml#summary

Link to project: http://www.sciencebuddies.org/science-f ... ml#summary

### Re: After the experiment.

Hi mickbrian,

Your conclusion section should state whether your data supported or disproved your hypothesis. Then, you should explain why the experiment turned out the way it did. The Beer-Lambert law is very important for this experiment, so you could definitely talk about how the Beer-Lambert law helps explain your results.

This section of the Science Buddies Project Guide has useful information about writing your conclusion section.

http://www.sciencebuddies.org/science-f ... ml#keyinfo

The conclusions checklist is particularly handy. Keep in mind, too, that your conclusion doesn't have to be a particularly long section of your project report or display board.

Post back if you have other questions!

Your conclusion section should state whether your data supported or disproved your hypothesis. Then, you should explain why the experiment turned out the way it did. The Beer-Lambert law is very important for this experiment, so you could definitely talk about how the Beer-Lambert law helps explain your results.

This section of the Science Buddies Project Guide has useful information about writing your conclusion section.

http://www.sciencebuddies.org/science-f ... ml#keyinfo

The conclusions checklist is particularly handy. Keep in mind, too, that your conclusion doesn't have to be a particularly long section of your project report or display board.

Post back if you have other questions!

All the best,

Terik

Terik

### Re: After the experiment.

Hello Terik Daly,

Thank you for your help. I apologize for not replying sooner. My presentation went well and I was able to write a suitable conclusion for the time being. I say time being because I would still like to add in something about the Beer-Lambert Law. I am still not sure how to do this. Do you have any suggestions? Like I said, I have the graphs generated by the samples but I do not know how to "get" the information to prove this (how do I know what the light intensity is or level of transmitted light). Or do I already have it in the graphs? Is there any direction you can steer me in that will give me a better sense of how to find these? I have about another week for this paper.

Thank you again for your previous help,

mickbrian

Thank you for your help. I apologize for not replying sooner. My presentation went well and I was able to write a suitable conclusion for the time being. I say time being because I would still like to add in something about the Beer-Lambert Law. I am still not sure how to do this. Do you have any suggestions? Like I said, I have the graphs generated by the samples but I do not know how to "get" the information to prove this (how do I know what the light intensity is or level of transmitted light). Or do I already have it in the graphs? Is there any direction you can steer me in that will give me a better sense of how to find these? I have about another week for this paper.

Thank you again for your previous help,

mickbrian

### Re: After the experiment.

Hi mickbrian,

I'm happy to hear that your presentation went well!

In your conclusion, you can state that the Beer-Lambert law is the equation that forms the foundation for interpreting your experiments.

The graphs (spectra) you already have contain the information you need. You should have two sets of plots. The absorbance*** spectra have wavelength on the x-axis and absorbance on the y-axis; transmission spectra have wavelength on the x-axis and transmittance on the y-axis. The absorbance spectrum is convenient because the Beer-Lambert law is linear with respect to absorbance. You can read the absorbance at a certain wavelength directly off an absorbance spectrum, if needed. You can do the same thing for transmittance at a certain wavelength. For example, if you wanted to know the absorbance at 550 nm, you could draw a vertical line at 550 nm. At the point where the vertical line intersects the spectrum, you would then draw a horizontal line through that point to the y-axis. The value where that line intersects the y-axis is the absorbance at that wavelength.

Here's the Beer-Lambert law:

In words:

Absorbance at a particular wavelength = (extinction coefficient of absorbing species at that wavelength)*(path length)*(concentration of absorbing species)

In variables:

A = e*l*c

A is unitless

e has units of (1/moles)*(1/cm)

l has units of cm

c has units of moles

In this experiment, l is the width of the cuvette and c is the concentration of food dye in the water. You can easily measure l, but you do not measure c in this experiment.

The key thing to notice is that there is a linear relationship between absorbance and each of the other variables in the equation. In your conclusion, you could talk about what would happen to absorbance if, for example, you doubled the width of the cuvette so that the path length was twice as long. You could also point out in your conclusion that if you knew the molar extinction coefficient, you could calculate the concentration of food dye in each solution. Alternatively, if you knew A, l, and c you could calculate e. Sometimes in the conclusion section people talk about what they would change if they were to do the experiment again. Perhaps you could say you would vary the concentration of dye if you were to do this experiment again.

