Optics

Reflection, refraction, lenses, and the wave nature of light

What is optics?

Optics is the branch of physics that studies light: how it travels, how it bounces off and bends through different materials, and how it forms the images we see in mirrors, lenses, cameras, and our own eyes.

Light travels in straight lines

In a uniform material like air, light travels in straight lines — this is why objects cast sharp shadows, and why you can't see around a corner without a mirror. This simple fact explains a great deal of everyday optics, from shadows to pinhole cameras.

Reflection: bouncing off a surface

When light hits a smooth surface like a mirror, it bounces off in a very predictable way: the angle at which it arrives equals the angle at which it leaves, measured from an imaginary line perpendicular to the surface. This is why a flat mirror shows a clear, undistorted reflection of whatever is in front of it.

Refraction: bending through a surface

When light passes from one material into another — say, from air into water — it bends. This happens because light travels at different speeds in different materials, and the change in speed at the boundary causes the direction to change. This is why a straw in a glass of water looks like it bends at the surface, and it's the basic principle behind how lenses work.

Lenses and everyday devices

A lens is simply a curved piece of transparent material, shaped so that refraction bends light rays in a controlled, useful way — bringing parallel rays together to a point (a converging lens, like in a magnifying glass or the human eye) or spreading them apart (a diverging lens). Cameras, eyeglasses, telescopes, and microscopes are all built from carefully shaped lenses using these same basic reflection and refraction principles.

Why optics matters

Optics explains how we see the world, and it underlies an enormous range of real technology: eyeglasses, cameras, microscopes, telescopes, optical fibers that carry the internet across oceans, and much more.