What is an image? Light changes its path after reflection or refraction. Thus when we see something, we may not seeing the REAL thing at its true location. Rather, we are seeing its IMAGE, which is formed by extrapolating the light in straight lines. Virtual image: if the light rays do not go through the image. Otherwise, the image is real.

How to get a plane mirror image? Consider an object in front of a mirror. Draw a ray from a point on the object to the mirror. It reflects according to the law of reflection. Draw another ray in the same way. Extend the rays to the back of the mirror. The intersection of the two rays is the image of the point on the object.

Plane Mirror Image optics/mirror_e.html

Properties of a plane mirror image Plane mirror image is a virtual image. Plane mirror image has the same size as the original object. Plane mirror image has the same distance from the mirror as the real object does. Plane mirror image is reversed front to back, NOT left to right.

Kaleidoscopes Plane mirror images plus multiple reflections. tw/java/optics/image_e.ht ml tw/java/optics/image_e.ht ml

Spherical Mirrors Pieces of reflecting material in the shape of part of the surface of a sphere. Convex mirror: mirror surface bulges out. Mirrors on your car, mirror in a store. Concave mirror: mirror surface caves in. Have you seen one? What are the images from these mirrors?

Characterization of a convex mirror Center of the sphere: C Focal point: the point where all the incident parallel rays are focused. It is the middle point between the surface of the mirror and the center. Focal length: the distance between the surface of the mirror and the focal point.

Three important rays: Ray I All rays incident parallel to the axis are reflected so that they appear to be coming from the focal point, F.

Three important rays: Ray II All rays that (when extended) pass through C are reflected back on themselves.

Three important rays: Ray III All rays that (when extended) pass through F are reflected back parallel to the axis.

Summary of three rays All rays incident parallel to the axis are reflected so that they appear to be coming from the focal point, F. All rays that (when extended) pass through C are reflected back on themselves. All rays that (when extended) pass through F are reflected back parallel to the axis. You need to draw only two rays to determine an image an image!

Image from a convex mirror

Properties of the image It is a virtual image It is an upright (or erect) image. It is closer to the mirror than the real object The image is smaller than the real object. Applications: store and car (allows to see wider angles)

Parallel Light From Faraway All rays are nearly parallel. Rays parallel to each other are imaged on the focal plane. The image comes from a ray passing through the center of the mirror, OR a ray going through the focal point.