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                                 Light pen

 Using a light pen , sketches or diagrams can be drawn directly on screen . The light pen interacts with the screen of the micro-computer (VDU) and can be used to draw on a screen or alter a display . The light pen works when a bright spot of light behind the screen alerts the pens light sensitive tip. the spot know as Raster spot moves in an orderly way across back of the screen . whit this the computer can work out where the spot was wen the tip of the pen was altered . Depending on how the computer has been programmed an action can them be set off.