Physics Lab & Aquariums!

[karatekid14]

Hello there,

I’m a senior in highschool and part of the International Baccalaureate (IB) program (similar to AP classes- kinda), and one my classes is a two year physics course that we take during Junior and Senior years. One of the assignments for the class is to conduct a lab and document it. Although I haven’t been as active with aquariums (freshwater aquascapes mostly) the last few years I am interested in conducting a lab relating to wavelength and water depth. Aquariums require so much tinkering and science I figured people may be interested in this lab, and I would like to hear everyone’s suggestions in terms of layout. But first I’ll explain it a little more in about it.

Please note these are just my initial thoughts, so excuse a few mistakes!

Research question: what is the relationship between water depth and light intensity at various wavelengths?

 

My variables are the depth of freshwater, the wavelength of light, and the intensity of light. The independent variable is the water depth d and the dependent variable is the wavelength of light (lambda). I will measure the light intensity (l) at various depths of water and wavelengths to determine two things; the relationship between water depth and light intensity at given wavelengths; the relationship between wavelength and light intensity at given water depths.

 

I predict that as the depth of water (d) increases the light intensity (l) decreases when wavelength (lambda) is held constant. Next, I predict that as the wavelength (lambda) of light increases, the light intensity (l) will increase, and then decrease (Bell curve?). I predict these two relationships from my experience of the physical world; in the ocean light disappears as one goes deeper and at depths the light that can be observed is mainly from the blue spectrum.

 

Lab Design

I will use a 10 gal aquarium filled with freshwater and will place a PAR meter sensor at the bottom of the aquarium to measure the intensity of light. Above the water will be the light source, which will include three different three-watt LEDs: True Violet (405nm), Turquoise (495nm), Deep Red (660nm). The first part of the experiment I will vary the depth of water for each of the three LEDs to determine the water depth vs light intensity relationship. For the second part of the experiment I will keep the water depth constant while the intensity of light is measured for each of the three LEDs.

I picked those three LEDs because they cover a wide range of the visible spectrum, and I also chose LEDs because they have fairly precise wavelengths. I’ve never used a PAR meter before, so do you think this set up would work?

I’m interested to hear any thoughts, or suggestions!

[seabass]

http://oceanexplorer.noaa.gov/facts/red-color.html

 

A ten gallon tank is fairly shallow (about a foot). It might be enough to measure, but the effects will not be as pronounced as they would be in a deeper tank (the deeper the better). The sunlit layer (euphotic zone) is 330 feet deep, although the majority of visible light is absorbed in the first 33 feet. But even red light can penetrate up to 49 feet.

http://www.waterencyclopedia.com/La-Mi/Light-Transmission-in-the-Ocean.html

 

I assume that particles in saltwater would affect light penetration. Even dissolved solids in unfiltered water might affect the results. It might be interesting to observe the same tests against distilled water and seawater (NSW). Also, I'd use blue, green, yellow, orange, and red lights.

 

While the glass wall will have an effect (less with acrylic, or even low iron glass), consider doing the test sideways, as gravity won't affect light waves. You might first perform the test on an empty tank to determine the effect of the glass. You could get a three or four foot long glass tank for a reasonable price. Again, the longer the better.

 

Also realize that distance itself will affect light spread (and therefore intensity). So I would do the test at the determined distances through air only, then through the glass wall (if performing the test through a side wall) at the determined distances, and then through water at the determined distances.

[jedimasterben]

Light is an inverse square -double the distance, 1/4 the PAR, half the distance, 4x the PAR.

 

Light photons of a higher wavelength are actually less energetic, but PAR is not weighted, so while a 660nm red photon has 1.88eV energy and a 430nm photon has 2.88eV, they have the same quantum efficiency in the photosynthetic process.

 

12" of water will have no effect on light intensity or wavelength beyond the effect that air does. I would recommend using something in the 36-48" range for height, it will give you much more data to work with and will start to show aa slight attenuation of red light.

 

What PAR meter and spectrometer will you be using?

[HarryPotter]

Hi from a fellow IB student. I'm doing my IA in HL Chemistry, not physics.

[spinycheek]

I like the sideways idea, easier to get more data and hence better measurements without having a giant aquarium. As long as you measure everything inside the glass from start to finish, it won't matter. Say start is 5 cm through water and then have 25, 50, 100, 200 cm measurements. Also have a way to check the wavelengths yourself (manufacturers lie) and make sure you account for differences in lumens. A brighter red light will penetrate deeper than a dim red light even if they have the same wavelength.

[Veng]

Light is an inverse square -double the distance, 1/4 the PAR, half the distance, 4x the PAR.

Just a note, but this is only true from a point source, sources such as T5HOs don't behave as a point source at the distances commonly used in aquariums.

