Question

For what range of object positions does a bi-convex lens of focal length f form a real image?

Answer 1

The Golden Gate Bridge refracted inrain droplets, which act as lenses

Image of a plant as seen through a biconvex lens.

If the distances from the object to the lens and from the lens to the image are S₁ and S₂ respectively, for a lens of negligible thickness, in air, the distances are related by the thin lens formula

.

This can also be put into the "Newtonian" form:

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where and .

What this means is that, if an object is placed at a distance S₁ along the axis in front of a positive lens of focal length f, a screen placed at a distance S₂ behind the lens will have a sharp image of the object projected onto it, as long as S₁ > f (if the lens-to-screen distance S₂ is varied slightly, the image will become less sharp). This is the principle behind photography and the human eye. The image in this case is known as a real image.