Speed of Light in Different Units
Convert the speed of light between mph, km/h, m/s, and Mach — with fascinating comparisons.
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Speed of Light in Different Units The speed of light — 299,792,458 metres per second — is a universal speed limit, a fundamental constant of nature, and a measuring stick for astronomical distances. Here is what it looks like in every unit humans use to describe speed. --- The Speed of Light in Every Common Unit | Unit | Value | |------|-------| | metres per second (m/s) | 299,792,458 m/s (exact) | | kilometres per second (km/s) | 299,792.458 km/s | | kilometres per hour (km/h) | 1,079,252,848.8 km/h | | miles per second (mi/s) | 186,282.397 mi/s | | miles per hour (mph) | 670,616,629 mph | | feet per second (ft/s) | 983,571,056 ft/s | | Mach number (sea level) | Mach 874,030 | | Astronomical Units/hour | 0.0201 AU/h | | Light-nanoseconds | 0.2998 m (per ns) | | Light-years per year | 1…
Frequently Asked Questions
What is the speed of light in mph?
The speed of light in a vacuum is 299,792,458 m/s, which equals approximately 670,616,629 mph (about 670 million mph). In km/h, that is 1,079,252,849 km/h (roughly 1.08 billion km/h). In miles per second: approximately 186,282 miles per second.
How does the speed of light compare to sound?
Light travels approximately 874,030 times faster than sound at sea level (where sound travels about 343 m/s or 767 mph). Light crosses the Atlantic Ocean (5,572 km) in about 18.6 milliseconds; sound takes about 4.5 hours. This is why you see lightning before hearing thunder — about 3 seconds per kilometer of distance.
What is Mach 1 vs speed of light?
Mach 1 is the speed of sound in air at sea level — approximately 343 m/s (1,235 km/h, 767 mph). The speed of light (299,792,458 m/s) is exactly 874,030 times faster than Mach 1. Mach 10 (3,430 m/s, used by the fastest experimental aircraft) is still only 0.001145% of the speed of light.
How is the speed of light measured?
Historically, Ole Rømer first estimated the speed of light in 1676 using observations of Jupiter's moon Io. James Bradley measured it more precisely in 1729 using stellar aberration. By the 20th century, laser interferometry achieved extreme precision. Since 1983, the metre has been defined using the speed of light: c = 299,792,458 m/s exactly, so the speed of light is now a defined constant, not a measured quantity.
Is the speed of light constant?
The speed of light in a vacuum (c) is a universal constant: 299,792,458 m/s, exact by definition. However, light slows down when passing through a medium. In water, light travels at about 0.75c (225 million m/s). In glass (optical fiber), about 0.67c. In air, essentially c (slowed by only 0.03%). The slowing of light through a medium is described by its refractive index (n = c/v).
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