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virtual images and convex lenses



I have two geometrical optics questions.

1. Why don't you see a real image without a screen? (Actually, sometimes you
do, why not in this case?)

Imagine this arrangement:

object ----------------------- focal point ---- convex lens ---- focal
point -real image ------ eye

That is, a self-illuminated object is placed much greater than the focal
length (d_object >> f) away from a convex lens. A real image should be
formed on the other side of the lens. So, if you look towards the lens, why
don't you see the image without a screen?

2. Virtual or real image?

Imagine this arrangement:

object ---------------------------------------------------- f -- convex lens
-- f --------------eye

When you hold a convex lens far away from your eye and other objects, you
see an inverted image in the lens. Is this a virtual or real image? What is
the ray diagram that describes this arrangement?

Tim Forth


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