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Refractive index formula

The refractive index formula as it is used in A Level Physics, taken from the A Level Physics formula sheet.

The formula

n=cv,n1sin⁡θ1=n2sin⁡θ2n = \dfrac{c}{v}, \quad n_1\sin\theta_1 = n_2\sin\theta_2

How to use it in the exam

  • Write the formula down before substituting — most mark schemes award a method mark for a correct, clearly quoted formula.
  • Check every quantity is in consistent units before you substitute, then substitute in one step so the working stays easy to follow.
  • Show the rearrangement rather than jumping to the answer, and give the final value to the accuracy the question asks for.

Related waves & optics formulas

Wave speed
v=fλv = f\lambda
Diffraction grating
dsin⁡θ=nλd\sin\theta = n\lambda
Young's double slit
w=λDsw = \dfrac{\lambda D}{s}
Photoelectric effect
hf=ϕ+Ek(max⁡)hf = \phi + E_{k(\max)}
de Broglie wavelength
λ=hp=hmv\lambda = \dfrac{h}{p} = \dfrac{h}{mv}

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