How Lenses Work for an Overhead Projectors Work

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Mirrors – Inside the lens, which is the large and moveable piece at the top of the overhead projector, there are two mirrors. The mirrors work together to reflect light and project an image onto a screen. The first mirror captures the light from the base of the projector; this is the flat surface where you place the transparencies. The first mirror reflects that light to the second mirror, which then reflects it through a magnifying lens for large projection onto the screen.

Light – The projector has only one light and it is not part of the lens. The light is an essential part of the lens function. The light in an overhead projector is found in the flat, glass surface where you place your transparencies on top of it. The light is needed for the lens to distinguish the difference between the clear transparency and the printed material on the transparency.

Magnifying Lens – Once the two mirrors have distinguished the transparency reflection and reflected it toward the screen, the actual image would be small without the use of a magnifying lens. The magnifying lens is what captures the image that is reflected by the mirror and then displays it on the screen so that it’s much larger and clearer. The size and clarity of the actual projection relies on how close the lens is to the transparency and how close the entire projector is to the screen.

Lens Movement – The entire lens construction on the overhead projector features a knob at the side. That plastic knob allows you to adjust the lens up and down the projector arm to change the focus of the lens.

The closer the lens is to the transparency, the sharper the image, but only when combined with the distance from the projector to the screen. You’ll likely need to adjust the lens up and down each time you use the projector to get the best combination of distance, size and clarity. In class I learned the closer the focal length the greater the magnification.


  1. Ireland, J.. (2011). How Lenses Work for an Overhead Projector.
  2. Retrieved February 4, 2012, from http://www. ehow. com/info_12172565_lenses-work-overhead-projector. html

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