Snell's law of refraction
- CBSE · NCERT Class 10, Ch 9 Light - Reflection and Refraction
Snell's law says that when light passes from one medium into another, sin i divided by sin r stays constant for a given colour and pair of media.
Refraction is the bending of light as it passes from one clear medium into another, such as from air into glass. The angle of incidence, i, is between the incoming ray and the normal. The angle of refraction, r, is between the bent ray and the normal.
NCERT gives two laws of refraction. First, the incident ray, the refracted ray and the normal all lie in one plane. Second, sin i / sin r is a constant. The second is Snell's law. The constant is the refractive index of the second medium with respect to the first.
A small example
A ray in air meets a glass block at i = 30°. Inside the glass, r is about 19.5°. Then sin 30° = 0.5 and sin 19.5° is about 0.334. So sin i / sin r = 0.5 / 0.334, which is about 1.5. Change the angle and both sines change, but the ratio stays about 1.5 for this glass.
Where marks go
Question 33(A) of CBSE's 2026-27 Science sample paper asks you to state Snell's law, for 1 mark. The 2026-27 Science marking scheme prints the full statement. It ends with the conditions: for light of a given colour and for the given pair of media.
Those closing words are part of the law. A statement that stops at "the ratio is a constant" leaves out what the constant depends on. Read what a marking scheme contains.
Sources
Facts last checked against these sources on 30 September 2026.
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