Download Lecture17 - UMD Physics

Who Invented the first
microscope?
 Credit for the first microscope is usually given to
Zacharias Jansen, in Middleburg, Holland, around the
year 1595. Since Zacharias was very young at that
time, it's possible that his father Hans made the first
one, but young Zach took over the production. Details
about these first Jansen microscopes are not clear,
but there is some evidence which allows us to make
some good guesses about them. The above early
microscope found in Middleburg, Holland,
corresponds to our expectations of the Jansen
microscopes.
Microscope
 Structure: Made of two lenses, Objective and
eyepiece
 Objective:
The object being viewed is
placed just outside the focal length of
the objective lens. the image thus
formed is real, inverted, and enlarged.
We will call this image the intermediate
image since it is not the final image
viewed in the eyepiece.
An example
 Suppose the focal length of the objective is
12mm, and the object is placed at 13cm. The
image is then 156cm away from the lens and
the magnification is
156/13 = 12 times!
 Eyepiece:
Work as a magnifying glass,
used to view the real intermediate image
formed by the objective lens.
To view the image with a relaxed eye (so
the light rays entering the eye is
parallel), the image must be located at
the focal point of the eyepiece.
 Shorter the focal length, larger the
magnification.

The example continued
 Now suppose the eyepiece has a focal length
of 50mm. Then it has a magnification power
of
25cm / 5cm = 5 times
 The total magnification is
12 x 5 = 60
 The distance between two lens S =
(156+50)mm = 20.6 cm
neuron
Cat flea
Fruit fly
Black ant
Telescope
 The incoming rays are parallel and the
outgoing rays are also parallel.
 Again need two lenses: the objective and
eyepiece.
 The intermediate image is on the focal plane
of the objective. It must be also on the focal
plane of the eyepiece.
 The total length of the telescope
is fo +fe.
Telescope Magnification
 To get a large image, fO must be large.
 To see the image up close, fE must be small.
 The telescope magnification is
 fo / fe
Negative because the image is inverted.
 A large intermediate image means that it is
dimmer. One needs a larger aperture. Thus
telescopes are usually specified by their
sizes.
Images from Hubble Telescope
How to make the image erect?
 Method I:
 Add
another lens
 Method II:
 Galilean

telescope:
the eye piece is a diverging lens. fe is
negative.
The magnification can be calculated the
same way!  f / f
o
e
Galilean Telescope
 A Galilean telescope is defined as having one convex
lens and one concave lens. The concave lens serves
as the ocular lens, or the eyepiece, while the convex
lens serves as the objective. The focal point of the
ocular lens is the same as the focal point for the
objective lens.
Magnification = fo/fe
Length of the telescope = fo-fe
 Small field of view. A Galilean telescope
typically has a field of view of about 15-18 arc
minutes. The moon has a diameter of about
30 arc minutes, so the Galilean telescope
only reveals approximately one-fourth of the
moon's surface at one time.