Bouncing Light
Reflection is one of the most basic ways that light behaves. It occurs when a light ray strikes a surface and bounces off. How the light reflects depends on the smoothness of the surface.
Regular Reflection: This happens when light reflects off a very smooth surface, like a mirror or a calm body of water. All the parallel incoming light rays bounce off at the same angle, creating a clear, sharp image.
Diffuse Reflection: This happens when light reflects off a rough or uneven surface, like a piece of paper or a brick wall. Each light ray still obeys the law of reflection, but because the surface is bumpy, the rays are reflected in many different directions. This is why you can see the wall, but you can't see your reflection in it.
The Law of Reflection
For any regular reflection, there is a simple and predictable rule called the Law of Reflection.
Imagine a line drawn perpendicular to the mirror's surface at the point where the light ray hits. This line is called the normal.
The incoming ray is the incident ray. The angle between the incident ray and the normal is the angle of incidence.
The outgoing ray is the reflected ray. The angle between the reflected ray and the normal is the angle of reflection.
The Law: The angle of incidence is always equal to the angle of reflection. (θi = θr)
Plane Mirrors
A plane mirror is a mirror with a perfectly flat surface. It's the type of mirror you have in your bathroom. Plane mirrors always produce the same kind of image:
The image is virtual. This means the light rays do not actually come from the image's location; they only appear to. A virtual image cannot be projected onto a screen.
The image is upright (right-side up).
The image is the same size as the object.
The image appears to be the same distance behind the mirror as the object is in front of it.
The image is laterally inverted (reversed left to right). This is why text appears backward in a mirror.
Curved Mirrors
Concave Mirror: A mirror that is curved inward, like the inside of a spoon. Concave mirrors cause parallel light rays to converge (come together) at a point called the focal point.
They can form real images (that can be projected on a screen) or virtual images (if the object is very close), and can be used to magnify images. They are used in reflecting telescopes and makeup mirrors.
Convex Mirror: A mirror that is curved outward, like the back of a spoon. Convex mirrors cause parallel light rays to diverge (spread out).
They always produce virtual, upright, and smaller images.
Because they make things look smaller, they provide a wider field of view. They are used for security mirrors in stores and for the passenger-side mirrors on cars.