***A couple definitions (which oddly don't appear in the background information for the project) might be helpful.

transmittance = the amount of light that passes through the sample and reaches the camera. If all of the light from that entered the sample makes it to the cell phone camera, then the transmittance would be 100%. The fraction of light absorbed is 1 - transmittance (e.g., if the transmittance were 80%, then 20% of the light was absorbed by the sample)

absorbance = -log(transmittance). That is, the absorbance is calculated by taking the negative of the log (base 10) of the transmittance. Because absorbance is logarithmic, each unit in absorbance corresponds to a change by a factor of 10 in the fraction of light transmitted through the sample. For example, for a transmittance of 10%, the absorbance would 1. For a transmittance of 1%, the absorbance would be 2.

Let me know if this is helpful and ask any other questions you have about how to discuss the Beer-Lambert law in your conclusion.

I'm happy to hear that your presentation went well!

In your conclusion, you can state that the Beer-Lambert law is the equation that forms the foundation for interpreting your experiments.

The graphs (spectra) you already have contain the information you need. You should have two sets of plots. The absorbance*** spectra have wavelength on the x-axis and absorbance on the y-axis; transmission spectra have wavelength on the x-axis and transmittance on the y-axis. The absorbance spectrum is convenient because the Beer-Lambert law is linear with respect to absorbance. You can read the absorbance at a certain wavelength directly off an absorbance spectrum, if needed. You can do the same thing for transmittance at a certain wavelength. For example, if you wanted to know the absorbance at 550 nm, you could draw a vertical line at 550 nm. At the point where the vertical line intersects the spectrum, you would then draw a horizontal line through that point to the y-axis. The value where that line intersects the y-axis is the absorbance at that wavelength.

Here's the Beer-Lambert law:

In words:

Absorbance at a particular wavelength = (extinction coefficient of absorbing species at that wavelength)*(path length)*(concentration of absorbing species)

In variables:

A = e*l*c

A is unitless

e has units of (1/moles)*(1/cm)

l has units of cm

c has units of moles

In this experiment, l is the width of the cuvette and c is the concentration of food dye in the water. You can easily measure l, but you do not measure c in this experiment.

The key thing to notice is that there is a linear relationship between absorbance and each of the other variables in the equation. In your conclusion, you could talk about what would happen to absorbance if, for example, you doubled the width of the cuvette so that the path length was twice as long. You could also point out in your conclusion that if you knew the molar extinction coefficient, you could calculate the concentration of food dye in each solution. Alternatively, if you knew A, l, and c you could calculate e. Sometimes in the conclusion section people talk about what they would change if they were to do the experiment again. Perhaps you could say you would vary the concentration of dye if you were to do this experiment again.

***A couple definitions (which oddly don't appear in the background information for the project) might be helpful.

transmittance = the amount of light that passes through the sample and reaches the camera. If all of the light from that entered the sample makes it to the cell phone camera, then the transmittance would be 100%. The fraction of light absorbed is 1 - transmittance (e.g., if the transmittance were 80%, then 20% of the light was absorbed by the sample)

absorbance = -log(transmittance). That is, the absorbance is calculated by taking the negative of the log (base 10) of the transmittance. Because absorbance is logarithmic, each unit in absorbance corresponds to a change by a factor of 10 in the fraction of light transmitted through the sample. For example, for a transmittance of 10%, the absorbance would 1. For a transmittance of 1%, the absorbance would be 2.

Let me know if this is helpful and ask any other questions you have about how to discuss the Beer-Lambert law in your conclusion.

Last edited by tdaly on Fri May 29, 2015 6:23 am, edited 2 times in total.

All the best,

Terik

Terik

### Re: After the experiment.

Hello Terick,

Thanks again. There are just a few questions I would like to ask. First is, would there be a way to measure c? Second, I actually only calculated the absorbency plots (the project page didn't mention the transmittance ones). Do you believe I need to discuss and show the transmittance ones as well? And finally, how would I prove that there is a linear relationship between A and the other variables? Is there any way I can actually test the Beer-Lambert Law?

Thank you,

mickbrian

Thanks again. There are just a few questions I would like to ask. First is, would there be a way to measure c? Second, I actually only calculated the absorbency plots (the project page didn't mention the transmittance ones). Do you believe I need to discuss and show the transmittance ones as well? And finally, how would I prove that there is a linear relationship between A and the other variables? Is there any way I can actually test the Beer-Lambert Law?

