{"product_id":"perota-fabry-perot","title":"Perota – The Fabry–Pérot Cavity \/ FSR = c\/(2nL) Physics Sticker","description":"\u003cp\u003ePerota, a hooded light-keeper, holds two mirrors face to face with red light bouncing between them.\u003c\/p\u003e\u003ch3\u003eThe Physics\u003c\/h3\u003e\u003cp\u003eTwo parallel, partly reflecting mirrors trap light that bounces back and forth. Only wavelengths that fit a whole number of half-waves into the gap build up and pass through; the rest cancel out. The transmission is a comb of sharp peaks, spaced by the free spectral range.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003eFSR = c\/(2nL)\u003c\/strong\u003e\u003c\/p\u003e\u003cul\u003e\n\u003cli\u003eFSR: free spectral range, the frequency spacing between peaks (Hz)\u003c\/li\u003e\n\u003cli\u003ec: speed of light (m\/s)\u003c\/li\u003e\n\u003cli\u003en: refractive index between the mirrors\u003c\/li\u003e\n\u003cli\u003eL: mirror spacing (m)\u003c\/li\u003e\n\u003c\/ul\u003e\u003cp\u003eThe sharpness of the peaks is set by the finesse, F = π√R\/(1 − R) for mirror reflectivity R. With R = 0.99, F ≈ 312.6: the peaks are about 300 times narrower than their spacing. The same physics sets the modes of a laser cavity.\u003c\/p\u003e\u003cp\u003e\u003cem\u003eHonest caveat:\u003c\/em\u003e These formulas assume ideal, lossless, perfectly parallel mirrors at normal incidence. Real cavities lose finesse to absorption, scattering and imperfect alignment.\u003c\/p\u003e\u003ch3\u003eOn the sticker\u003c\/h3\u003e\u003cp\u003ePerota holds two silver mirrors with a red beam glowing between them; the transmission peaks are not drawn. Printed text: \u003cstrong\u003ePEROTA\u003c\/strong\u003e with the line \u003cstrong\u003eFSR = c\/(2nL)\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":50964019904615,"sku":"51662712533390884041","price":4.99,"currency_code":"USD","in_stock":true},{"title":"3\" × 3\" \/ White","offer_id":50964019937383,"sku":"28043273502205734692","price":6.99,"currency_code":"USD","in_stock":true},{"title":"4\" × 4\" \/ White","offer_id":50964019970151,"sku":"20021480483126454350","price":8.99,"currency_code":"USD","in_stock":true},{"title":"6\" × 6\" \/ White","offer_id":50964020002919,"sku":"13743764155913683406","price":11.99,"currency_code":"USD","in_stock":true}],"thumbnail_url":"\/\/cdn.shopify.com\/s\/files\/1\/0596\/8746\/9159\/files\/2955416461904579964_1200.jpg?v=1793658693","url":"https:\/\/pixelatedphysics.com\/products\/perota-fabry-perot","provider":"Pixelated Physics","version":"1.0","type":"link"}