AP Night Lecture #4: Optics, Part II

Online Video Lessons:

For animated lessons with narration and problems, visit HippoCampus's AP Physics B II website and scroll down to the "Physical Optics" and "Geometric Optics" topics.

  1. Refraction
    1. Light bends toward the normal as it enters a denser substance where it travels slower, and away from the normal as it enters a less dense substance where it travels faster (car and mud analogy).
    2. The index of refraction of a substance is the ratio of the speed of light in vacuum to its speed in that substance (n=c/v).
    3. Snell's law says n1sinθ1=n2sinθ2 where n=index of refraction and θ is the angle from the ray to the normal.
    4. Light can bend so far from the normal when leaving a dense substance that its angle of refraction exceeds 90°. Thus the light doesn't really leave the substance, but totally internally reflects. The critical angle is the angle of incidence in the substance for which the angle of refraction is 90°; thus any angle of incidence greater than the critical angle leads to internal reflection.
    5. Lenses
      1. Each lens type forms images just like the opposite type of mirror (convex lenses form images similar to those of concave mirrors; concave lenses form images like that of a convex mirror)
      2. An image is virtual if it forms on the same side of the lens as the object, whereas virtual mirror images form behind the mirror.
      3. Lens ray diagrams use the following rays:
        1. Draw a ray from the top of the object to the lens, parallel to the axis, which refracts through the focal point on the far side of the lens.
        2. Draw a ray from the top of the object that passes straight through the center of the lens on the axis.
        3. Where the refracted ray (from a) crosses ray b is where the image will form.
  2. Mirror/Lens Math
    1. If f=focal length (and is negative for convex mirrors and concave lenses that diverge light instead of physically focusing it), di=distance from image to mirror/lens, and do=distance from object to mirror/lens, then 1/f = 1/di + 1/do
    2. If hi=height of image and ho=height of object, then hi / ho = di / do

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