# Flashlights for prism demo?



## entoptics (Sep 19, 2010)

Guy at work needs to do a prism demonstration for optical mineralogy, and asked me for a "bright ***" flashlight for the job.

Please discuss. I have no idea, and I'm the go to flashahaulic around the lab, so help me out.

*Helpful details*

1" x 4" prism.
He wants to shine the rainbow on a wall in a mildly lit classroom (projector and dim ambient to compete with) so students in the back can see it.
We can make a slotted shield if necessary
At first I thought a nice bright LED would work, but we crudely tested my D10, and it wasn't even close.

He only needs it once a year, but he's willing to spend $50 out of pocket if it's a good flashlight for other personal use. I have a MagCharger that I'll lend him if it's suitable, but we haven't had a chance to test it yet.

Any tips on using a flashlight for a prism demo?


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## wyager (Sep 19, 2010)

I'm fairly sure that flashlights are not good for refractometry demonstrations... the light is all jumbled up, unfocused, and has a large starting diameter. This is not even close to ideal for refractometry. I suppose the best you could do is something with an optic.... an E2B (that has a TIR lens, right?) or something with an aspheric. However, even better would be something laser-based. Lasers will clearly refract at different angles through a prism. They will also maintain a high lux rating and tight focus to keep the "rainbow" visible.


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## ^Gurthang (Sep 19, 2010)

Probably an incan light would get him some of what he wants. LEDs have more blue output, their spectrum is not close to sunlight. An incan would be more balanced.


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## carrot (Sep 19, 2010)

Due to the optics built into most flashlights they are not ideal for physics light experiments, or so I learned firsthand in the lab.


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## sappyg (Sep 19, 2010)

my wife does this once a year in her classroom. the 1st year she took both LED and incans. the report from her was that the LED (admittedly cool) was a very poor performer. the incan was the star of the show. in this case it was a SF p90 lamp assembly.


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## HKJ (Sep 19, 2010)

wyager said:


> However, even better would be something laser-based. Lasers will clearly refract at different angles through a prism. They will also maintain a high lux rating and tight focus to keep the "rainbow" visible.



You will have some problems getting a rainbow from lasers, they are monochromatic light, i.e. there are only one color to show.




sappyg said:


> my wife does this once a year in her classroom. the 1st year she took both LED and incans. the report from her was that the LED (admittedly cool) was a very poor performer. the incan was the star of the show. in this case it was a SF p90 lamp assembly.



Could it be that the limited spectrum of a led is part of the reason for this.


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## Yucca Patrol (Sep 19, 2010)

Why use a flashlight when you are in a classroom with access to 110V AC power? I'd use a spotlight bulb installed in one of those bendy gooseneck clip-on lamps. Will cost only a few dollars and will produce MUCH MUCH more light than a cheap flashlight.


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## wyager (Sep 19, 2010)

HKJ said:


> You will have some problems getting a rainbow from lasers, they are monochromatic light, i.e. there are only one color to show.



That's the point... they are also coherent. The light from a flashlight, non-coherent diode or other source will always be jumbled up, bouncing all over the place in a very unclean beam. This will result in a low quality "rainbow" with lots of overlapping. Lasers very clearly demonstrate the "rainbow", as different color lasers will put dots in different spaces along the wall if multiple lasers are shined in to a prism.


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