{"product_id":"halotra-22-degree-halo","title":"Halotra – The 22° Halo \/ Ice-Crystal Optics Physics Sticker","description":"\u003cp\u003eHalotra, an ice spirit, wears a ring of light above her head, a playful nod to the real 22° halo that rings the Sun or Moon on a cold, hazy day.\u003c\/p\u003e\u003ch3\u003eThe Physics\u003c\/h3\u003e\u003cp\u003eThe real halo is made by millions of tiny hexagonal ice crystals in thin, high cirrus cloud, not by water droplets. Light entering one side face of a crystal and leaving by another passes through what is in effect a 60° prism. Every such prism has a minimum angle of deviation, and light piles up near it, so a bright ring appears at that angle from the Sun.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003eD\u003csub\u003emin\u003c\/sub\u003e = 2 arcsin(n sin 30°) − 60°\u003c\/strong\u003e\u003c\/p\u003e\u003cul\u003e\n\u003cli\u003eD\u003csub\u003emin\u003c\/sub\u003e: minimum deviation, the halo radius (degrees)\u003c\/li\u003e\n\u003cli\u003en: refractive index of ice, about 1.31 for visible light\u003c\/li\u003e\n\u003cli\u003e30°: half the 60° prism angle between alternate faces\u003c\/li\u003e\n\u003c\/ul\u003e\u003cp\u003eWith n = 1.31, D\u003csub\u003emin\u003c\/sub\u003e = 2 arcsin(0.655) − 60° ≈ 21.8°, which is the 22° halo. No light is deviated by less than this, so the sky inside the ring is darker. Ice bends red light slightly less than blue, so the inner edge of a real halo is reddish.\u003c\/p\u003e\u003cp\u003e\u003cem\u003eHonest caveat:\u003c\/em\u003e The halo is a ring around the Sun or Moon, about 22° in radius (roughly a hand-span at arm's length), not a ring above someone's head. It needs randomly oriented crystals; crystals that fall in a preferred orientation make sundogs and arcs instead.\u003c\/p\u003e\u003ch3\u003eOn the sticker\u003c\/h3\u003e\u003cp\u003eHalotra, an icy-blue spirit, has a stylised halo ring floating above her head. Printed text: \u003cstrong\u003eHALOTRA\u003c\/strong\u003e with the line \u003cstrong\u003e22°\u003c\/strong\u003e. Pixel-art kiss-cut vinyl sticker, white, in 2″, 3″, 4″ and 6″ sizes.\u003c\/p\u003e","brand":"Printify","offers":[{"title":"2\" × 2\" \/ White","offer_id":50963996967015,"sku":"87911801665517817982","price":4.99,"currency_code":"USD","in_stock":true},{"title":"3\" × 3\" \/ White","offer_id":50963996999783,"sku":"21187266072262687609","price":6.99,"currency_code":"USD","in_stock":true},{"title":"4\" × 4\" \/ White","offer_id":50963997032551,"sku":"76748644819209303490","price":8.99,"currency_code":"USD","in_stock":true},{"title":"6\" × 6\" \/ White","offer_id":50963997065319,"sku":"19780683265749904033","price":11.99,"currency_code":"USD","in_stock":true}],"thumbnail_url":"\/\/cdn.shopify.com\/s\/files\/1\/0596\/8746\/9159\/files\/5419900769453856190_1200.jpg?v=1793657864","url":"https:\/\/pixelatedphysics.com\/id\/products\/halotra-22-degree-halo","provider":"Pixelated Physics","version":"1.0","type":"link"}