An Introduction to Concave Mirrors

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Object Outside the Mirror Curvature

 Consider an object outside the curvature of a spherical concave mirror as shown. This is like looking at your reflection inside of a shiny spoon as you pick it up. The image is located where the the light rays from an object intersect or focus. A minimum of two light rays are needed to find the image. Since any two rays will work, we will pick the easiest two. To draw the first light ray, start at the top of the object. Draw a straight line to middle of the mirror as shown in the diagram. Applying the principle that the incident angle is equal to the reflection angle, sketch the outgoing light ray. Think of the concave mirror as a part of a sphere. The second ray will pass through the sphere center. Begin at the top of the object and draw through the center of curvature extending the line to the mirror surface. The light ray is square with the mirror. Therefore, the outgoing ray's path traces over the incoming path and on past the object. This light ray is shown in red in the schematic. The top of the image is located where the light rays meet. Since this is underneath the bottom of the object, the image is inverted. Note that the image is reduced in size. This image is physically different from that of a flat mirror. Here the real light rays are focused, producing a real image.* Go to next page.
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* Interested in proving that this type of image is real yourself? Click here to find out how you can do it with things found in your kitchen!