What is the Speed of Light?
The speed of light is scientifically proven to be the universe's maximum speed — nothing in the universe can travel faster. In a vacuum, light travels at exactly 299,792,458 meters per second (m/s), which is approximately 670,616,629 mph or 1,079,252,849 km/h. This constant, denoted by the letter c, is one of the most fundamental constants in physics.
How Fast Is the Speed of Light?
To put the speed of light into perspective:
- Light can travel around the Earth's equator (40,075 km) more than 7.5 times per second.
- It takes light only about 1.3 seconds to travel from the Moon to Earth.
- Sunlight takes approximately 8 minutes 20 seconds to reach Earth from the Sun.
- Light from the nearest star (Proxima Centauri) takes about 4.24 years to reach us.
- In 1 minute, light travels about 17,987,547,480 meters — enough to circle Earth over 448 times.
How to Calculate Distance Traveled by Light
Since the speed of light is constant in a vacuum, calculating how far light travels is straightforward:
distance = speed of light × time
d = c × t = 299,792,458 m/s × t (seconds)
Example Calculation
How far does light travel in 100 seconds?
- Speed of light: c = 299,792,458 m/s
- Time: t = 100 seconds
- Distance: d = 299,792,458 × 100 = 29,979,245,800 m ≈ 29,979,246 km
Measurement Units Supported
Our Speed of Light Calculator displays results in both metric and American (imperial) units:
Metric System
- Meters (m) — the SI base unit of length
- Kilometers (km) — 1 km = 1,000 m
- Centimeters (cm) — 1 m = 100 cm
American / Imperial System
- Miles (mi) — 1 mile = 1,609.344 m
- Feet (ft) — 1 foot = 0.3048 m
- Inches (in) — 1 inch = 0.0254 m
Astronomical Units
- Light-years (ly) — the distance light travels in one year ≈ 9.461 × 10¹⁵ m
- Astronomical Units (AU) — the average distance from Earth to the Sun ≈ 1.496 × 10¹¹ m
- Parsecs (pc) — 1 parsec ≈ 3.262 light-years
Does Light Always Travel at the Same Speed?
The speed of light is constant only in a vacuum. When light passes through a medium (like glass or water), it slows down. The ratio of c to the speed of light in a medium is called the refractive index (n = c/v). For example:
- In air: ≈ 299,702,547 m/s (nearly identical to vacuum)
- In water: ≈ 224,900,000 m/s (~75% of c)
- In glass: ≈ 199,860,000 m/s (~67% of c)
- In diamond: ≈ 123,900,000 m/s (~41% of c)
This slowing-down of light in different media is what causes the phenomenon of refraction — the bending of light when it passes from one medium to another.
FAQs
What is the speed of light in mph?
The speed of light in a vacuum is 670,616,629 mph (miles per hour). This is equivalent to 299,792,458 m/s or 1,079,252,849 km/h.
Is the speed of light always constant?
Yes, the speed of light is constant for a given medium. In a vacuum, it is always exactly 299,792,458 m/s. The speed changes when light travels through different materials.
How far can light travel in 1 minute?
Light travels 17,987,547,480 meters (about 17,988,000 km or 11,176,944 miles) in 1 minute. This means light can circle Earth more than 448 times in just one minute.
How far does light travel in 1 year (a light-year)?
One light-year is the distance light travels in one Julian year (365.25 days): approximately 9.461 × 10¹⁵ meters (about 5.879 trillion miles or 9.461 trillion kilometers).