Naked Science Forum

General Science => General Science => Topic started by: cedfyfergo on 08/11/2017 15:23:24

Title: Can you make a huge prism, and disperse light in different colors?
Post by: cedfyfergo on 08/11/2017 15:23:24
Can you make a huge prism, project a light beam on it and have it clearly disperse in different colors on a wall or across the floor?

By huge I mean around 2x2 meters, depending on what's possible and how much it would cost.

I'm not the world's greatest scientist. So any help would be much appreciated.

Thanks!
Title: Re: Can you make a huge prism, and disperse light in different colors?
Post by: chiralSPO on 08/11/2017 17:03:34
It is possible, but unnecessary, and probably wouldn't work very well.

If you want to project a large rainbow on a wall (or floor), you just need a long but otherwise normal-sized prism (or multiple prisms lined up end to end), and place it farther away from the surface you want the rainbow on. The length of the prism will determine the hight of the rainbow, and the distance between the prism and the surface will determine the width (spread) of the rainbow. Of course you will need a brighter light to get the same apparent brightness across a larger area.

It might be cheaper and easier to use a large diffraction grating. (something along these lines: https://www.carolina.com/catalog/detail.jsp?prodId=755227&s_cid=ppc_gl_products&utm_source=google&utm_medium=cpc&scid=scplp755227&sc_intid=755227&gclid=EAIaIQobChMIttrRkbqv1wIVwrXACh31pA6pEAQYASABEgI6ffD_BwE)

This is basically a thin plastic sheet with very closely spaced grooves that separate out the spectrum of light by a somewhat different, but equally effective process. You can either shine the light directly through it, or mount the grating on a mirror and bounce light off of it (just the way shining light off of a CD or DVD makes rainbows)

Good luck!
Title: Re: Can you make a huge prism, and disperse light in different colors?
Post by: chiralSPO on 08/11/2017 17:07:09
Also: if you want a continuous spectrum (whole rainbow with no gaps) use a high temperature incandescent bulb or sunlight (you might be able to see some very fine gaps in the sunlight spectrum if you expand it a lot). Fluorescent and LED lights will only give a few bright lines.
Title: Re: Can you make a huge prism, and disperse light in different colors?
Post by: chiralSPO on 04/02/2018 18:25:51
This tech is even better at dispersing light that prisms or diffraction gratings:

https://gizmodo.com/this-giant-rainbow-was-made-with-tech-thats-used-to-stu-1671593483
Title: Re: Can you make a huge prism, and disperse light in different colors?
Post by: Bored chemist on 05/02/2018 19:45:50
This tech is even better at dispersing light that prisms or diffraction gratings:

https://gizmodo.com/this-giant-rainbow-was-made-with-tech-thats-used-to-stu-1671593483
So, this diffraction grating is better than a diffraction grating.
Would you like to expand on that?
Title: Re: Can you make a huge prism, and disperse light in different colors?
Post by: JMerrill on 28/05/2019 19:57:37
It is possible, but unnecessary, and probably wouldn't work very well.

If you want to project a large rainbow on a wall (or floor), you just need a long but otherwise normal-sized prism (or multiple prisms lined up end to end), and place it farther away from the surface you want the rainbow on. The length of the prism will determine the hight of the rainbow, and the distance between the prism and the surface will determine the width (spread) of the rainbow. Of course you will need a brighter light to get the same apparent brightness across a larger area.

It might be cheaper and easier to use a large diffraction grating. (something along these lines: (LINK)

This is basically a thin plastic sheet with very closely spaced grooves that separate out the spectrum of light by a somewhat different, but equally effective process. You can either shine the light directly through it, or mount the grating on a mirror and bounce light off of it (just the way shining light off of a CD or DVD makes rainbows)

Good luck!

Love this discussion! Found it because I'm an art teacher trying to help a student explore light & reflection in art. She has built a sculpture of re-purposed CDs, and the reflective qualities are beautiful. The downside is the durability of the piece.

