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LENSES. 30.1 – Converging & Diverging Lenses. Light is refracted as it passes through glass Converging (Convex) lens – light intersects (converges) on backside, thicker in middle Diverging (Concave) lens – light spreads out (diverges) on backside, thinner in middle.

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30 1 converging diverging lenses
30.1 – Converging & Diverging Lenses
  • Light is refracted as it passes through glass
  • Converging (Convex) lens – light intersects (converges) on backside, thicker in middle
  • Diverging (Concave) lens – light spreads out (diverges) on backside, thinner in middle
slide3

Focal point = the point that parallel rays of light cross after going through lens

  • Principle axis – a perpendicular line going through center of lens
  • Focal length – the distance from center of lens to focal point
30 2 image formation by a lens
30.2 – Image Formation by a Lens

Converging Lenses

  • Consider an object very far away (at infinity: f  ∞)
  • The spreading is so small, the rays are considered to be parallel
  • After passing through lens, they intersect at focal point on other side and produce an upside down image
  • Object at F  no image (forms at infinity)
slide5

Between F and ∞  real, inverted

  • Object closer than F = virtual image –
    • cannot be projected onto screen,
    • rays of light do not intersect
    • Can only be seen in lens

Diverging Lenses

  • Regardless of location of object image is:
    • Virtual
    • Upright
    • smaller
30 5 common optical instruments
30.5 – Common Optical Instruments

Camera

  • Uses lens (or lenses) to produce  real, inverted image on light sensitive film
  • Exposure of light is regulated by shutter/diaphragm

Telescope

  • A second lens is used (eyepiece) as a magnifying lens to enlarge real image from first lens
slide7

A third lens or prisms can be used to make image upright

C0mpound Microscope

  • First lens (objective) produces real image (inverted)
  • Second: inside focal length of image producing  virtual, larger, inverted
30 6 the eye
30.6 – The Eye
  • Light enters through transparent cover – cornea
  • The amount of light allowed in through the pupil is regulated by the iris – colored part of eye
  • Light is focused on tissue on back of eye - retina
slide9

The nerves that carry light information away (optic nerve) produce a blind spot

  • Muscles surrounding the lens of the eye constantly change its shape to adjust for objects close and far away
  • Recall: the image produced is upside down
30 7 some defects in vision
30.7 – Some Defects in Vision

Farsightedness

  • Eyeball is too small, the lens focuses light behind the eye
  • Eye cannot focus on objects too close
  • A converging lens is placed in front of the eye to begin the curving process
slide11

Nearsightedness

  • Eye is too long or lens is curved too much causing focal point to be before the retina
  • Cannot see objects far away
  • Must wear diverging lenses to “slow the convergence”

Astigmatism

  • The cornea is not uniformly curved
  • Causes light to focus well at certain locations on retina, but not all
  • Corrective lenses are specially designed
30 8 some defects of vision
30.8 – Some Defects of Vision
  • Distortions of an image – abberation
  • Spherical abberation
    • Light does not focus at same location
    • Due to varying refraction angles at different distances from center
    • Corrected by: thin covering at edges or combination of lenses
  • Chromatic Abberation
    • Due to dispersion of colors as they pass through lenses
    • Therefore each color has its own focal point