Absorption of Radiant Energy by Different Colors

[JCLown]

Hi!

At school, my group is doing this experiment from the Science Buddies Website and we have a question on the procedure.

We aren't able to get an infra-red thermometer as required. Instead we thought that is we got different coloured jars, all of the same size, and stuck a thermometer inside.

One of the equations involved with the experiment needed a value for area. In our case, would we use surface area instead?

Would this work? If not, how can we improve.

Thanks for your help in advance.

I hope I am understandable.

Here is the link again --> Click Here

[deleted-141593]

Hi there,

I think these changes would alter the experiment substantially. This will no longer be a black body radiation demonstration. The original experiment uses the infrared thermometer to measure the amount of energy being radiated from pieces of colored paper in the form of infra-red photons. This then allows you to calculate the amount of energy initially absorbed. If you use colored jars you are going to affect the wavelengths of light that can get into the jar. This will then alter the energy the air in the jar receives from the light and thus the temperature inside. I suppose you can modify the procedure and do the experiment in this way. You will need to estimate the surface area of the jars in order to compare the amounts of energy entering the jars. You will also need to find a light source that illuminates the jars evenly. Some other elements of the equations will also change, but we might be able to work with this setup. I would appreciate input from other experts on this.

Cheers,
Colin

[JCLown]

Hi Colin.

Thank you for your reply.

since the set-up with the jars would change the experiment too much, how can I keep the same experiment without using an infra-red thermometer? We do not have access to this item. If the experiment can be somehow completed without one, that would be good. If not, how could we design a similar experiment with the same level of difficulty?

I appreciate your time and help.

~ Jessica

[deleted-141593]

Hi Jessica,

I think you can do the experiment they way you proposed, it's just different, as I described before. In simple terms, in the jar setup, the only light that gets through to the inside will be the light that is the color of the jar. From looking at the temperature of the thermometers inside you can determine what color light imparts the most energy. It's a clever way to get around the limitation of not having an infra-red thermometer. When you are working on your results the calculations will be different. You can calculate the total amount of energy transmitted through the glass of the jar but I don't think you can estimate the number of photons involved. On the other hand, another way to do the experiment would be to put clear jars containing your thermometers over the colored paper squares in order to trap the thermal energy from infrared photons emitted from the paper. Then, do the calculations the same way the experiment originally described.

Cheers,
Colin

[JCLown]

Colin - Thank you for clearing that up for me. I now understand.

I have another question about this experiment.

Would a suitable hypothesis for this experiment be :- It is hypothesised that colours with a lower wavelength will absorb and re-emit the most light energy.

Also, would my experiment be affected if a heat lamp was used instead of sunlight? I understand that I would have to do one jar/coloured square at a time. But would anything else like my results and/or equations be affected?

And does it matter what coloured squares I use? Is there a particular reason why white, yellow, blue, red and black are suggested? What about using the 3 primary colours plus maybe white and black instead?

[deleted-141593]

Would a suitable hypothesis for this experiment be :- It is hypothesised that colours with a lower wavelength will absorb and re-emit the most light energy.

Well, I don't want to make your hypothesis for you, but if you are doing the colored paper experiment, the hypothesis should be similar to that presented in the original experiment. In general, a hypothesis based on your knowledge of what wavelengths of light the different colored papers will absorb would be a good hypothesis. The more energy the paper absorbs, the more it should emit as infrared photons. Remember, with light longer wavelength means lower energy and vice versa.

Also, would my experiment be affected if a heat lamp was used instead of sunlight? I understand that I would have to do one jar/coloured square at a time. But would anything else like my results and/or equations be affected?

Yes, I think this is not a good idea. The heat lamp will emit many more infrared photos than other kinds of lamps and heat the jars too much directly.

And does it matter what coloured squares I use? Is there a particular reason why white, yellow, blue, red and black are suggested? What about using the 3 primary colours plus maybe white and black instead?

I see no problem with this variation. As long as you can make an inference about the wavelengths of light absorbed based on the color of the paper it should be fine.

Good luck!

Cheers,
Colin