Lecture 8: Optical instruments
Lecture aims to explain:
1. Magnifying power
2. Magnifying glass
3. Microscope, principle of operation
4. Telescope, principle of operation
Magnifying power
Magnifying power
The magnifying power or angular magnification of a visual instrument
is defined as:
the ratio of the size of the retinal image as seen through the instrument
over the size of the retinal image as seen by the unaided eye at normal
viewing distance (or the near point)
This is equivalent to the ratio of the angles (assisted over unaided)
αa
MP =
αu
αu
do
do distance to the near point (~25 cm)
Magnifying glass
Magnifying glass
Use a positive lens to increase the size of the
retinal image relative to that with the unaided eye:
magnified virtual image is formed
do
f
l
Magnification is given by
do
MP =
L
 1

1 + f ( L − l )


L distance to the image, do
distance to the near point (~25 cm)
Example 8.1
Find magnification of a magnifying
glass, if the object is positioned at
the focal point of the lens
Microscope
Compound microscope
Two convex lenses, usually with fixed distance between them
1. Objective near the specimen, creates a real image in the focal
plane of the eyepiece. Specimen placed near the focal point of the
objective
2. Eyepiece near the eye, works as magnifying glass, creates virtual
image for viewing with an unaccommodated eye
Magnification when the final image
is viewed at infinity:
L do
MP = −
fo f e
L distance from the focal point of the
objective to the image, do near point
(~25 cm)
Objective
Eyepiece
fe
fo
L
Telescope
Refracting telescope
Two convex lenses with adjustable distance between them
1. Objective creates a real image in its focal plane as objects
are very far away.
2. Eyepiece, works as magnifying glass, creates a virtual image
for viewing with an unaccommodated eye
Magnification is given by
fo
MP = −
fe
Objective
Eyepiece
fo
fo + f e
fe
Example 8.2: Galilean telescope
A Galilean telescope has an objective lens of 12cm focal length and
an eyepiece of 5cm focal length. It is focused on a distant object so
that the final image seen by the eye appears to be at a distance of
30cm from the eyepiece. Determine the angular magnification.
Figure shows an
example of Galilean
telescope when the
image is viewed at
infinity: objective is
convex and eyepiece
is concave
Example 8.3: microscope
The eyepiece and objective of a microscope are 20.6cm apart, and
each has a focal length of 0.6cm. Find a) the distance from objective
to the object viewed b) the linear magnification produced by the
objective c) the overall magnifying power. Assume throughout the
final image is formed at infinity.
SUMMARY
The magnifying power: the ratio of the size of the retinal image as seen
through the instrument over the size of the retinal image as seen by the
unaided eye at normal viewing distance (or the near point)
Magnifying glass: a single positive lens.
Magnification for an unaccommodated eye:
do
MP =
fe
Microscopes and telescopes can be assembled using two convex lenses.
Microscope: objective creates a real magnified image in the focal plane
of the eyepiece. Telescope: objective creates a real image in its focal
plane, coinciding with the focal plane of the eyepiece.
For viewing with an unaccommodated eye the magnifying power for
telescope:
microscope:
o
o
f
MP = −
fe
L d
MP = −
fo f e