Optical Instruments

1
Optical Instruments

• Chapter 25

2
Introduction

• We will discuss
• Cameras
• The eye
• The simple magnifier
• The compound microscope
• The telescope
• The Michelson interferometer
• The diffraction grating

3
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
  between the lens and the film.
• A shutter
• 25.1

4
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

5

• 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

6

• The equation for lateral magnification

7
The London Eye
8
The Human Eye

• Parts of the eye
• Cornea
• Aqueous humor
• Iris and pupil
• Crystalline lens
• Vitreous humor
• Retina
• Rods and cones
• B13

9

• Accommodation
• Ciliary muscles
• Relaxed when viewing distant objects

10

• The focal length of the eye is about 1.7 cm.

11

• The cornea does over 70 of the refracting.
• The lens makes the final adjustment.

12
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

13
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.

14
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

15
Astigmatism Test

16
Diseases of the Eye

• Cataracts
• The lens is surgicallyreplaced

17

• Glaucoma
• Corrected by medication or YAG solid state laser
  surgery

18
The Power (P) of a Lens

• Measured in diopters (no abbreviation)
• Used in lens prescriptions
• Formula
• f must be in meters

19
The Simple Magnifier

• Consists of a single converging lens
• Increases the apparent size of an object
• 25.6a, b

20
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

21

• Maximum angular magnification occurs when the
  image formed by the lens is at the near point of
  the eye (more eye stress).

22

• Angular magnification (m) when the image formed
  by the lens is at infinity (relaxed eye)

23
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 or
• 25.7a

24
Microscope Images

• The image formed by the objective becomes the
  object for the eyepiece.
• The image is inverted with respect to the object.

25
Microscope Equations

• Magnification equations

26
The Electron Microscope

• Atoms could never be seen with a compound
  microscope by using visible light
• The wavelength of light is too large

27
The Telescope

• There are two fundamentally different types of
  telescopes.

28

• The Refracting Telescope

29

• The Reflecting Telescope

30
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

31
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.

32
The Reflecting Telescope

• Reflecting telescopes are preferred in astronomy.
  
• A parabolic mirror replaces the objective.
• Spherical aberration is reduced.
• Chromatic aberration is
• virtually eliminated.

33
The Newtonian Focus

• Reflecting Telescopes use the Newtonian focus.

34
Large Telescopes

• The largest telescopes in the world are
  reflecting telescopes.
• 10 meter lens in Hawaii
• 5 meter lens on Mt. Palomar in California

35
Canada-France-Hawaii
36
Mt. Palomar
37

• The largest refracting telescope in the world is
  at Yerkes Observatory in Wisconsin
• Only 1 meter in diameter

38
Yerkes Observatory
39
Images

• Microscopes and telescopes can only form virtual
  images.

40
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

41
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

42
Rayleighs 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 (q is in radians)
• ???????????????????
• 266

43
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.

44
The Michelson Interferometer

• It splits a light beaminto two parts and then
  recombines them toform an interferencepattern.
• 262

45
The Michelson Equation

• L is the length of travel of the adjustable
  mirror.
• N is the number of fringe shifts.

46
Questions

• 1, 3, 4, 6 - 9, 11, 13
• Pg. 842