Study Guide

Optics Terms

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Optics Terms

Convex, Converging Lens

These lenses are thicker in the middle than at the edges, such that when parallel light rays hit the lens the light is bent together, converging them, on the other side of the lens.

Concave, Converging Mirror

The inside of a spoon. A way to remember which mirror is concave is that the surface of concave mirrors "cave" in. This causes it to bring light together, or in fancy physics speak, it converges light rays.

Concave, Diverging Lens

These lenses are thicker at their edges and thinner in the center. Concave lenses take parallel rays of light and spread them outward away from each other so that the rays are further apart on the other side of the lens.

Constructive Interference

Interacting waves that come together in phase, and combine (add) to form a bigger wave.

Convex, Diverging Mirror

Convex mirrors are mirrors whose surface bulges out at us. These types of mirrors spread out, or diverge, parallel light rays that hit their surface. What results is always a virtual image that is reduced in size.

Destructive Interference

Waves which come together in such a way that they cancel each other out by adding a wave crest to a wave trough. Surfers call this "a major bummer."


The spread of a beam of light after passing through a narrow slit or bending around an opaque edge.

Electromagnetic Spectrum

In their obsessive quest to classify everything in existence, scientists have organized light by wavelength. While most people think of only visible light as the electromagnetic spectrum, there are actually more wave types on it, including radio, microwave, infrared, visible, ultraviolet, x-ray, and gamma-ray. Just because you can't see them, doesn't mean they aren't there.

Geometric Optics

The model considering light as being made up of bajjillions of tiny particles—little balls bouncing minding their own business, obeying Newton's laws of mechanics. They're called photons, for the record.

Huygens' Principle

In essence, a wave front consists of an infinite number of points. Each of these points acts as the point source of a new wave. The sum of all of these point-source wavelets gives us a brand-spanking new wave front, or a "secondary" wave front.

Image Distance, di

The distance between a mirror or lens and the image (not object). Image distances are negative for virtual images and positive for real images

Image Height, hi

The height of the image formed by a mirror or a lens. An image height is considered positive if it has the same orientation as the object and negative if it is upside down.

Index of Refraction, n

The index of refraction, n, tells us something about how light moves through that object. It is a measure of how much slower light travels through a given material than when it travels through a vacuum


That football foul. Oh, not that kind. Interference happens when two of the same kind of waves (light, sound, oceanic) interact with each other.

Magnification, m

The ratio of the height of the "image" object, hi, to the height of the real object, ho, or

Mirror Equation

The mirror equation relates object distance, do, and image distance, di, to the focal distance, f, of a mirror:

Object Distance, do:

The distance between an object and a mirror or lens. An object distance is always positive unless we have more than one lens or mirror to deal with so that one mirror's image becomes the next one's object

Object Height, ho

The height of an object. This value is always positive.

Physical Optics

Physical optics treats light as a wave. This can explain all of the phenomena that geometric optics can, but in addition, it explains why light can interact with itself to form the diffraction patterns we see in Young's Double Slit Experiment.

Plane Mirror

A flat, smooth-surfaced mirror like the one found in the bathroom.

Ray Diagram

The picture of light's path from an object to a lens or mirror to create an image.

Real Image

An image from light rays converging where the image appears to be.


Flipping a point or figure over a line. Think of comparing your left and right hands in a mirror. It's also what Mulan wishes would show who she is inside.


This refers to the light which bends to move through an object.

Snell's Law

n1sinθ1 = n2sinθ2, used to calculate how light bends for refraction as it moves from one material into another

Total Internal Reflection

Light trapped inside of an object because of the angle light travels. If light hits the edge of an object at just the right angle or beyond it, then all of the light is reflected within the material and none of it is refracted.

Virtual Image

The appearance of an image at some distance di even though light rays don't converge there in reality. It's one of those things we have to see to believe.

Young's Double Slit Experiment

An experiment illustrating the wave properties of light, first performed by someone named Thomas Young.

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