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WonderScience fact

In a vacuum, light travels 299,792 kilometres (186,282 miles) every second. At that speed it reaches us from the Moon in about 1.3 seconds and from the Sun in about 8 minutes 20 seconds.

Physics

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The story behind the card

The exact figure is 299,792,458 metres per second, and it is exact by definition: since 1983 the metre has been defined as the distance light travels in a vacuum in 1/299,792,458 of a second. The first estimate came in 1676, when the Danish astronomer Ole Rømer noticed that the eclipses of Jupiter's moon Io came later when Earth was farther from Jupiter, because the light had further to travel. Light slows down inside materials such as water or glass. Einstein's theory of relativity showed that nothing with mass can reach this speed, which is why it is called the universe's speed limit. Scientists still time laser pulses bounced off mirrors that Apollo astronauts left on the Moon; the round trip takes about 2.5 seconds.

Türkçe oku
Işık boşlukta saniyede 299.792 kilometre yol alır. Bu hızla Ay'dan bize yaklaşık 1,3 saniyede, Güneş'ten ise yaklaşık 8 dakika 20 saniyede ulaşır.

Tam değer saniyede 299.792.458 metredir ve bu sayı tanım gereği kesindir: 1983'ten beri metre, ışığın boşlukta saniyenin 1/299.792.458'inde aldığı yol olarak tanımlanıyor. Işık hızına ilişkin ilk tahmini 1676'da Danimarkalı gökbilimci Ole Rømer yaptı. Rømer, Jüpiter'in uydusu İo'nun tutulmalarının, Dünya Jüpiter'den uzaktayken daha geç görüldüğünü fark etti; çünkü ışığın alması gereken yol uzuyordu. Işık su ya da cam gibi maddelerin içinde yavaşlar. Einstein'ın görelilik kuramı, kütlesi olan hiçbir şeyin bu hıza ulaşamayacağını gösterdi; ışık hızına evrenin hız sınırı denmesi bu yüzdendir. Bilim insanları bugün hâlâ Apollo astronotlarının Ay'a bıraktığı aynalara lazer gönderiyor; ışığın gidip gelmesi yaklaşık 2,5 saniye sürüyor.

Sources and further reading

Background reading: Speed of light
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