In materials with high refractive indices, the speed of light decreases compared to its speed in a vacuum.
- Refractive index definition: The refractive index nn of a material is defined as the ratio of the speed of light in a vacuum cc to the speed of light in the material vv:
n=cvn = \frac{c}{v}
So, a higher nn means a lower vv. - Light slows down: For example, in glass with a refractive index of 1.5, light travels at c/1.5c/1.5. In diamond (with n≈2.4n ≈ 2.4), it travels at less than half its vacuum speed.
- Cause of slowing: As light enters a dense medium, its electric field interacts with atoms or molecules, causing them to oscillate and re-emit the wave. This repeated absorption and re-emission causes an effective delay, reducing the net speed of propagation.
- Wavelength change: While the speed and wavelength of light decrease in such materials, its frequency remains constant.
- No loss of energy: The light isn’t losing energy; it’s just being delayed by microscopic interactions. When it exits the material back into a vacuum or air, it returns to its original speed.
Thus, materials with higher refractive indices cause greater reductions in the speed of light due to stronger interactions with the electromagnetic wave.