How does an overhead projector work?

How does an overhead projector work? — SR, Hartford, CT

An overhead projector uses a converging lens and a mirror to project a real image of your transparency onto a screen. A lamp brightly illuminates the transparency and a special surface under the transparency (actually a Fresnel lens) directs the light from the transparency through the projector’s main lens. This lens bends the light rays in such a way that all of the rays spreading outward from one point on the transparency bend back together and merge to one point on the screen. For example, if you make a green dot on the transparency, light rays spread outward from that green dot and some of them pass through the main lens. The lens bends these rays back together so that they form a single green dot on the screen. There is a single point on the screen for the light rays from each point on the transparency.

The pattern of light that forms on the screen is called a real image because it looks just like the original object—in this case the transparency—and it’s real, meaning that you can touch it with your hand. Real images are usually upside-down and backward, but the overhead projector uses its mirror to flip the image over so that it appears right side up. Because of this vertical flip, the side-to-side reversal is a good thing—the right side of the transparency becomes the left side of the screen image (as viewed by the same person) and the screen image is readable.

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