25 Optical Instruments

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Optical Instruments

Chapter 25


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Introduction

We will discuss:

Cameras

The eye

The simple magnifier

The compound microscope

The telescope The Michelson interferometer

The diffraction grating


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The Camera

A single lens camera consists of:

A light tight box

A converging lens

Produces a real, inverted image

Focusing is done by adjusting the distance betweenthe lens and the film.

A shutter

25.1


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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


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The equation for lateral magnification:

p

q

h

hM

'


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The London Eye


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The Human Eye

Parts of the eye: Cornea

Aqueous humor

Iris and pupil

Crystalline lens

Vitreous humor

Retina

Rods and conesB13


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Accommodation

Ciliary musclesRelaxed when viewing distant objects


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The focal length of the eye is about 1.7

cm.


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The cornea does over 70 % of the

refracting.The lensmakes the final adjustment.


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The Near Point

The near pointis the closest distance for

which the lens will produce a sharpimage of a nearby object on the retina.

About 18 cm at age 10

About25 cm at age 20

About 50 cm at age 40

500 cm or greater at age 60


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The Far Point

The far pointis the farthest distance for

which the lens will produce a sharpimage of a far away object on the retina.

Objects locatedbeyondthe far point will

be out of focus.


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Defects of the Eye

Hyperopia(farsightedness)

Corrected with a converging lensMyopia(nearsightedness)

Corrected with a diverging lens

Astigmatism Corrected by using cylindrical lenses

25.3, 25.4, 30-1


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Astigmatism Test


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Glaucoma

Corrected by medication or YAG solidstate laser surgery


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The Power (P) of a Lens

Measured in diopters (no abbreviation)

Used in lens prescriptions Formula:

f must be in meters

f

1

P


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The Simple Magnifier

Consists of a single converging lens

Increases the apparentsize of an object

25.6a, b


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Angular Magnification (m)

Angular Magnificationis the ratio

of the angle subtended by the objectwhen the lens is in use to the anglesubtended by the object when it is

placed at the near point with no lens

m

o


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Maximum angular magnification

occurs when the image formedby the lens is at the near point of

the eye (more eye stress).

f

25cm1m


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Angular magnification (m)when

the image formed by the lens isat infinity (relaxed eye):

f

25cmm


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The Compound Microscope

Uses two lensesto achieve greatermagnification than the simple magnifier

An objectivewith 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 or

25.7a

of

ef

of ef


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The Electron Microscope

Atoms could never be seen with a compoundmicroscope by using visiblelight

The wavelength of light is too large


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The Telescope

There are two fundamentally different

types of telescopes.


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The Refracting Telescope


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The Reflecting Telescope


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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.

25.8ae

o

ff

meo ff L


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Refracting Telescope

Difficulties

Large refracting telescopes are difficult

to manufactureand are expensive. In addition, their large masses would

lead to saggingwhich would cause

distortion.


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The Reflecting Telescope

Reflecting telescopes are preferred in

astronomy. A parabolic mirror replaces the objective.

Spherical aberration is reduced.

Chromatic aberration isvirtually eliminated.


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The Newtonian Focus

Reflecting Telescopes use the Newtonian

focus.


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Canada-France-Hawaii


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Mt. Palomar


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The largest refractingtelescope in the world

is at Yerkes Observatory in Wisconsin

Only 1 meter in diameter


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Yerkes Observatory


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Images

Microscopes and telescopes can only form

virtual images.


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Resolution Of Single-Slit And

Circular Apertures It is important to be able to distinguishbetween two closely spaced objects.

This ability is limited by the diffractionoflight

25.10


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Maximum Resolution

If the two sources are separated so thattheir central maximado not overlap,their images can be distinguished andthey are said to be resolved.

Maximum resolution occurs at the shortest

possible wavelength.

265


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Rayleighs Criterion

When the central maximum of oneimage falls on the first minimum ofanother image, the images are said to beresolved.

Equations for the resolution of a circular

aperture (is in radians)

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min 1.22

D

s

r


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The Diffraction Grating

Diffraction gratings are preferred over single

or double slits when accuratewavelength

measurements are needed. They have a much higher resolutionand provide a

brighterpattern.

sin m

dsin

y

L


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The Michelson Interferometer

It splits a light beaminto two parts and thenrecombines them to

form an interferencepattern.

262


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The Michelson Equation

L is the length of travel of the adjustablemirror.

N is the number of fringe shifts.

LN 4


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