MOTION MOUNTAIN

156 4 images and the eye – optics
f
focus
object and
light departing
from it
real
image
with
optional
screen
do
di
f
Challenge 165 s
Ref. 114
Challenge 166 e
Ref. 115
object and
light departing
from it
focus
do
virtual
image
di
F I G U R E 104 A real image produced
by a converging lens (if used in the
way shown) and the virtual image
produced by a diverging lens.
nifyingglass. Figure 104 also allows one to deduce thethinlensformula that connects the
lengthsd o ,d i andf. What is it?
Even though glasses and lenses have been known since antiquity, the Middle Ages had
to pass by before two lenses were combined to make more elaborate optical instruments.
The various effects that can be observed with one or two lenses are shown in Figure 105.
Thetelescope was invented – after a partial success in Italy by Giambattista della Porta –
just before 1608 in the Netherlands.The most well-knownofatleastthreesimultaneous
inventors was the lens grinder Johannes Lipperhey (b.c. 1570 Wesel, d. 1619 Middelburg)
who made a fortune by selling his telescopes to the Dutch military. When Galileo heard
about the discovery, he quickly took it over and improved it. Already in 1609, Galileo
performed the first astronomical observations; they made him world-famous.TheDutch
telescope design has a short tube yielding a bright and upright image, and its magnification
is the ratio of the focal distances of the two lenses. It is still used today in opera
glasses. Over the years, many other ways of building telescopes have been developed;
in particular, many modern high-performancetelescopesusemirrors. Sincemirrorsare
cheaperandeasiertofabricate for high-precisionimaging, mostlarge telescopeshave a
mirror instead of the first lens.
By the way, telescopes also exist in nature. Most spiders have several types of eyes, and
some up to 6 different pairs. For example, the jumping spider genus Portia (Salticidae)
has two especially large eyes, made to see distant objects, which have two lenses behind
each other; the second lens and the retina behind it can be moved with muscles, so that
such spiders can effectively point their telescope in different directions without moving
Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net