[dandelion]

Interesting how I noticed quite a lot of us on this forum are younger people. Looks like the hobby is Ttracting young kids too

 

Hi from a fellow IB student. I'm doing my IA in HL Chemistry, not physics.

[Rural]

Looks like the hobby is Ttracting young kids too

 

 

there are some oldies too ! just not that many are as tech savvy...

 

also, since you will get wavemakers and such, you could do fluid dynamics analysis and use something other than water?

 

also dont be afraid to reach out to local universities, national companies and research pubmed as there are many places that have already done this similar project and could give you some pointers.

 

interestingly, you could calculate the varying intensity of light as a product of wave dynamics of the water wave and correlate that variance to photosynthetic advantages. you would need to sync and record water height, turbulence, par on a fairly short time and could extrapolate the sine wave?

 

rambling

[karatekid14]

Thanks for everyone's suggestions!

 

My main concern was with the shallow depth of water, so I like the idea of going lengthwise. I think I can tweak my question to be more along the lines of "What is the relationship between wavelength and light intensity at shallow depths (<24in)?" This lab doesn't, and honestly shouldn't be super complicated, we just need to demonstrate an ability to plan and execute a lab, so it's okay if the results are inconclusive, as long as the method is correct.

 

It seems like my school might not have a PAR or light meter, so I might actually change my lab to something a little simpler and more likely to succeed such as investigating the index of refraction at different salinities of water.

[TFish77]

Thanks for everyone's suggestions!

 

My main concern was with the shallow depth of water, so I like the idea of going lengthwise. I think I can tweak my question to be more along the lines of "What is the relationship between wavelength and light intensity at shallow depths (<24in)?" This lab doesn't, and honestly shouldn't be super complicated, we just need to demonstrate an ability to plan and execute a lab, so it's okay if the results are inconclusive, as long as the method is correct.

 

It seems like my school might not have a PAR or light meter, so I might actually change my lab to something a little simpler and more likely to succeed such as investigating the index of refraction at different salinities of water.

 

Do you have a local tinker/makerspace that you can get a student membership for? That's where I was lucky enough to get access to everything from a par meter to a laser cutter.

[ajmckay]

Cool idea. Obviously there is a lot of data already available on the subject, but I think there are plenty of ways to spin it.

 

In addition to not having a PAR meter though I see some other (possibly easy to overcome) hurdles to the original experiment.

 

- You would need a light source that can be easily and exactly repeated.

- A light source that can cover a wide nm range

- A light source powerful enough to penetrate depth

 

The sun seems to be the obvious answer - but also introduces other variables.

 

Were someone to try this experiment I think it could be cool to build a sort of water tube instead of an aquarium where you take PVC - say 10' tall 4" diameter. Make a stand so it stays upright. Then drill a large enough hole in an end cap to fit the PAR meter sensor and glue a piece of acrylic to the inside of the cap - then glue the cap onto the bottom of the pipe. I haven't used a PAR meter before but I would assume this would be enough to gather a sample?

 

Good luck with whatever way you decide to take this. It's still possible you could gain access to a PAR meter either through a reef club or possibly someone local to you that has one and would let you borrow it.

[seabass]

For your purposes, a lux meter (which is relatively inexpensive, as little as $14) is probably sufficient: http://www.amazon.com/Light-Meter-LX1010B-Luxmeter-display/dp/B000JWUT6O/ref=sr_1_1/191-1860440-4018814?s=hi&ie=UTF8&qid=1447133545&sr=1-1&keywords=lux+meter

If using an aquarium, I'd at least use a 20 gallon long (30" length). Sometimes you can get one on sale for as little as $20 at Petco.

However, ajmckay's PVC pipe idea is very good. You could get a 10 foot long, 2" diameter pipe pretty cheap (then you could cut it to any manageable length), and fabricate an end cap with an acrylic panel (for the meter): http://www.reefcentral.com/forums/showthread.php?t=1808627

You might want to make two or three tubes (like a 1', 3', and 6') for the tests. An advantage of the tubes is that they will help reduce light loss and stray light from entering.

I'd use a blue, green, yellow, orange, and red light (blue, yellow, and red at a minimum). Light filters on a white LED source (even a LED flashlight) is probably good enough to achieve colored light: http://www.amazon.com/gp/product/B005QEH5YO?keywords=Light%20filters&qid=1447138040&ref_=sr_1_8&sr=8-8

You might be able to do the experiment without an additional light source (using just the room lighting and light filters). On the bottom, make a cover for the lux meter (to keep out stray light). Then place the light filters over the PVC pipe opening (on top) and read the values on the meter. Perform the test using the different tubes, and compare the decrease in intensity of the various colors at various depths. Hopefully it will show that the red light loses more (a higher percentage of) intensity at a greater depth than the other light colors.

