# HARVARD PHYS 11b - Lecture 20 (24 pages)

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## Lecture 20

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## Lecture 20

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Pages:
24
School:
Harvard University
Course:
Phys 11b - Electricity, Magnetism, and Waves
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Physics 11b Lecture 20 Lenses Mirrors and Images S J Chapter 36 What We Did Last Time Law of reflection 1 1 Huygens principle Index of refraction n c v 1 1n1 2 n2 Snell s law of refraction n1 sin 1 n2 sin 2 n2 Total internal reflection if 1 arcsin n1 Fermat s principle The actual path between Wavelength is affected two points taken by a beam of light is the one which is traversed in the least time 1 Dispersion and chromatic aberration 1 1 n1 n2 n1 n2 2 1 n2 n1 n1 n2 2 Today s Goals Discuss simple optical devices Lenses and mirrors Building blocks of optical devices Use Geometrical Optics approximation Convex and concave lenses Focal points focal lengths Spherical aberration Concave mirrors Lens formula for ideal lenses Analyze a magnifying glass Geometrical Optics Optical devices are combination of lenses and mirrors Telescopes microscopes cameras binoculars Assume all elements lenses mirrors are much larger than the wavelength in aperture and thickness We can treat light as if it s a particle Trajectory in each medium is a straight line At boundaries it either reflects or refracts Refraction angle given by Snell s law Everything is determined by the elements shapes indices of refraction and their geometrical arrangement 1 2 sin 1 n2 sin 2 n1 Geometrical Optics Analysis of optical devices using this approximation Optical Elements There are 4 major types Concave and convex lenses Concave and convex mirrors Lenses may have different radii on two surfaces Consider them as a combination of two lenses with one side flat Focal Points Lenses mirrors turn plane waves into spherical waves Real FP Light may or may not actually go through the focal point Origin of the spherical waves is the focal point If yes real focal point If not virtual focal point Distance between the lens and its focal point focal length f If real f 0 If virtual f 0 f Virtual FP convention f Diopter Your optometrist prescribes your glasses using diopter It s just 1 f called the optical power Larger

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