What Is Snell's Law and How Do You Use It?

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Here, $n_1$ and $n_2$ are the refractive indices, while $\theta_1$ is the angle of incidence and $\theta_2$ is the angle of refraction. Both angles must be measured from the normal, an imaginary line perpendicular to the boundary.

The frequency is fixed by the source and does not change at the boundary. Since $v=f\lambda$, a lower speed therefore produces a shorter wavelength. The wavefront changes direction because the side entering the second medium first changes speed first; this rotation of the wavefront is refraction.

How to Use It

1. Identify the incident and refracted media.
2. Measure or read both angles from the normal.
3. Substitute known values into Snell's law.
4. Rearrange before using the inverse sine function.

For light travelling from air, $n_1=1.00$, into glass, $n_2=1.50$, at $\theta_1=30.0^\circ$:

The light bends toward the normal because it enters the medium with the higher refractive index. Its frequency remains constant, while its speed and wavelength decrease.

Exam Technique

For a Calculate question, show the equation, substitution, rearrangement, and final value. In degree mode, not radian mode, use inverse sine only after isolating the sine term. A common misconception is measuring angles from the surface; IB questions define them from the normal. Also check that your answer's direction of bending agrees with the refractive indices.