optical instruments chapter 25. introduction we will discuss: cameras the eye the simple magnifier...
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TRANSCRIPT
Introduction
We will discuss: Cameras The eye The simple magnifier The compound microscope The telescope The Michelson interferometer The diffraction grating
The Camera
A single lens camera consists of: A light tight box A converging lens
Produces a real, inverted imageFocusing is done by adjusting the distance between
the lens and the film. A shutter
25.1
Camera Variables
Shutter speeds Control exposure times Faster shutter speeds for faster objects or
for bright light Aperture size
Larger aperture needed for low light conditions
f-number The ratio of the focal length to the diameter of the
lens
Determines the speed of the lens A measure of the light concentrating power of the
lens A small f-number means that the aperture is wide
open€
f - number =f
D
The Human Eye
Parts of the eye: Cornea Aqueous humor Iris and pupil Crystalline lens Vitreous humor Retina
Rods and conesB13
The Near Point
The near point is the closest distance for which the lens will produce a sharp image of a nearby object on the retina. About 18 cm at age 10 About 25 cm at age 20 About 50 cm at age 40 500 cm or greater at age 60
The Far Point
The far point is the farthest distance for which the lens will produce a sharp image of a far away object on the retina. Objects located beyond the far point will
be out of focus.
Defects of the Eye
Hyperopia (farsightedness) Corrected with a converging lens
Myopia (nearsightedness) Corrected with a diverging lens
Astigmatism Corrected by using cylindrical lenses
25.3, 25.4, 30-1
The Power (P) of a Lens
Measured in diopters (no abbreviation) Used in lens prescriptions
Formula:
f must be in meters
f
1P =
The Simple Magnifier
Consists of a single converging lens Increases the apparent size of an object
25.6a, b
Angular Magnification (m)
Angular Magnification is the ratio of the angle subtended by the object when the lens is in use to the angle subtended by the object when it is placed at the near point with no lens
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m =θ
θo
Maximum angular magnification occurs when the image formed by the lens is at the near point of the eye (more eye stress).
f
25cm1m +=
The Compound Microscope
Uses two lenses to achieve greater magnification than the simple magnifier An objective with a short focal length ( ) An eyepiece (ocular) with a focal length ( ) of a few centimeters Lenses are separated by a distance (L)
which is greater than either or25.7a
of
ef
of ef
Microscope Images
The image formed by the objective becomes the object for the eyepiece.
The image is inverted with respect to the object.
The Electron Microscope
Atoms could never be seen with a compound microscope by using visible light The wavelength of light is too large
The Refracting Telescope
It uses a combination of lenses. The image (q1) formed by the first lens becomes the
object (p2) for the second lens.
The image is inverted.
25.8a e
o
f
f−=meo ff +=L
Refracting Telescope Difficulties
Large refracting telescopes are difficult to manufacture and are expensive.
In addition, their large masses would lead to sagging which would cause distortion.
The Reflecting Telescope
Reflecting telescopes are preferred in astronomy. A parabolic mirror replaces the objective.
Spherical aberration is reduced.Chromatic aberration is
virtually eliminated.
Large Telescopes
The largest telescopes in the world are reflecting telescopes. 10 meter lens in Hawaii 5 meter lens on Mt. Palomar in California
The largest refracting telescope in the world is at Yerkes Observatory in Wisconsin Only 1 meter in diameter
Resolution Of Single-Slit And Circular Apertures
It is important to be able to distinguish between two closely spaced objects. This ability is limited by the diffraction of
light
25.10
Maximum Resolution
If the two sources are separated so that their central maxima do not overlap, their images can be distinguished and they are said to be resolved. Maximum resolution occurs at the shortest
possible wavelength.
265
Rayleigh’s Criterion
When the central maximum of one image falls on the first minimum of another image, the images are said to be resolved.
Equations for the resolution of a circular aperture ( is in radians)
266
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min = 1.22λ
D
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= s
r
The Diffraction Grating
Diffraction gratings are preferred over single or double slits when accurate wavelength measurements are needed. They have a much higher resolution and provide a
brighter pattern.
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sinθ = mλ
d
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sinθ = y
L
The Michelson Interferometer
It splits a light beaminto two parts and then recombines them toform an interferencepattern.
262
The Michelson Equation
L is the length of travel of the adjustable mirror.N is the number of fringe shifts.
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ΔL = Nλ
4
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