View Single Post
Old 2007-09-15, 08:33   #10
davieddy
 
davieddy's Avatar
 
"Lucan"
Dec 2006
England

2×3×13×83 Posts
Default Optics

Treat the projector as a simple convex lens,
and take the distance of the screen to be measured from the
optical centre of this lens.
If we assume the images are accurately focussed, then we
find that the object slide/film is closer to the lens for an
image distance of 15 feet. So the angle subtended is larger
in the second case giving a larger image than you have
all been assuming.

David

Last fiddled with by davieddy on 2007-09-15 at 08:38
davieddy is offline   Reply With Quote