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Object within the Focal Length

Object within the mirror focal length.

What happens when the object is inside the focal length of spherical concave mirror as shown? The procedure here will follow the two Light Ray Diagram rules that we have successfully employed previously.

    Ray Diagram Rules
  1. Draw two light rays.
  2. Choose the easiest two!
Ray from object top to mirror and optic axis intersection.

To draw the first light ray, start at the top of the object. Draw a straight line to middle of the mirror. Sketch the outgoing ray using the principle that the incident reflection angles are equal.
Ray from object top through center of curvature.

The second ray passes through the center of curvature. Begin at the top of the object and draw through the center of curvature extending the line to the mirror surface.

Since the light ray is square with the mirror, the outgoing path traces over the incoming path and on past the object. This light ray is shown in green.
Virtual, erect, and enlarged image. Here, the real outgoing light rays diverge. Since the actual light rays do not focus, we have a virtual image. Extend the light rays on the back side of the mirror.

The top of the image is located where these virtual rays meet. This image is upright and enlarged. It is virtual image since it is formed by virtual, rather than real, light rays.


Go on to convex mirrors.

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