Geometrical Optics

Reflection

When light traveling in a given direction hits a smooth surface obliquely, (latin obliquus, not at right angles) it is reflected in some different direction. Watch! The direction of the reflected light is determined by the angle of reflection, . It is the angle measured from the perpendicular (normal) of the surface to the ray of the reflected light. Similarly, we call the angle of incidence, , the angle between the normal and the incident ray. The Law of reflection states that the two angles are equal. That is, they are equal in magnitude but on opposite sides of the normal. The law of reflection describes how a plane mirror forms images. Reflection creates an Image By tracing the reflected rays, mirror images are located where the rays intersect or appear to intersect behind the mirror. The apparent image is very important when designing optical equipment. Tutorial: Construct the image in a plane mirror

A single mirror can form several images of an object (e.g. a candle). Since most mirrors have a glass surface covering the reflecting silver, one can get several reflections from the mirror. Most light is reflected from the silver, but some is reflected from the front glass surface. Corner Mirrors If mirrors are not parallel but at some angle to each other, an incident ray will be reflected just a few times. For example, if the mirrors are perpendicular to each other, one gets two reflections (see figure ). The interesting thing about these perpendicular mirrors is this: No matter what the direction of incidence, the light always is reflected back into the same direction (at least for light in the plane of the paper). No matter from where you look into the corner, you always see yourself. Such a corner mirror does not reverse right and left. Try 3 plane mirrors! Such a device reflects all incoming rays.

Retroreflector reflects all rays back into incoming direction. Kaleidoscopes These are long tubes containing two mirrors running the length of the tube and set at an angle to each other. One looks through a peephole between the mirrors at objects at the other end. If the angle between the mirrors is 60o, 45o or 30o (or any other angle angle whose multiple is 360o) one sees a symmetric array of virtual images. Diffuse Reflection Reflection from smooth surfaces, such as metal, glass, water, is called specular reflection. However, if the surface is rough, the reflection is smeared out, the light is reflected in many directions and does not form a `mirror image'. The result of the reflections from all the irregularities of a rough surface is called diffuse reflection. Example: Compare driving at night on a dry road and on a wet road.

Ch. Elster
Aug 26 14:27:03 EDT 2019