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Optical prism question

by Ascendant78
Tags: optical, prism
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Ascendant78
#1
Feb26-13, 09:30 PM
P: 281
I am trying to gain a better understanding of physics outside of my classes. As such, I decided to buy myself an optical prism to learn more about the spectrum of specific light sources, light filtering, etc. I just received my prism in the mail today, but am severely disappointed. When I was a child, I remember my parents having a prism, and that it was relatively easy to get a rainbow from sunlight. However, the prism that I received today doesn't seem to refract the light properly. It is extremely difficult to get even the slightest hint of a spectrum, and even then, it is very faint and at the edge of a large section of pure white light.

I wasn't sure how reliable the reviews on the site were, but the company overall seemed to be ok from the research I did into them. This is the prism I bought: http://dx.com/p/optical-triple-trian...urce=affiliate. I would appreciate your thoughts.

One thing to note is that if I put my eye close to it and look at certain light sources, I can see a pretty good separation of the spectrum. For some reason though, it just doesn't seem to want to do the same when reflecting it onto a white surface.

I am not sure if there are certain things in particular to look for in a prism, so any feedback on the matter would be appreciated.
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Drakkith
#2
Feb26-13, 10:35 PM
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Take a piece of cardboard or thick paper, cut a thin slit in it, and attach it to one side of the prism. Then hold the prism with the paper side towards a light source and see what happens. I'd guess that you aren't using a slit or a thin 'beam' of light. If you aren't this would cause the split colors to simply blend in with all the other split colors next to them.
Ascendant78
#3
Feb26-13, 11:01 PM
P: 281
Quote Quote by Drakkith View Post
Take a piece of cardboard or thick paper, cut a thin slit in it, and attach it to one side of the prism. Then hold the prism with the paper side towards a light source and see what happens. I'd guess that you aren't using a slit or a thin 'beam' of light. If you aren't this would cause the split colors to simply blend in with all the other split colors next to them.
Ah, thanks for the information. I will give that a try tomorrow after class and work.

Bobbywhy
#4
Feb27-13, 02:54 AM
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Optical prism question

Quote Quote by Drakkith View Post
Take a piece of cardboard or thick paper, cut a thin slit in it, and attach it to one side of the prism. Then hold the prism with the paper side towards a light source and see what happens. I'd guess that you aren't using a slit or a thin 'beam' of light. If you aren't this would cause the split colors to simply blend in with all the other split colors next to them.
I respectfully disagree that the above procedure is necessary to obtain a spectrum of colors in sunlight. In our Museum we have several kinds of prisms, including one that appears identical to the OP's. Visitors take them outside, hold them in the direct sunlight, and cast brilliant spectrums on the white wall there. We do not use any slit and get these results.

Cheers,
Bobbywhy
sophiecentaur
#5
Feb27-13, 11:02 AM
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A 'good' prism will need to be made of optically dense glass with high dispersion - the sort of crystal glass that posh cut-glass tumblers are made of. A plastic prism will work far worse - and so will one made of cheap, window glass. The essential thing is the dispersion in the optical region.

BTYW, I notice that the advert in that link says that the prism will "reflect" light and produce a spectrum. Whilst prisms can be used as good reflectors, they are not used in 'reflecting mode' to disperse a beam of light. What you bought could just be junk, for your purpose as the company selling it may just be 'box shifters'.
Drakkith
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Feb27-13, 03:23 PM
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Quote Quote by Bobbywhy View Post
I respectfully disagree that the above procedure is necessary to obtain a spectrum of colors in sunlight. In our Museum we have several kinds of prisms, including one that appears identical to the OP's. Visitors take them outside, hold them in the direct sunlight, and cast brilliant spectrums on the white wall there. We do not use any slit and get these results.

Cheers,
Bobbywhy
*Shrug*
Next time you're there try the slit method and see if it has an effect.
Ascendant78
#7
Feb27-13, 05:08 PM
P: 281
Thanks for all the feedback. Considering how easy it was to get a full spectrum from the one I used during my youth, I am definitely leaning towards this being very shoddy. At least I can use it by holding my eye close to it while looking directly at light sources, so I can still get a little bit of use out of it.

Since there was an overcast today, I am going to try tomorrow, if it clears up by then of course.
sophiecentaur
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Feb27-13, 05:26 PM
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I think you'll need to buy a posh one from a spectrometer supplier - that should blow yer socks off.
Bobbywhy
#9
Feb27-13, 07:36 PM
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Quote Quote by Drakkith View Post
*Shrug*
Next time you're there try the slit method and see if it has an effect.
Excuse me, please. The choice of the phrase "I disagree..." was unfortunate and unwarranted. I simply intended to say that the cardboard slit you mentioned was not necessary to obtain brilliant full spectrums with our prisms in the sunlight. We will experiment with your suggestion this weekend if there is any sunlight. Our rainy season hasn't ended yet. Thank you for your suggestion.

edit: Sophiecentaur is quite right: the quality of the glass makes a big difference. Our best spectrums are produced by high optical quality prisms. The OP probably has a common, cheap glass prism.

Cheers, Bobbywhy
Drakkith
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Feb27-13, 07:59 PM
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Quote Quote by Bobbywhy View Post
Excuse me, please. The choice of the phrase "I disagree..." was unfortunate and unwarranted. I simply intended to say that the cardboard slit you mentioned was not necessary to obtain brilliant full spectrums with our prisms in the sunlight. We will experiment with your suggestion this weekend if there is any sunlight. Our rainy season hasn't ended yet. Thank you for your suggestion.
Please, let me know how it turns out. I do not have any prisms here to test myself and I am quite curious to know if it has any real effect. You could also hold it up to a bright light source and observe the effect as well. Try the old incandescent bulbs and then try a newer fluorescent ones and see how well you can make out the spectral lines in the latter. I do happen to have a little plastic handheld spectroscope I bought used off of ebay that has marks on it that allow you to match the lines up with the frequency/wavelength. It's pretty neat! I can even make out some absorption lines from the Sun just by looking at bright concrete in the middle of the day.

edit: Sophiecentaur is quite right: the quality of the glass makes a big difference. Our best spectrums are produced by high optical quality prisms. The OP probably has a common, cheap glass prism.

Cheers, Bobbywhy
Agreed. A poor prism may not disperse the light well enough and may be scattering it everywhere if the quality is poor.
Bobbywhy
#11
Feb27-13, 08:52 PM
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Just as an added observation: One of our Museum visitors took a natural (unpolished) very clear quartz crystal outside into the sunlight and manipulated it just like he had with the prisms. To our surprise, the spectrum cast on the white wall was mostly pink!

Cheers, Bobbywhy
sophiecentaur
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Feb28-13, 05:45 PM
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Quote Quote by Bobbywhy View Post
Just as an added observation: One of our Museum visitors took a natural (unpolished) very clear quartz crystal outside into the sunlight and manipulated it just like he had with the prisms. To our surprise, the spectrum cast on the white wall was mostly pink!

Cheers, Bobbywhy
The spectrum is spread out due to the change in refractive index with wavelength (dispersion).
I found a link (lost it again cos I was on my mobile) with graphs of refractive index of many different transparent substances and, whilst many glasses seem to have an almost straight line variation across the optical band, others have a distinct curve. For the right substance (quartz seems to be one), it looks as if the angular spread in the reds would be greater. Also, quartz is birefringent (different refractive index for different polarisations). This could well produce a spectrum from a slit source that will be a mixture of two colours in any direction, depending on the cut of the prism. I haven't any details but I think this may the direction to go if you want to explain the pinkish effect with the quartz prism due to this mixing of wavelengths.


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