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Light
rays are what we deal with in Sections 7.1 – 7.4. This is geometric
optics. Light is represented by a
straight line with an arrow on it to show direction of travel of the light beam
or “wavefront.” Sections 7.5 and 7.6
deal with physical optics, the wave
nature of light.
7.1
Reflection: The law of reflection is “the angle of
reflection is equal to the angle of incidence.” Snell’s law, dates from about… when? (Baghdad 984, says Wikipedia.) It applies to
all surfaces, but the reflection from most surfaces is diffuse, meaning the
individual light rays go off in all directions away from the surface. When you have a polished surface, it’s
“regular reflection” and reflected rays travel parallel to each other.
7.2
Refraction and dispersion:
Refraction: (get some equipment do demo
or lab!) “the deviation of light from
its original path because of a change in speed.” In other words, a beam of
light bends when going from one medium to another. What is a medium? The substance that a wave travels
through. Electromagnetic radiation can
travel in a vacuum, without a medium. The speed of light in the absence of a medium (in free space) is 299792458 meters per second, a universal constant.
The "index of refraction" of a medium tells how much bending of a light beam will occur when light goes from one medium into another, such as from air into water (making a pencil in a glass of water appear bent). Index of refraction is defined as (speed of light in vacuum / speed of light in medium), and given by the letter n .
The "index of refraction" of a medium tells how much bending of a light beam will occur when light goes from one medium into another, such as from air into water (making a pencil in a glass of water appear bent). Index of refraction is defined as (speed of light in vacuum / speed of light in medium), and given by the letter n .
Density
variations: oasis and illusion of water on hot roadway = refraction by different air density (due to temperature difference). Also, just
above the surface of a hot object, you can see a shimmering from the changing
index of refraction if you look along the surface.
Reflection by refraction: also known as total internal reflection. When going from higher density material to lower density material --- higher n to lower n --- light is refracted “away from the normal.” Draw a picture. When incidence angle gets big, the refracted ray can be totally reflected back into the substance, fig. 7.10.
Dispersion: prisms and rainbows: White light has spectrum of red (longest wavelength) through violet (shortest), with mnemonic ROY G. BIV. The index of refraction varies with the wavelength of light, so that there is more bending for shorter wavelengths. The “phenomenon of colors” or the “spectrum” of light is seen. The different wavelengths are spread out or dispersed, thus giving the effect the name “dispersion.” The higher the index of refraction the more the light is spread out, or dispersed. Thus a prism made of crown glass or of leaded crystal, both of which have higher n than regular glass, can disperse the colors better.
The best material is diamond, with index of refraction 2.42. (From comparison, water is 1.33, and normal glass is about 1.5.) Besides the good separation of the colors, the total internal reflection of diamond is more than for other materials.
Rainbow,
see highlight, page 160. You can see
rainbows in the sky when there are enough water droplets and when the sun is in
the right position, which is about 42 degrees above the horizon. Total internal reflection AND refraction are
occurring in a rainbow. Two internal
reflections produce a secondary rainbow.
Where else do you see rainbows?
Fooling around with the water hose—spraying fine mist of water. Oil on water, soap bubbles.
7.3 Spherical Mirrors. Concave, convex. Introduction to focal length. Rays hitting a concave mirror converge at what’s called the focal point. (Draw it.) Spherical and parabolic reflectors are used to collect light or radio waves at a point. Convex mirror = diverges light away from a point.
Ray diagrams. Won’t do these too muchly.
7.4 Lenses. Do all this as if it’s an introduction and they will continue on in physics… no, forget it.
Human eye: The main thing I want you
to know is the lens of the eye, the "crystalline lens,” can change shape and is
able to focus light on the retina. Know the difference between nearsightedness
and farsightedness in terms of where the image appears—in front of the retina
or behind it. Near: in front of
retina. Far: behind retina. And know which type lens corrects for each.
Diverging for nearsightedness, converging for farsightedness. Also, what happens to near point with
age? Recedes=gets further away. Inability of the eye’s lens to focus.
7.5 Polarization: Light is a transverse wave, its oscillation is perpendicular to its direction of travel. Looking at this wave I draw on the board, you can see its oscillation is up and down in the plane of the board. That means it is polarized vertically. Most light in the room is unpolarized—no preferred direction of oscillation. P. 171, fig 7.28. Polarization is the preferential orientation of the field’s oscillation (book says field vectors).
A Polaroid or polymer sheet will block all polarization directions except one. The light coming out of a Polaroid will be linearly polarized, as shown in fig. 7.29
"Crossed" Polaroids will block all the incoming light. Polaroid sunglasses reduce glare off of water and off of car windshields because in both cases reflected light is horizontally polarized and the sunglass lenses only pass light with a vertical polarization.
7.6 Diffraction and Interference
Diffraction—formation
of a new wave front because of waves striking a small object or going through a
small opening.
Interference—the
addition of the amplitudes of two or more waves. Constructive and destructive.