Thank you,

mickbrian

### Re: After the experiment.

Hi mickbrian,

Yes, you can use the Beer-Lambert law to determine c. To do so, you need to know A, e, and l. In this experiment you measure A and l; you do not find e in this particular project. Usually people determine e by measuring solutions with known concentrations (e.g., 0.1 molar, 0.5 molar, 1 molar, etc.). You can then draw a line through the data points. The slope of that line is e*l. Since you can easily measure l, you can then calculate e. Often you can also look up e for a particular molecule in reference books. Once you know e, you can calculate the concentration, c, of the species in an unknown solution. But, doing this analysis is beyond the scope of this project.

Step 13 of the procedure says to make both wavelength vs. absorbance graphs and wavelength vs. transmittance graphs. But, I think showing your plots in absorbance is just fine - no need to recalculate everything into transmittance. Just be sure to explain how transmittance and absorbance are related mathematically (see my previous post).

You could show a linear relationship between absorbance and the other variables in the Beer-Lambert law in a couple of different ways (see below). But, this particular project is focused on identifying the wavelength of the absorbance maximum, not directly testing the Beer-Lambert law. You could do these kinds of tests on your own, however, if you want to:

1) changing the path length, l, without changing the concentration, c

2) changing the concentration, c, without changing the path length, l

Post back with any other questions you have!

Yes, you can use the Beer-Lambert law to determine c. To do so, you need to know A, e, and l. In this experiment you measure A and l; you do not find e in this particular project. Usually people determine e by measuring solutions with known concentrations (e.g., 0.1 molar, 0.5 molar, 1 molar, etc.). You can then draw a line through the data points. The slope of that line is e*l. Since you can easily measure l, you can then calculate e. Often you can also look up e for a particular molecule in reference books. Once you know e, you can calculate the concentration, c, of the species in an unknown solution. But, doing this analysis is beyond the scope of this project.

Step 13 of the procedure says to make both wavelength vs. absorbance graphs and wavelength vs. transmittance graphs. But, I think showing your plots in absorbance is just fine - no need to recalculate everything into transmittance. Just be sure to explain how transmittance and absorbance are related mathematically (see my previous post).

You could show a linear relationship between absorbance and the other variables in the Beer-Lambert law in a couple of different ways (see below). But, this particular project is focused on identifying the wavelength of the absorbance maximum, not directly testing the Beer-Lambert law. You could do these kinds of tests on your own, however, if you want to:

1) changing the path length, l, without changing the concentration, c

2) changing the concentration, c, without changing the path length, l

Post back with any other questions you have!

All the best,

Terik

Terik

### Re: After the experiment.

Hello Terik.

Just a few more questions:

So if I were to double the width of the cuvette, would that double the absorption because the equation is linear? Also I want to talk about the value in having a homemade spectrophotometer. Is this something that you believe is worth talking about? And I looked at the graphs created and I analyzed them by seeing where they peaked and whether or not they corresponded to that solution's complementary color (so for instance, green is complementary to red so in a green solution the graph for absorption should peak at about 720 or above). Is there any other ways I should analyze the data from my graphs?

Thank you,

mickbrian

Just a few more questions:

So if I were to double the width of the cuvette, would that double the absorption because the equation is linear? Also I want to talk about the value in having a homemade spectrophotometer. Is this something that you believe is worth talking about? And I looked at the graphs created and I analyzed them by seeing where they peaked and whether or not they corresponded to that solution's complementary color (so for instance, green is complementary to red so in a green solution the graph for absorption should peak at about 720 or above). Is there any other ways I should analyze the data from my graphs?

Thank you,

mickbrian

### Re: After the experiment.

Hi mickbrain,

Yes, doubling the width of the cuvette - keeping everything else constant - would double the absorbance you would measure.

Talking about the value of homemade spectrophotometer is a fine thing to include. You could discuss possible ways they could be used (e.g., determining what type of dye was in a food coloring)

No other types of graphs come to mind. I think you have done a nice job of tying the different parts of this project together.

Yes, doubling the width of the cuvette - keeping everything else constant - would double the absorbance you would measure.

Talking about the value of homemade spectrophotometer is a fine thing to include. You could discuss possible ways they could be used (e.g., determining what type of dye was in a food coloring)

No other types of graphs come to mind. I think you have done a nice job of tying the different parts of this project together.

All the best,

Terik

Terik