# Total Internal Reflection

## Key Questions

• Snell's Law can be used to calculate the critical angle for total internal reflection.

${n}_{1} \sin {\theta}_{1} = {n}_{2} \sin {\theta}_{2}$

where ${n}_{1}$ is the index of refraction of the incident substance
${\theta}_{1}$ is the unknown critical angle
${n}_{2}$ is the index of refraction of the refracting substance
${\theta}_{2}$ is the angle of refraction (${\theta}_{2} = {90}^{o}$ in a critical angle question)

Isolate ${\theta}_{1}$ by dividing both sides by ${n}_{1}$

Then once you have solved for $\sin {\theta}_{1}$, remember to take the inverse sine to find the angle of incidence.