I wanted to ask, if one were to purchase a sheet of the diffraction grating and attach it to a mirror for natural light reflection, what sort of adhesive would you recommend in order to create a secure bond while minimizing disruption of the light and reflection? Would you suggest sandwiching the diffraction grating with some sort of clear plastic or laminate for protection from the elements?

Hoping this could turn into a permanent piece of public art if done properly.

Thanks in advance for your patience with this non-scientist!
Title: Re: Can you make a huge prism, and disperse light in different colors?
Post by: Bored chemist on 28/05/2019 20:06:38
I suspect that most adhesives would wreck the diffraction grating.
However, sandwiched between acrylic and a mirror might work nicely.
Title: Re: Can you make a huge prism, and disperse light in different colors?
Post by: JMerrill on 28/05/2019 20:12:06
I suspect that most adhesives would wreck the diffraction grating.
However, sandwiched between acrylic and a mirror might work nicely.

So then the key would be finding a way to encase the grating between the mirror and the acrylic without disrupting the area over the grating itself? I may have to spend some time thinking this through, but it could be a fun project. Maybe I'll do this with my kids over the summer to work out the kinks. I only have about a week of turnaround time before the student would need to be able to complete and install her work, and I have a feeling that might be cutting it close.
Title: Re: Can you make a huge prism, and disperse light in different colors?
Post by: Colin2B on 29/05/2019 07:46:58
So, if the grating is attached to the mirror light passing through the grating to the mirror will be diffracted. What happens when the diffracted light passes back through the grating? What effect do you get? Has anyone tried it?

PS I thought the CD rainbow effect was similar to oil on water, a thin film interference pattern rather than a grating effect.
Title: Re: Can you make a huge prism, and disperse light in different colors?
Post by: evan_au on 29/05/2019 10:57:33
Quote from: Colin2B
I thought the CD rainbow effect was similar to oil on water, a thin film interference pattern rather than a grating effect.
Googling certainly produces a mixture of answers - even when they identify interference from the ridges as the source of interference, they still use "oil on water" or "soap bubbles" as an analogy...

I got out a CD, and tried angling it different ways. I haven't quite got my head around it, but:
- There are multiple spectra at different angles, with gaps between them
- The colors generally seem to be at right-angles to the track
- I expect that thin-film effects like a soap bubble would not maintain this angle to the tracks on the disk. I would not expect to see the same gaps between the colors
- So I am guessing that it is a diffraction grating effect

The CD track spacing is 1.6um, which is comparable to visible light (near-infra-red wavelengths are used for reading). DVDs use 0.4um spacing, and 0.14um for Blu-Ray disks.

I don't have any Blu-Ray disks handy, but CD and DVDs seem to have different angles of refraction, which would be expected if they acted as diffraction gratings.

https://en.wikipedia.org/wiki/Compact_disc#Physical_details
https://en.wikipedia.org/wiki/Blu-ray#Laser_and_optics
Title: Re: Can you make a huge prism, and disperse light in different colors?
Post by: Bored chemist on 29/05/2019 19:03:58
What happens when the diffracted light passes back through the grating?
One thing that can't happen is that it "reassembles" the colours back into white light.
From  the point of view of the beautiful colours, I don't think it matters exactly what happens.
Title: Re: Can you make a huge prism, and disperse light in different colors?
Post by: chiralSPO on 29/05/2019 20:15:39
The dispersion obtained from a CD (or similar) is due to diffraction. This can be verified by shining light with different spectral makeup on it and looking at the reflected patterns. If one has access to red and/or green laser pointers, one can easily see that a single laser point generates a whole diffraction pattern (multiple bright spots), and that the spacing of the diffraction pattern is different for red (typically 600-700 nm, depending on source) and green (typically 532 nm doubled Nd:YAG). One can also look at the "rainbows" produced from light from an incandescent source (continuous), and from fluorescent or LED lighting (a few narrow bands).
Title: Re: Can you make a huge prism, and disperse light in different colors?
Post by: Bored chemist on 29/05/2019 21:12:27
Incidentally, it's not very practical, but you can make a big prism.
The easy way is to make a prism shaped tank (the same way they make fish tanks) and then fill it with water.

