Visual Angle

1
Visual Angle
http//www.microscopy.fsu.edu/primer/java/humanvis
ion/accommodation/index.html

• How large an object appears, and how much detail
  we can see on it, depends on the size of the
  image it makes on the retina.

• This, in turns, depends on the angle subtended
  by the object at the eye.

2
Magnifying Glass (Simple Microscope)
http//www.microscopy.fsu.edu/primer/java/scienceo
pticsu/microscopy/simplemagnification/index.html

• A magnifying glass allows us to place the object
  closer to our eye so that it subtends a greater
  angle.
• The object is placed within the focus of the lens
  so as to produce a virtual image, which must be
  at least 25 cm (least distance of distinct vision
  or near point) from the eye.

3
Magnifying Power of a simple microscope(Angular
Magnification)
?

• Where ? is the angle subtended by the object at
  the near point of the eye and
• ? is the angle subtended by the image to the
  lens.
• M is the ratio of the apparent sizes of the image
  and the object.

4
Compound microscope

• A microscope is used to produce an image on the
  retina larger than that obtainable by placing a
  small accessible object at the near point.
• The overall magnification of a microscope is the
  product of the magnifications produced by the two
  lenses.

5
Compound Microscope in Normal Adjustment
http//www.hazelwood.k12.mo.us/grichert/optics/in
tro.html

• In normal adjustment an enlarged virtual image is
  formed at the near point, 25 cm from the normal
  eye.

6
Magnifying Power of a compound Microscope

• Where ? is the angle subtended by the object at
  the near point of the eye and
• ? is the angle subtended by the final image at
  the eye.

• In normal adjustment, the angular magnification
  equals the linear magnification

7
Resolution of Lens

• The ability of a lens to produce distinct images
  of two point objects very close together is
  called the resolution of the lens.
• The closer the two images can be and still be
  seen as distinct, the higher the resolution.

Image of pollen grain with good resolution (left)
and poor resolution (right)
8
Resolving Power of a Microscope

• The resolving power of a microscope is its
  ability to enable detail in the image to be made
  out.
• The resolving power depends on
• The aperture of the objective
• (The larger the aperture, the better the
  resolution.)
• The wavelength of the light
• (The shorter the wavelength, the better the
  resolution.)

9
The Eye Ring for a Microscope

• The eye ring is the optimum position for the
  observers eye to gather most light that passing
  through the objective.

• The image is then brightest and the field of view
  greatest.
• The eye ring is also the image of the objective
  formed by the eyepiece.
• An observer should ideally have a pupil diameter
  equal to the eye ring.

10
Modern Microscope Component Configuration
11
Refracting Telescope

• A telescope is used to produce an enlarged
  retinal image of a distant inaccessible object.
• The job of a telescope
• Light gathering power
• Magnifying power
• Resolving power

12
Magnifying Power of a Refracting Telescope

• Where ? is the angle subtended at the eye by the
  object without the telescope,
• ? is the angle subtended by the final image at
  the eye.

13
Refracting Telescope in Normal Adjustment

• In normal adjustment the final image seen through
  the eyepiece is adjusted to line at infinity so
  that the eye is the most relaxed.
• In normal adjustment,

• The length of a telescope in normal adjustment
  fofe

14
Resolving Power of a Telescope (1)

• Resolving power of a telescope is the ability to
  separate two closely positioned stars.

• Diffraction by the objective is a factor that
  limits the resolving power of a telescope.

15
Resolving Power of a Telescope (2)

• The resolving power of a telescope
• depends on the quality of the optical surfaces,
• depends on the wavelength observed,
• increases as the diameter of the objective
  increases.
• Large lenses are difficult to make and they tend
  to sag under their own weight.

16
The Eye Ring for a Telescope

• In normal adjustment, it can be shown that

17
Reflecting Astronomical Telescope

• Advantages of reflecting telescope
• No chromatic aberration
• A mirror can have a much larger diameter than a
  lens
• No spherical aberration if paraboloidal mirror is
  used

18
Hubble Space Telescope
Eskimo nebula
Eagle nebula
HSTs primary mirror
19
Terrestrial Telescope

• An erecting lens is inserted between the
  objective and the eye piece to erect the inverted
  image formed by the objective.

• This system has the disadvantage of increasing
  the length of the telescope.
• An advantage is that it makes it possible to vary
  the magnification of the telescope.

20
Galilean Telescope

• Advantages
• The final image is erect so it is useful for
  terrestrial purposes.
• It is shorter than the terrestrial telescope
• Disadvantages
• Small field of view

21
Spectrometer

• The spectrometer is an instrument used for
• Producing, viewing and taking measurements on a
  pure spectrum using either a prism or a
  diffraction grating.
• Measuring accurately the refractive index of a
  material in the form of a prism.

22
Construction of a spectrometer

• The essential parts are
• The collimator which is fixed to the base of the
  instrument, consisting of a slit of variable
  width, and an achromatic lens.
• The turntable, which can be rotated, and to which
  a prism or grating can be attached. The circular
  edge of the table has a scale graduated in
  degrees.
• The telescope, which can also be rotated. A
  vernier scale is fitted to the telescope where it
  adjoins the table, enabling their relative
  orientation to be measured to 0.1o, or less.

23
Functions of the collimator and the telescope (1)

• Spectrometer used to measure wavelength of light

24
Functions of the collimator and the telescope (2)

• The collimator is set to produce a parallel beam
  of light from the light source near the slit.
• The telescope is set to receive parallel beam of
  light and hence measures the angle of deviation
  of light through the diffraction grating or the
  prism.

25
Adjustments of the spectrometer

• The eyepiece is focussed on the crosswires.
• The objective lens of the telescope is focussed
  so that the crosswires are in its focal plane.
• Using a slit of width appropriate to the source
  brightness, the collimator lens is moved so that
  the slit is in its focal plane.
• Using the table levelling screws, the axis of the
  table is made perpendicular to the plane
  containing the principal axes of the collimator
  and telescope lenses.