Speed of Light
Light travels at c ≈ 3 × 10⁸ m/s, the cosmic speed limit.
About this topic
Light travels at c = 299,792,458 m/s in a vacuum, about 3 × 10⁸ m/s. It takes 8 minutes to reach us from the Sun and 1.3 seconds from the Moon, and a light-year is the distance it travels in a year, about 9.46 × 10¹² km.
According to relativity, nothing with mass can reach it and it is the same for all observers regardless of their motion, a surprising fact that led Einstein to rethink space and time. Light slows down in materials: in water about 2.25 × 10⁸ m/s, in glass about 2 × 10⁸ m/s. The refractive index n = c/v tells how much.
Worked example: the nearest star beyond the Sun, Proxima Centauri, is about 4.2 light-years away, so we see it as it was 4.2 years ago. Looking far into space is also looking back in time. A signal to a Mars rover takes between 3 and 22 minutes depending on the planets' positions.
Early measurements by Rømer used the moons of Jupiter, and Michelson used rotating mirrors. Today the metre is defined from c. Applications include GPS timing corrections, fibre-optic communications and astronomy. A common misconception is that a light-year is a unit of time; it is a unit of distance. Another is that light travels instantly; the speed is huge but finite.
Key ideas
- Time = distance ÷ speed
- Sunlight takes about 8 minutes
- Slower in glass
Quick check
Use cases: where Speed of Light helps
- Homework: review the key idea behind Speed of Light before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Speed of Light without notes.
- Visual learning: play with the interactive lab to see how Speed of Light changes when you adjust its inputs.
- Teaching a friend: explain Speed of Light in your own words, then compare your version with the key ideas.
The same laws describe car crashes, sports, satellites and everyday machines.
Frequently asked questions
What is Speed of Light?
Light travels at c ≈ 3 × 10⁸ m/s, the cosmic speed limit.
What will I learn in Speed of Light?
Time = distance ÷ speed. Sunlight takes about 8 minutes. Slower in glass.
Is Speed of Light part of Physics?
Yes. It is one of 150 Physics topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Speed of Light lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Speed of Light?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.