A light ray is a concept, not a physical thing. It is the line along which light energy flows.

• Rays travel in straight lines. Two rays can cross without disturbing one another.

• Objects are sources of light rays.

• Reflection and refraction by mirrors and lenses create images of objects. Points to which light rays converge are called real images. Points from which light rays diverge are called virtual images.

• The eye sees an object or an image when diverging bundles of rays enter the pupil and are focused to a real image on the retina.

You’ll use both graphical and mathematical techniques to analyze how light rays travel and how images are formed.

Light rays bounce, or reflect, off a surface.

• Specular reflection is mirror like.

• Diffuse reflection is like light reflecting from the page of this book.

The law of reflection says that the angle of reflection equals the angle of incidence. You’ll learn how reflection allows images to be seen in both flat and curved mirrors.

Light rays change direction at the boundary when they move from one medium to another. This is called refraction, and it is the basis for image formation by lenses. Snell’s law will allow you to find the angles on both sides of the boundary.

《 LOOKING BACK Section 16.5 Index of refraction

Lenses form images by refraction.

• We’ll start with ray tracing, a graphical method of seeing how and where images are formed.

• We’ll then develop the thin-lens equation for more quantitative results.

The same methods apply to image formation by curved mirrors.

Optics is everywhere, from your smart phone camera and your car headlights to laser pointers and the optical scanners that read bar codes. Our knowledge of the microscopic world and of the cosmos comes through optical instruments. And, of course, your eye is one of the most marvelous optical devices of all. Modern optical engineering is called photonics. Photonics does draw on all three models of light, as needed, but ray optics is usually the foundation on which optical instruments are designed.