A projector is a device that uses light and lenses to take an image and project a magnified image onto a larger screen or wall. Projectors can magnify still or moving images depending on how they are built. The image must be shone through a sealed tube or frame when passing through the lenses to maintain focus.
The projector sends the lighted image through at least two lenses - one concave and one convex - to focus the image and then magnify it out to the screen. Because it is several lenses fused into one, a fresnel lens can sometimes work by itself (though the image can be inverted).
Overhead and slide projectors are among the projectors that shine light through a still image and uses its lenses to project a larger image onto the screen. A film or video projector sends moving pictures across the light beam to play a movie in the same way.
The one, and most important, feature shared by almost all projectors is the ability to adjust the lenses and thus focus the projected image to get a clear picture (depending on the projector's proximity to the screen).
Most projectors use a straight tube or frame that sends the image in one direction. An overhead projector, however, first shines the image upward and then bounces it off a mirror to send it in a forward direction.
Still and video projectors are used by both business and educational institutions to showcase important presentations and videos. Movie theaters to this day use projectors to show films on a large screen.
Any device with a video screen, like a TV or media player, can be turned into a projector by constructing a tube-like frame around the screen and inserting lenses into the frame.