Real vs. Virtual Images
Plane Mirror Rays emanating from an object at point P strike the mirror and are reflected with equal angles of incidence and reflection. After reflection, the rays continue to spread. If we extend the rays backward behind the mirror, they will intersect at point P’, which is the image of point P. To an observer, the rays appear to come from point P’, but no source is there and no rays actually converging there . For that reason, this image at P’ is a virtual image. Object Virtual Image P P’ O I d o d i The image, I, formed by a plane mirror of an object, O, appears to be a distance d i , behind the mirror, equal to the object distance d o . Animation Continued…
Object Image P B M P’ d o d i h h’ Mirror Two rays from object P strike the mirror at points B and M. Each ray is reflected such that i = r . Triangles BPM and BP’M are congruent by ASA (show this), which implies that d o = d i and h = h’. Thus, the image is the same distance behind the mirror as the object is in front of it, and the image is the same size as the object. With plane mirrors, the image is reversed left to right (or the front and back of an image ). When you raise your left hand in front of a mirror, your image raises its right hand. Why aren’t top and bottom reversed? object image Plane Mirror (cont.)
Concave and Convex Mirrors Concave and convex mirrors are curved mirrors similar to portions of a sphere. light rays light rays Concave mirrors reflect light from their inner surface, like the inside of a spoon. Convex mirrors reflect light from their outer surface, like the outside of a spoon.
Concave Mirrors • Concave mirrors are approximately spherical and have a principal axis that goes through the center, C, of the imagined sphere and ends at the point at the center of the mirror, A. The principal axis is perpendicular to the surface of the mirror at A. • CA is the radius of the sphere,or the radius of curvature of the mirror, R . • Halfway between C and A is the focal point of the mirror, F. This is the point
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- Fall '09
- Light, Total internal reflection, Geometrical optics, rays