New trends in physics teaching, v.4; The ... - unesdoc - Unesco

New Trends in Physics Teaching IV
is convex. Hence, a convex lens can magnify in the same way as a magnifying glass, producing an
image larger than the object. As the water leaks out, the surface caves in, and we say it is concave.
Look at the object through the water now, and you see it appears much reduced in size, as it
would on looking through the wrong end of a pair of binoculars. Concave lenses therefore give an
image reduced in size. What do we learn?
Qualitative: Convex lenses (bowed out) magnify; concave lenses (bowed in) give images
reduced in size.
p
\ I
\ I
Quantitative: The ray trace of the system is shown in figure 20. Look at the little square
(figure 2 1 a) through the water lens. When the meniscus is convex, it looks like figure 2 1 b. This
is known as barrel distortion, because the square image is distorted to look like a barrel. Distortion
of this kind occurs with all lenses having spherical surfaces, such as this. When the lens
becomes concave, the distortion changes and becomes pincushion distortion (figure 2 IC).
radius of
surface
no magnification
\ I
\ I
\ I
\ I
\ I
A
large virtual image
n- the refractive index.
the ratio of the speed
of light in air to that
in the medium.
flat surface
\ /
+ I
I H
I
H/n
Figure 20.
-image of
reduced size
o
tal
Figure 21.
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