The refractive index of a medium directly affects the speed of light in that medium. It tells us how much slower light travels in the material compared to its speed in a vacuum.
Key Idea:
- Higher refractive index = Slower light speed
- Lower refractive index = Faster light speed
Light travels fastest in a vacuum (with a refractive index of 1). When it enters a material like water, glass, or plastic, it interacts with the particles in that medium, which causes it to slow down.
Examples:
- Vacuum: Speed = maximum (about 300,000 km/s), refractive index = 1
- Air: Almost same as vacuum, refractive index ≈ 1.0003
- Water: Refractive index ≈ 1.33 → light slows down
- Glass: Refractive index ≈ 1.5 → light slows down even more
This slowing of light causes it to bend or refract when entering or exiting the material, which is the basis for lenses, prisms, and many optical effects. The greater the refractive index, the more significant the change in light’s speed and direction.