LECTURE 15
OPTICAL INSTRUMENTS
Instructor: Kazumi Tolich
Lecture 15
2
¨
Reading chapter 27.1 to 27.6
¤ Optical
Instruments
n Eyes
n Cameras
n Simple
magnifiers
n Compound microscopes
n Telescopes
¤ Lens
aberrations
Quiz: 1
3
Cameras
4
¨
¨
The camera lens moves closer to or farther away
from the film in order to focus.
The amount of light reaching the film is
determined by shutter speed and the 𝑓-number:
𝑓-­‐‑number =
*+,-. .01234
56-70308 +* -9083:80
=
;
<
Quiz: 2 & 3
5
Example: 1
6
¨
A nearsighted person cannot
focus clearly on objects that are
more distant than 2.25 m from
her eye. What refractive power
lenses are required for her to
see distant objects clearly?
Apparent size
7
¨
The apparent size of an object is determined by the actual size of the
image on the retina.
Magnifying glass
8
¨
The angular magnifications of a magnifying glass (𝑓 < 𝑁) are by
𝑀=
𝑀=
@A
@
@A
@
=
B
;
(image at infinity)
=1+
B
;
(image at near point)
Example: 2
9
¨
In your botany class, you
examine a leaf using a convex
12-D lens as a simple magnifier.
What is the angular
magnification of the leaf if the
image formed by the lens is at
infinity? Assume your near point
distance is 𝑁 = 25 cm.
Compound microscope/Demo: 1
10
¨
¨
¨
¨
The object is placed just outside of the focal length of
the objective.
The objective forms a real, enlarged, and inverted
image of the object.
The image formed by the objective falls at the focal
length of the eyepiece.
The total magnification is the product of the
magnification of each lens:
𝑀3+3-. = 𝑚+FG0,36H0 𝑀0I0960,0 = −
𝑑6 𝑁
𝑓+FG0,36H0 𝑓0I0960,0
Quiz: 4
11
Refracting telescopes
12
¨
¨
¨
Refractive telescopes work by creating a real, inverted, and diminished
image of an object that is much closer than the object.
An eyepiece at its focal length away from the image is used as a simple
magnifier to view that image.
The total angular magnification of the telescope is
𝑀3+3-. =
@A
@
=
;LMNOPQRSO
;OTOUROPO
Quiz: 5
13
Example: 3
14
¨
An astronomical telescope has a
total angular magnification of 7.0.
The two lenses are 32 cm apart.
Find the focal length of each lens.
Demo 3
15
¨
¨
¨
Galilean Telescope: Eyepiece is a diverging lens placed before the focal point of the objective lens. The
resulting magnified image is upright.
Keplerian Telescope: Eyepiece is a converging lens placed after the focal point of the objective lens. The
resulting magnified image is inverted.
Reflecting Telescope
Lens aberration
16
¨
¨
¨
Spherical aberration occurs when light striking the
lens far from the axis does not focus properly. It
can be fixed by grinding the lens to a nonspherical shape.
Chromatic aberration occurs when different colors
of light focus at different points.
Chromatic aberration can be improved by
combining two or more lenses that tend to cancel
each other’s aberrations. This only works perfectly
for a single wavelength, however.