Start with an empty sandbox.
Fill it, not with sand, but with tiny glass spheres, the type which are sprinkled on to road marking paint to make it retroreflective.
These marking spheres cost as little as 20 dollars for a 50 pound bag. Given that ordinary sand costs 3 to 6 dollars for that amount, it's not insanely expensive.
More importantly, since these glass spheres reflect and refract light in the same manner as water spheres, they produce a rainbow when viewed in the sunlight from the proper angle.-- goldbb, Oct 28 2015 Glass Bead Rainbow http://www.atoptics.co.uk/fz822.htm [goldbb, Oct 28 2015] sand in a bottle https://www.etsy.co...?q=sand+in+a+bottledifferent colors of sand in bottles. [travbm, Oct 29 2015] http://photo.accuwe...7/500/0aaaf0bee.jpg [goldbb, Oct 04 2016] http://photos1.blog.../glass_bead_bow.jpg So I lied: The rainbow here surrounds the shadow of the camera taking the picture, not the shadow of the head of the person holding the camera. [goldbb, Oct 04 2016] Sounds beautiful.-- blissmiss, Oct 28 2015 Beach glass is dull due to years of being moved around by ocean waves. Even the most rambunctious child would take decades or centuries to make enough of these glass microbeads dull enough for the rainbow to not be seen.-- goldbb, Oct 28 2015 Would there be any advantage to varying the shapes or alternating surface colors to produce a variety of rainbow shapes?-- 4and20, Oct 29 2015 Make this into a fluidized bed, then shine lasers onto it ...
Call it Art. Then some idiot will pay a fortune for it.-- 8th of 7, Oct 29 2015 Why not buy clear sand and food coloring. Make the sand the colors of the rainbow and have sections of the sandbox with a different color? By the way they do this with sand in a bottle already.-- travbm, Oct 29 2015 travbm, A sandbox filled with glass spheres will produce a rainbow which will always appear to encircle the viewer's shadow's head.
Can your colored sand in a bottle do the same?-- goldbb, Oct 04 2016 Kids tend to put everything in their mouths. I suppose glass spheres would be no more harmful than their parent material. [+]-- whatrock, Oct 04 2016 I like this!
if you make them out of quartz they will transmit UV spectra, causing fluorescent things to be fluorescent, although not more fluorescent-- beanangel, Oct 05 2016 Not sure how this one got by me. (+)-- 2 fries shy of a happy meal, Feb 06 2022 It was the dazzle.-- pertinax, Feb 06 2022 Should be able to "Gorilla Glass" the outside surface of the spheres, to extend the useful life (possibly already done in some fashion for the road use ones...).-- neutrinos_shadow, Feb 06 2022 SCREW YOU DAZZLE!!!-- 2 fries shy of a happy meal, Feb 06 2022 random, halfbakery