Blackbody Curves

theodog (Jeff) · #1 27-09-2011, 07:03 AM

Hi All,
Spent some down time at work looking into demonstrating blackbody radiation. By using a Vernier High school spectroscope, a 12v light globe and some computer programs I managed this.

At least I think they have the right shape and trends.

Merlin66 (Ken) · #2 27-09-2011, 04:57 PM

Very interesting!
Which camera were you using?
If you could generate an instrument response curve, this could be used to better determine the actual black body curves...

theodog (Jeff) · #3 27-09-2011, 06:56 PM

This image is of the set I used. The spectroscope is a 'cheap' instrument we use to demonstrate spectral lines in flame tests to high school students.
The experiment wasn't to measure BB curves so much as to see if the concept could be demonstrated.
I plan to map instrument responses later but this is beyond our requirements for demonstration.
Other considerations include the composition of the light-globe filament. I assumed it was tungsten and used this in the temperature calculation.

When time allows, will do more.

GeoffMc (Geoff) · #4 08-10-2011, 07:33 AM

Here's a set of curves I obtained using a 12V downlight for the same reason (demonstrating the concept). The next step will be to test the transmission properties of different types of glass and see if the glass in the globe is responsible for the absorption features.

How did you work out the temperature curve knowing the composition of the filament?

Geoff Mc

theodog (Jeff) · #5 08-10-2011, 12:36 PM

Hi Geoff,
The image attached is a screen shot of an example of my calculations.
The formula was referenced from the web below.

http://www.physicsforums.com/showthread.php?t=163355

I assumed tungsten for the filament. My major concern was its purity for calculations but figure it is close enough for the purpose of the investigation.

I too thought of the glass and absorption but figure it would be a similar shape for each measurement.

Keep us informed of any developments

GeoffMc (Geoff) · #6 08-10-2011, 02:52 PM

Hi Jeff,

thanks for the calculations; never seen it before.

My set up is a little different from yours: I mounted a 12V down light on an optical bench and varied the voltage, taking readings from that. Yours is probably a cleaner reading, as I made no attempt at shielding the spectrometer from ambient light sources. The intensity of the light was such that I kept having to reduce it either by varying the distance to the spectroscope (the fibre optic feed, that is) or using a diaphragm to stop the signal being saturated. It didn't seem to make that much difference to the shapes of the curves as the light was so bright. We seem to be using the same spectroscope.

I looked at both your curves and mine and they seem to match well. Your absorption features and mine seem roughly the same, suggesting a common glass composition? (I'm new to this game.) In my curves, I adjusted all three sets so they were the same amplitude, allowing me to look at the shape of the curve more easily.

What are your thoughts?

Incidentally, where do you teach? I'm at Melrose High in Canberra.

Cheers,

Geoff Mc

theodog (Jeff) · #7 09-10-2011, 11:50 AM

Hi Geoff,

I'm at Coonabarabran HS.
Seems we are on a similar track.
Light bench seems a good idea, to be honest I never thought of that. My set up has prob's with saturation with the higher temperatures.
Maybe use a cardboard tube (old post-pak tube) that can be extended to block incident light. I will try this after the break.

My ultimate goal is to demonstrate the process behind B-V temperature measures for stars by mearuring the intensity through these filters and comparing this to the BB curves.

As a side the metal-work guys are interested for tempering metals.

tonybarry (Tony) · #8 12-10-2011, 09:39 PM

Hi Jeff,

I have been looking at the Vernier spectroVis Plus for a while; you are the first person I know who has access to one!

How long will it accumulate for? The literature from Vernier is quite ambiguous on this point. Some ways of reading it imply that it will accumulate for about 65 sec.

And how sensitive is it to light?

I would assume the fibre light guide has an entry orifice of about 400 micron and a cone angle of about ten degrees - is there any info on the fibre?

Regards,
Tony Barry

theodog (Jeff) · #9 13-10-2011, 05:08 PM

Hi Tony,

I had a quick read through the paper work that came with the device today. All I could find is that the exposure time may be manually set between 15 and 1000ms.

The setting I used I assume was automatic, but it seemed to update the graph around every second.

I don't think it is suitable for astro stuff except the sun. There is no way of cooling the detector (linear CCD I think) easily. The resolution stated at about 2.5nm? What ever it is it's fairly coarse.

We use it mostly to demonstrate coarse lines in "flame tests" (for ions), discharge tubes and fluro' tubes (mercury) and transmission through solutions.

I don't have any info on the optical fibre.

It works well for High School demo's of general spectroscopy.