# Chapter 20: Geometric Optics

**At Grade**Created by: CK-12

Light refracted through water.

Optics is the study of light. Geometric optics is the classical science of mirrors, glass, and lenses. It explains how light bends in water, and how images are formed in mirrors, telescopes, microscopes, and other devices.

- 20.1.
## Light as a Ray and the Law of Reflection

- 20.2.
## Concave and Convex Mirrors

- 20.3.
## Index of Refraction

- 20.4.
## Thin Lenses

- 20.5.
## Summary

### Chapter Summary

1. The law of reflection states that the angle of incidence is equal to the angle of reflection \begin{align*}\theta_i=\theta_r\end{align*}

The incident ray, the reflected ray and the normal are also coplanar.

2. For the object distance, \begin{align*}d_o\end{align*}

3. For spherical mirrors, \begin{align*}f=\frac{r}{2}\end{align*}

4. The lateral magnification \begin{align*}m\end{align*}

\begin{align*}m=\frac{h_i}{h_o}=-\frac{d_i}{d_o}\end{align*}

5. We define the ratio of the speed of light through vacuum \begin{align*}c\end{align*} to the speed of the light through a particular medium \begin{align*}v\end{align*} as the index of refraction \begin{align*}n\end{align*}

\begin{align*}n=\frac{c}{v}\end{align*}

Since \begin{align*}c > v\end{align*}, the index of refraction is always greater than 1 when traveling through any medium other than vacuum.

6. Snell’s law can be defined in terms of the indices of refraction \begin{align*}n\end{align*} or the speeds of light in the two media. It expresses the relationship between the angle of incidence \begin{align*}\theta_1\end{align*} and the angle of refraction \begin{align*}\theta_2\end{align*} at the interface of two media.

\begin{align*}n_1 \sin \theta_1=n_2 \sin \theta_2\end{align*}

The index of refraction \begin{align*}n_1\end{align*} is the medium from which light originates and \begin{align*}n_2\end{align*} is the index of refraction of the medium to which light passes. Angle \begin{align*}\theta_1\end{align*} is measured with respect to the normal at the interface of medium 1 and angle \begin{align*}\theta_2\end{align*} is measured with respect to the (same) normal at the interface of medium 2. Snell’s law can be expressed in terms of the velocities of light in the respective media as

\begin{align*}v_2 \sin \theta_1=v_1 \sin \theta_2\end{align*}

where \begin{align*}v_1\end{align*} is the speed of the light in medium 1 and \begin{align*}v_2\end{align*} is the speed of light in medium 2.

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