Ultraviolet, usually shortened to UV, is light your eyes cannot see. Picture a rainbow: red at one end, violet at the other. Ultraviolet is the light just past the violet end — still light, still travelling from the sun, but your eyes have no way to catch it.

Ultraviolet, usually shortened to UVUltraviolet，通常簡稱UV, is light your eyes cannot see是你眼睛看不到的光。Picture a rainbow想像一道彩虹: red at one end, violet at the other紅色在一端，紫色在另一端。Ultraviolet is the light just past the violet endultraviolet是超過紫色那一端的光 — still light, still travelling from the sun它仍然是光，仍然是從太陽來的, but your eyes have no way to catch it但你的眼睛就是抓不到它。

It is an adjective (ultraviolet light, ultraviolet rays, ultraviolet radiation) and also a noun for the light itself ("protects you from ultraviolet"). In real speech people almost always say "UV" — UV index, UV protection, UV rays — and "ultraviolet" written out sounds more like a textbook or a label.

It is an adjective它可以是形容詞 (ultraviolet light, ultraviolet rays, ultraviolet radiation)（ultraviolet light紫外光、ultraviolet rays紫外線、ultraviolet radiation紫外輻射） and also a noun for the light itself也可以是名詞，指這種光本身 ("protects you from ultraviolet")（像「protects you from ultraviolet」）。In real speech people almost always say "UV"實際說話時大家幾乎都直接說「UV」 — UV index, UV protection, UV raysUV index紫外線指數、UV protection防曬、UV rays紫外線 — and "ultraviolet" written out把「ultraviolet」整個字寫出來 sounds more like a textbook or a label反而比較像課本或標籤上的用字。

UV is the part of sunlight that changes your skin: it tans you, burns you if you get too much, and over years it wrinkles and damages skin and can cause cancer. That is what sunscreen blocks, and why the weather report here gives a UV level even on a cool day — the temperature and the UV strength are two different things, which is why you can burn badly on a windy spring afternoon.

UV is the part of sunlight that changes your skinUV是陽光裡會改變你皮膚的那部分: it tans you它會讓你曬黑, burns you if you get too much曬過頭會曬傷, and over years it wrinkles and damages skin and can cause cancer而且經年累月會讓皮膚長皺紋、受損，甚至可能致癌。That is what sunscreen blocks這就是防曬乳擋的東西, and why the weather report here gives a UV level even on a cool day也是為什麼這裡的氣象報告連涼涼的天氣也會給紫外線指數 — the temperature and the UV strength are two different things氣溫和UV強度是兩回事, which is why you can burn badly on a windy spring afternoon這也是為什麼春天有風的下午你也可能曬得很慘。

UV is also used on purpose: it kills germs, so it is used in hospitals and water purifiers; it makes some things glow, which is how banknotes are checked and how a "black light" works; and it hardens gel nail polish.

UV is also used on purposeUV也會被故意拿來使用: it kills germs它能殺菌, so it is used in hospitals and water purifiers所以用在醫院和淨水器裡; it makes some things glow它能讓某些東西發光, which is how banknotes are checked and how a "black light" works這就是驗鈔和「黑光燈」的原理; and it hardens gel nail polish它也能讓光療凝膠指甲油變硬。

One warning worth remembering: UV goes through cloud and through water, so a grey day and a swimming pool both give less protection than they feel like they do.

One warning worth remembering有一個值得記住的提醒: UV goes through cloud and through waterUV能穿透雲層和水, so a grey day and a swimming pool both give less protection than they feel like they do所以陰天和游泳池給你的防護，其實都比感覺上的少。