Refractive Index
Simple Explanation
A medium's refractive index (n) is a number that measures how much slower light travels in that medium compared to in a vacuum. A higher refractive index means light travels more slowly (and bends more) in that medium.
Why Do We Need It?
Refractive index is the key material property that determines how strongly a substance refracts light, and is essential for designing lenses, prisms, and optical fibres.
Formula
Refractive Index
n = c / v
A medium's refractive index measures how much slower light travels in it compared to in a vacuum.
- n
- — Refractive index of the medium (no units, always ≥ 1)
- c
- — Speed of light in a vacuum, ≈ 3 × 10⁸ m/s
- v
- — Speed of light in the medium, in m/s
When to use it: Use to find a medium's refractive index from the speed of light within it, or to find that speed if the refractive index is known.
Worked Example
Finding the speed of light in a medium
Diamond has a refractive index of 2.42. Find the speed of light inside diamond (c = 3 × 10⁸ m/s).
Why Does This Work?
Light slows down in a medium because it continuously interacts with the medium's atoms (being absorbed and re-emitted, or interacting with electron clouds) — denser materials, or materials with more strongly interacting electrons, slow light down more, giving them a higher refractive index.
Real-Life Example
Diamond's brilliant sparkle
Diamonds are famous for their bright, sparkling appearance compared to glass imitations.
Diamond has an unusually high refractive index (2.42, versus about 1.5 for glass) — this causes light to bend and reflect much more dramatically inside the diamond, contributing significantly to its characteristic sparkle and brilliance.
Practice
Light travels at 2 × 10⁸ m/s inside a medium. Find its refractive index (c = 3 × 10⁸ m/s).
MediumCommon mistake
Thinking a higher refractive index means light travels FASTER in that medium — it is the opposite: a higher n means light travels SLOWER (n = c/v, so larger n means smaller v for a fixed c).
Quick Review
- n = c/v: refractive index compares light's speed in vacuum to its speed in a medium.
- Higher refractive index means light travels more slowly (and bends more) in that medium.
- Diamond's very high refractive index contributes to its characteristic sparkle.