Refractive index of glass: 1.52
The refractive index of crown glass is 1.52, so light travels at about 1.97 × 10⁸ m/s in it. Dense flint glass is 1.65, and ordinary glass is often taken as 1.5.
Refraction
Snell's law: n₁ sin i = n₂ sin r, so sin i / sin r = n₂₁ = n₂/n₁. Angles are measured from the normal. Going from a denser to a rarer medium beyond the critical angle C (sin C = n₂/n₁), there is no refracted ray. Drag the incoming ray or type an angle.
Angle of refraction r27.72°sin r = 0.4652
n₂₁ = n₂/n₁1.52light in 2 at 1.97 × 10⁸ m/s
Critical angle Cnoneonly from denser to rarer
The ray bendsTowards normalinto a denser medium
Light from n = 1 into n = 1.52 at 45° refracts at 27.72°, bending towards the normal.
Show the working
- n₁ sin i = n₂ sin r, so sin r = n₁ sin i / n₂ = 1 × sin 45° / 1.52 = 0.4652
- r = sin⁻¹(0.4652) = 27.72°
- n₂₁ = n₂/n₁ = 1.52/1 = 1.52. Light travels at c/n: 3 × 10⁸ m/s in medium 1 and 1.97 × 10⁸ m/s in medium 2.
n = 1.52
- Speed of light in crown glass: c/n = 3 × 10⁸ / 1.52 = 1.97 × 10⁸ m/s.
- Dense flint glass (1.65) bends light more and is used where strong bending is wanted.
- From glass into air the critical angle is sin⁻¹(1/1.52) = 41.14°, which is why a 45° prism reflects light totally.
- Glass with respect to water: n = 1.52/1.33 = 1.14.
Snell's law with these numbers
- Light goes from air (n taken as 1) into crown glass at 45°.
- n₁ sin i = n₂ sin r, so sin r = n₁ sin i / n₂ = 1 × sin 45° / 1.52 = 0.4652
- r = sin⁻¹(0.4652) = 27.72°
- n₂₁ = n₂/n₁ = 1.52/1 = 1.52. Light travels at c/n: 3 × 10⁸ m/s in medium 1 and 1.97 × 10⁸ m/s in medium 2.
Refractive index of common media
| Medium | n | Speed of light |
|---|---|---|
| Air | 1.0003 | 3 × 10⁸ m/s |
| Ice | 1.31 | 2.29 × 10⁸ m/s |
| Water | 1.33 | 2.26 × 10⁸ m/s |
| Alcohol | 1.36 | 2.21 × 10⁸ m/s |
| Kerosene | 1.44 | 2.08 × 10⁸ m/s |
| Fused quartz | 1.46 | 2.05 × 10⁸ m/s |
| Turpentine oil | 1.47 | 2.04 × 10⁸ m/s |
| Benzene | 1.5 | 2 × 10⁸ m/s |
| Crown glass | 1.52 | 1.97 × 10⁸ m/s |
| Canada balsam | 1.53 | 1.96 × 10⁸ m/s |
| Rock salt | 1.54 | 1.95 × 10⁸ m/s |
| Carbon disulphide | 1.63 | 1.84 × 10⁸ m/s |
| Dense flint glass | 1.65 | 1.82 × 10⁸ m/s |
| Ruby | 1.71 | 1.75 × 10⁸ m/s |
| Sapphire | 1.77 | 1.69 × 10⁸ m/s |
| Diamond | 2.42 | 1.24 × 10⁸ m/s |
Speed of light = c/n, with c = 3 × 10⁸ m/s. Air is taken as 1 in most sums.
Quick answers
What is the refractive index of glass?
Crown glass is 1.52 and dense flint glass 1.65 in NCERT table 10.3. Many sums take glass as 1.5.
What is the speed of light in glass?
About 1.97 × 10⁸ m/s in crown glass.
What is the critical angle of glass?
About 41.14° for crown glass going into air.