Ray optics

  • Ray model of light
  • Ray diagrams
  • Reflection
  • Index of refraction
    • Snell's law
  • Color and dispersion
  • Thin lenses
    • Real images
    • Virtual images
    • Magnification
    • Converging and diverging lenses

The ray model of light

  • Light can be modeled as a particle (photoelectric effect)
  • Light can be modeled as a wave (interference)
  • Light can be modeled as a ray


Assumption: Light travels in straight lines

  • Valid when apertures are large compared to the wavelength of light

A light ray is a line in the direction along which the light energy is flowing.


  • A light ray continues forever unless it interacts with matter
    • At an interface it can reflect or refract
    • Within a material it can be scattered or absorbed

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  • Light rays travel in straight lines
  • Light rays can cross without affecting each other
  • An eye sees by focusing a bundle of diverging rays
  • A converging lens uses refraction to focus the rays
  •  Angles are measured normal to the surface
  •  The angle of incidence equals the angle of reflection

Plane mirrors

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Multiple rays from the point P reflect from the mirror. Each ray obeys the law of reflection: the angle of reflection equals the angle of incidence.

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An image is formed at point P'. The reflected ray seems to have traveled from the point P', tracing the reflected ray backward through path "inside the mirror."


The image created is called a virtual image, since it cannot be seen on a screen. We define the distance s from the point P to the mirror. We define the distance s' as the apparent distance from the mirror to the image, or the "image distance."


For a flat mirror, the distance s' = s.


For a flat mirror, the image size is the same as the object size. The magnification, m = 1.

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Your eye detects the rays which seem to be diverging from the point source P'. The lens in your eye causes the rays to converge and focus on your retina.

Two plane mirrors form a right angle. How many images of the ball can you see in the mirrors from this position?

You are standing a distance s from a mirror, looking at the reflection of your toes. How far is it from your eyes to the point of reflection s’, if your eyes are a distance D from the floor?

If your height is h and your eyes are l1 from the top of your head, what is the shortest mirror on the wall in which you can see your full height? Where must the top of the mirror be?