BTW, any chance of pictures of the artwork?
Title: Re: Can you make a huge prism, and disperse light in different colors?
Post by: JMerrill on 29/05/2019 21:27:05
Incidentally, it's not very practical, but you can make a big prism.
The easy way is to make a prism shaped tank (the same way they make fish tanks) and then fill it with water.

BTW, any chance of pictures of the artwork?

I'd like to, but it's definitely still a work in progress. Took two people to hold and angle to CDs properly the other day just to get the reflections and spectrum to display on the walls. Also a adds to the difficulty because the CDs are brittle (student super-glued them all together).

I think my next step for now will be suggesting she visit Home Depot to get a large sheet of acrylic to mount the CDs on for stability purposes. I'd feel better taking a picture at that point, so you'd be able to see the reflected light on the wall.
Title: Re: Can you make a huge prism, and disperse light in different colors?
Post by: JMerrill on 29/05/2019 21:36:33
Here's a picture of the arrangement on the table at least. She was originally going for a cresting ocean wave shape, but would have needed many more CD's to effectively create that form (in my opinion). The reflected light/spectrum work has grown out of trying to make lemonade from this batch of lemons.

(https://previews.dropbox.com/p/thumb/AAeNqsaa8y-uEGCfzzoG7lT2iPrb95V26DqVlx3U2nKv7ASPoKYQyIkh3oEllaVbaAQ5lC_f96AOBjXEP-P_VkFSfIHJ2wvRqAtCE8G-LXBg6l7U3cL-EBKXlXWhyBDbq-ks9fqxmVCxlTtt_DOzjjgsr8UHj3aqcFVA0LKJaV_ezOdk_uE1u0DV4iG512StTeVYUFtAmMhntV3uBtCVTYJYb8pZ1zBNGnqy4hsDivyHFj7P1UDxMY-gwF44lJUrHHg9Zns6S5CSm1jdlyhiJO8K7SrldoWd-2FxQc2CEUfRbTOe7-9aRTmy_Es7ohK3ScaV2OWWL4NwTKN-3QvDug9pvMSsY-Eqjun48bF7sKh5UQ/p.jpeg?fv_content=true&size_mode=5)
Title: Re: Can you make a huge prism, and disperse light in different colors?
Post by: Colin2B on 29/05/2019 22:35:22
If one has access to red and/or green laser pointers, one can easily see that a single laser point generates a whole diffraction pattern (multiple bright spots), and that the spacing of the diffraction pattern is different for red (typically 600-700 nm, depending on source) and green (typically 532 nm doubled Nd:YAG).
Thatís really interesting, I had assumed thin plastic layer over reflective surface = interference, but the laser test sounds conclusive.
Iím tempted to get some grating and try the double diffraction back through mirror. As @Bored chemist says it wonít reassemble, but it might be an interesting effect especially if you cut shapes out of the grating so some single others double diffraction. Flexible mirror might be fun!
Title: Re: Can you make a huge prism, and disperse light in different colors?
Post by: chiralSPO on 04/06/2019 02:45:57
I just found this reference online: https://www.nnin.org/sites/default/files/files/Karen_Rama_USING_CDs_AND_DVDs_AS_DIFFRACTION_GRATINGS_0.pdf

The CD peeling methods might be of interest to those, likeJMerrill, who wish to incorporate rainbows in artwork, and the equations shown might be of interest to those, like Colin2B, who wish to examine the mechanism of forming those rainbows--it turns out that one can even calculate the spacing of the ridges with some accuracy using a laser and a ruler (I am certain the error bars would drop if one used multiple lasers, each with a different wavelength...