Speed of light in different media
Fastest in vacuum, nearly as fast in air, much slower in glass and water.
A race of light
Light races through different materials: vacuum, air, water and glass. They all start together.
In which of these does light travel fastest?
What this lesson covers
The idea
Light travels with different speeds in different media: fastest in vacuum, at 3×10⁸ m s⁻¹, only marginally slower in air, and considerably slower in glass or water.
A race of light
Light races through different materials: vacuum, air, water and glass. They all start together.
In which of these does light travel fastest?
- Vacuum
- Water
- Glass
Compare the bars
The bars are worked out from the refractive indices of Table 9.3 (you will meet them soon). Pick air, water and glass and compare each bar with the one for vacuum.
Speeds differ
The bar for air was almost as long as the one for vacuum. Water and glass had much shorter bars.
Light travels with different speeds in different media. It travels fastest in vacuum, at 3×10⁸ m s⁻¹. In air the speed is only marginally less, and it is considerably less in glass or water.
Notes
Light travels with different speeds in different media: fastest in vacuum (3×10⁸ m s⁻¹), marginally less in air, considerably less in glass or water.
Check yourself
In which of these does light travel fastest?
What is the speed of light in vacuum?
Compared with its speed in air, the speed of light in glass or water is
- Water. Light travels considerably slower in water than in vacuum.
- Glass. Light travels considerably slower in glass than in vacuum.
- Vacuum — correct. Yes!
- 3×10⁸ m s⁻¹ — correct. Yes!
- 3×10⁶ m s⁻¹. The speed of light in vacuum is 3×10⁸ m s⁻¹, a hundred times more than 3×10⁶.
- 3×10¹⁰ m s⁻¹. The speed of light in vacuum is 3×10⁸ m s⁻¹, not a hundred times more.
- marginally less. That describes air compared with vacuum. In glass or water the speed reduces considerably.
- considerably less — correct. Yes!
- greater. Light is slower in glass and water than in air.