An overhead projector is a variant ofslide projector that is used to display images to an audience.
An overhead projector works on the same principle as a 35mm slide projector, in which a focusing lens projects light from an illuminated slide onto a projection screen where a real image is formed. However some differences are necessitated by the much larger size of thetransparencies used (generally the size of a printed page), and the requirement that the transparency be placed face up (and readable to the presenter). For the latter purpose, the projector includes a mirror just before or after the focusing lens to fold the optical system toward the horizontal. That mirror also accomplishes a reversal of the image in order that the image projected onto the screen corresponds to that of the slide as seen by the presenter looking down at it, rather than a mirror image thereof. Therefore the transparency is placed face up (toward the mirror and focusing lens), in contrast with a 35mm slide projector or film projector (which lack such a mirror) where the slide's image is non-reversed on the sideopposite the focusing lens. .How to use the OHP effectively
Often mobile digital audio players are marketed and sold as "portable MP3 players", even if they also support other file formats and media types. Other types of electronic devices which can also act as a like mobile digital media player, but only have those media playback features as a secondary function, are cellphones, smartphones, tabletss, and sometimes even digital cameras are referred as portable media players because of their playback capabilities. This article focuses on portable devices that have the main function of playing media.
In addition to VCR-like functions such as playing, pausing, stopping, rewinding, and forwarding, some common functions include zooming/full screen, audio channel selection, subtitle selection, and frame capturing.
Many of the video players also support simple playback of digital audio and 3D playback of 2D video.