[karatekid14]

Oh I really like the pipe idea. I can use 3" black pipe in 4, 3, 2, 1 ft lengths for a 10 ft. I think that would work, and with black pipe I don't need to worry about light reflecting as much. So for the LEDs do you think I should get 405nm, 495nm, and 660nm? That seems like the best range I can find, well at least on LEDgroupbuy

[seabass]

Black pipe might be fine, but it will probably absorb more light. Plus, I don't think the light reflecting internally would necessarily be a bad thing, as it would still have to pass through the length of the pipe in order to register on the meter. So I think you could use white PVC.

When using specific colored lights, you'll want to make a cover for the top too (to avoid stray light from affecting the results). However, it would be simpler to use one light source (and use 4" x 4" color filters for the color).

If you are making a light for the filters, choose a white light. Like I said, even a white LED flashlight would work. However, if you are just using the room lighting as your light source, all you'll need are the colored filters (which would be cheaper and easier than making a light).

A rough guide to LED colors:

• Blue: 475nm
• Green: 510nm
• Yellow: 570nm
• Orange: 590nm
• Red: 650nm

[karatekid14]

The reason I wanted to use different wavelength LEDs is because they have fairly exact wavelengths. If I were to use the colored gels I'm not sure how I would measure the wavelength of light. If I do use LEDs should I use lenses? Or leave those off?

 

I'm not great at soldering, so I like using the solderless LEDs. I looked on RapidLED and this line up seemed to be the best range for the most precise wavelengths.

 

True UV 400-410nm

Royal Blue 450-465nm

Lime Green 566-569nm

Green 520-535nm

Deep Red 660nm

 

In terms of power I have a Makersdriver, two channels at 700 mA, and a power supply.

 

Thanks again!

[jedimasterben]

Lenses will focus the light and will give you larger numbers, so honestly may show clearer results.

 

Be aware that lime is not a direct color LED but phosphor converted, so its peak is 567nm but it's spectral half-width covers from ~505-620nm, so it's much less exact

[karatekid14]

Lenses will focus the light and will give you larger numbers, so honestly may show clearer results.

 

Be aware that lime is not a direct color LED but phosphor converted, so its peak is 567nm but it's spectral half-width covers from ~505-620nm, so it's much less exact

Oh good to know. So would just this Green 520-535nm one be better? And in terms of lenses 40 degrees?

[jedimasterben]

Yes, as it has a MUCH more narrow half-width

[seabass]

If I were to use the colored gels I'm not sure how I would measure the wavelength of light.

By looking at it. While it won't give you an absolute quantitative number, your eyes do a reasonable job of determining wavelength by color. For the sake of the experiment, an exact wavelength wouldn't be necessary as your aren't determining a formula, but are simply trying to determine if certain colors penetrate deeper than others.

 

According to NASA.gov: http://science-edu.larc.nasa.gov/EDDOCS/Wavelengths_for_Colors.html

Blue: 475nm

Green: 510nm

Yellow: 570nm

Orange: 590nm

Red: 650nm

 

White light contains all of the different wavelenths; however, the gels only allow certain wavelenghths to pass through.

[karatekid14]

For the sake of the experiment, an exact wavelength wouldn't be necessary as your aren't determining a formula, but are simply trying to determine if certain colors penetrate deeper than others.

I didn't make this clear but the lab needs to explore something that you can graph, so in this case it would be water depth vs wavelength. So I think it is important to have specific wavelengths.

[seabass]

I didn't make this clear but the lab needs to explore something that you can graph, so in this case it would be water depth vs wavelength. So I think it is important to have specific wavelengths.

Simple:

Blue: 475nm

Green: 510nm

Yellow: 570nm

Orange: 590nm

Red: 650nm

 

It's probably as accurate as what's listed on the LED diodes.

[karatekid14]

So I did the lab and the results were very consistent, but... not what I expected. I ending up using UV 410-420nm, RB 450-465nm, B 465-485nm, G 520-535nm, R 620-630nm.

 

Here's my setup;

 

 

And here are the results. I realized RB is 1500mA, while the driver is 700mA, so I should probably throw that data out. Otherwise it's a bit confusing. Since I was measuring light, the results were very consistent, +/-1. However, the order of intensity wasn't what I expected, RB, B, G, UV, R. The actual results were B, R, G, RB, UV.

 

[ajmckay]

Kick ass dude - pretty cool experiment. Anything having to do with LEDs is cool IMO

 

Might as well get credit for something you enjoy doing, right?

[karatekid14]

I am going to do the lab again, but just through air so I can compare the ratio between everything. I hope that will help things make sense

[seabass]

I am going to do the lab again, but just through air so I can compare the ratio between everything. I hope that will help things make sense

Good, you want to see how much loss is due to the water (as apposed to other factors)