Introduction to Lenses

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Introduction to Lenses
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Definition of a lens

• A lens is transparent piece of plastic or glass
  with at least one refractive surface.

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How does a lens work?

• A lens works by refracting (bending) light rays
  as they pass through it so they change direction
• Lenses work by bending light at a glass-air
  boundary.  Although refraction occurs at the
  boundary, we will treat all lenses as bending the
  rays at the lens axis.

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Converging Lens

• The lens in the eye is a convex or converging
  lens.  This means that the lens makes rays of
  light come together, or converge.

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Converging Lens

• The rays parallel to the principal axis are
  converged onto the principal focus.  
• The focal length is the distance between the lens
  axis and the principal focus.

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Diverging Lens

• The concave lens cause rays traveling parallel to
  the principal axis to diverge, which is why it is
  called a diverging lens.
• The principal focus is virtual because the rays
  do not pass through it, but diverge as if they
  had come from it.

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Homework

• Copy out terminology diagrams on p.552 and p.553
  for Converging and Diverging Lenses
• Answer Questions p.553 3 6