Physics 212 lecture 34: Applications of Thin Lenses

1
Physics 212 lecture 34 Applications of Thin
Lenses

• The camera
• Why are f-stops often used to describe the
  speed for a film exposure?
•         The intensity of light falling on the
  film depends on the
• D2 diameter of the aperture squared (size of
  opening through which light passes through the
  lens). Bigger aperture lets more light through.
• f2 focal length of the camera squared (because
  di ? f and the image size is proportional the
  di2). Smaller focal length focuses light onto
  smaller film area.
• f-stop number is f/D
• I Intensity is proportional to 1/(f/D)2
•         The intensity of the light is Watts/m2
• So, since a certain amount of energy exposes the
  film and
• Power is energy/time
• you need shorter exposure times for larger
  intensities
• Therefore, short exposure times, due to large
  intensities, come from small f-stop numbers

2

• The eye (and glasses)
• How do you correct near and far-sighted vision?
•         Near Sightedness
• Occurs when the lens focuses the image in front
  of the retina. Light needs to be spread out more
  to correct this, and a diverging lens is used.
•         Far Sightedness
• Occurs when the lens focuses the image behind the
  retina. Light needs to be brought together
  earlier, and a converging lens is used.

3

• The magnifying glass
• Why do you place a magnifying glass close to the
  object, not your eye?
• A larger image is the desired result, so the
  image must be magnified.
• A virtual image is the type of image needed since
  we are not using a screen.
• Magnifying glasses are converging, which brings
  light together.
• Therefore, if the object is placed
• inside the focal length (near the glass), an
  enlarged virtual upright image is formed
• outside the focal length (near the eye), a
  reduced real inverted image is formed

4

• The microscope
• Can you see atoms with a really powerful optical
  microscope?
• No.
• The wavelength of visible light (400-700nm) is
  much larger than that of the atom, so we cannot
  see them with a regular microscope no matter how
  powerful it is.

• The telescope
• Why do some telescopes use mirrors while others
  use lenses?
• Cost (mainly).
• Small telescopes using lenses are cheaper to
  produce than they are using mirrors. However, at
  large sizes mirrors become more cost effective to
  produce.