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Author: Subject: More spectroscope questions
jgourlay
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[*] posted on 24-7-2008 at 07:51
More spectroscope questions


This is a continuation of another thread. I'm starting this thread as that one kinda got off its original target. But I appreciate the bunny trail!

Okay, I'm feeling a bit the retard. I've got an equilateral prism I wanted to make into a spectroscope for the boy. As I said in the earlier thread, it's easier for me to 'splain the prism than the DVD diffraction grating concept to an 8 year old.

Anyhoo, I get this thing last night, and it IS an equilateral glass prism (1"x6" long). Damn thing won't make a rainbow. Now, granted, I was using an LED flashlight (white), not the sun. But still? What in the world could I doing wrong?

My purpose in getting the prism also was so that it would cast a big enough rainbow from my burning salt source that my kid could really see it. With young kids, they have real trouble focusing their attention on "small" things. The image from a DVD spectroscope was so small that right away I could tell he would have trouble distinguishing what I was trying to tell him about the diffraction grating rainbow.

What I'm trying ultimately to get to is a cruciable with burning alchohol plus the salt, in dark room, where the flame light is shining through a slit and the dvd or prism will project onto a piece of paper that I can then point to with a tooth pick to show the features of different salts.

But this prism not making a rainbow from white light is really throwing me..... Does it only work with light from a really huge ball of fire 98 million miles away?
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ScienceSquirrel
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[*] posted on 24-7-2008 at 08:24


I think you will need a stronger source of light.

I would try using slide projector as shown here;

http://www.physics.montana.edu/demonstrations/apparatus/5_el...

Alternatively a powerful Maglite torch might work well.
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-jeffB
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[*] posted on 24-7-2008 at 12:35


You need collimated light -- light that's forming a parallel beam, rather than a fan. One slit in front of an extended source (like a flashlight) won't make a collimated beam.

Using lenses, multiple slits, and/or ingenuity, build a setup that will project a thin line of light onto a wall. Letting the sun shine through a slit works sort of okay, but since the sun's about half a degree across, the line will broaden as you move the slit further away from the wall.

Once you have that, put the prism in the way, and you'll get a rainbow. Note that it'll be much dimmer than the line of light, since you're spreading the light over a much larger area.

Getting a salted flame to project a spectrum onto a sheet of paper will be challenging. Put your sheet of paper in a draped box, then duck your head under the drape; otherwise, the general illumination from the flame will swamp the spectrum.
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[*] posted on 24-7-2008 at 14:04


A Maglite torch has a reflective mirror built in that allows focusing to a point.
You would need to focus the torch to a point and pass the beam through a slit onto the face of the prism in a darkened room.
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jgourlay
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[*] posted on 26-7-2008 at 12:31


Quote:
Originally posted by -jeffB
You need collimated light -- light that's forming a parallel beam, rather than a fan. One slit in front of an extended source (like a flashlight) won't make a collimated beam.


How do you collimate?
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-jeffB
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[*] posted on 26-7-2008 at 12:50


Quote:
Originally posted by jgourlay
How do you collimate?


It's hard to explain without diagrams, and I can't draw worth anything.

You have to make the light parallel, and if you have an extended (wide) source, that's hard to do. Laser light is typically collimated when it comes out, but it's pretty boring to send through a prism. Sunlight is collimated to with 1/2 degree (the apparent width of the Sun in the sky), which is not too bad, and it's also intense, which is helpful.

To make an artificial light source that's collimated, you first send the light through a slit or a pinhole; the smaller the slit or pinhole, the better the collimation. Then, you focus it with a convex lens or a concave mirror. The problem is that by sending the light through a pinhole, you're blocking (wasting) most of the light, so you need to start with a really intense source, or end with a really sensitive detector, or both.
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