A compound lens made with lenses of different indices of refraction can counteract this effect. This type of achromatic lens is found in high
quality optical equipment.33.16 - Gotchas
“Real” means the same thing with lenses as it does with mirrors. On the one hand, yes. Real images can be projected onto paper with both
lenses and mirrors. On the other hand, no. With lenses, a real image is on the opposite side of the lens from the object. With mirrors, a real
image is on the same side of the mirror as the object.
Signs. You need to be careful with signs when using the lens equations, and we have provided a table to help you do so.
The ray diagrams for lenses are entirely accurate and represent physical reality. The diagrams in this (and other textbooks) employ a few
conventions in order to make them easier to understand: Objects are shown fairly close to lenses that are relatively thick compared to the
distance to the objects. For the approximations inherent in the theory of thin lenses to hold true, the objects should be quite far from the lenses
and the lenses should be much thinner. Applying these changes to diagrams would make for illustrations that stretch far across the page or
computer screen.33.17 - Summary
Lenses redirect light by refraction. Converging lenses are thicker in the middle and
bring light rays together. Diverging lenses are thinner in the middle and spread light
out.
Lens terminology is very similar to that of mirrors. An important difference is that
when we consider a single lens, virtual images appear on the same side of the lens
as the object, and real images appear on the opposite side. Another difference is
that lenses have a focal point on each side.
Ray diagrams for converging and diverging lenses are also similar to those for
mirrors.
And like mirrors, lenses have equations that quantify the relative size, orientation
and distance of the images they produce. In addition, the lensmaker’s equation
determines the focal length of a lens with differing radii of curvature on its two sides.
Sign conventions can be trickier for lenses, so pay special attention to them.
Several lenses can be used together to enhance their magnification properties, such
as in a refracting telescopes.
The human eye contains a lens that can change shape in order to create a focused
image on the light-sensitive retina at the back of the eye. A person’s near point and
far point are the closest and farthest distances on which she can focus.
Lenses can exhibit spherical aberration just as mirrors can. They also exhibit chromatic aberration due to the differing refractive indices of
different wavelengths of light. Both kinds of aberration can cause an image to look blurry.Thin lens equationLateral magnificationLensmaker’s equation, air(^622) Copyright 2007 Kinetic Books Co. Chapter 33