Much as I venerate the name of Newton, I am not therefore obliged to believe he was infallible. I see... with regret that he was liable to err, and that his authority has, perhaps, sometimes even retarded the progress of science. Thomas Young (c. 1829), quoted in Mason's A History of the Sciences

Assignments: Read Chapter 4 in your text

In Class: -------------------------------------------------------------- review: waves are the propagation of energy through a medium medium sets the propagation speed - usually has something to do with how well bits of the medium are connected; communication periodic trains of waves can be defined by their frequency,-- number a waves to pass a given point per unit of time number per sec, or /sec , or Hz period, -- time betwen adjacent crests note: period = 1/frequency and by wavelength - distance between crests of a wave Note: wavelength/period = speed we will more often see this as: wavelength x frequency = speed allows for conversion between frequency and wavelength, IF you know the wave speed ------------------------------------------ The real wave phenomenon we're interested in is light Actually, the idea that light is a wave phenomenon is pretty new - Newton firtst showed that white light was a mixture of colored lights - prism (everyone knew that, though <-- New age fairs) - all thought that the prism changed the light didn't just split it up. - Newton showed that the prism could also be used to make white light from the colors - proved that white light really was a mixture - Newton decided that light was a made of elementary colored particles, which traveled through space - explained a lot - prisms split up the particles - light could travel through empty space - light stopped or bounced off objects - however, Newton's corpuscular theory could not explain diffraction and interference. - diffraction you've seen (and heard) - phenomenologically, it's the ability of waves to go around corners - see it in harbors, sometimes - waves, breakwater - waves come in and can wrap around breakwater - essentially, the opening is like a point source because the water is pushed up and down at the opening - NOTE: That's a wave explanation - this is what happens with light, too. and it can't be explained by the corpuscular theory - light bullets should just pass through no wrapping around the edges. If that's not bad enough, you've got interference to deal with too. - make a breakwater with two breaks - each creates a diffraction pattern - but a bit downstream, the waves overlap - start by looking at the line in the center of the pattern - here, wave peaks from each opening meet at the same time (cuz they travel the same distance to get here) - therefore, the combined effect of the waves from both openings is a really big peak. - likewise valleys meet at the same time, so the combined effect is really big valleys. - result: big amplitude waves bad place to park your boat. called CONSTRUCTIVE INTERFERENCE - now consider one of the grey lines, offset a bit from center - here, the distance from one opening is shorter than the distance from the other opening, so wave peaks from both openings DON'T arrive at the same time - instead, a wave peak from one opening arrives at the same time as a valley from the other opening. - when the waves combine, result is nothing. - later when a valley from one opening combined with a peak frmo the other, still nothing. - result: no waves here. good place to park your boat. called DESTRUCTIVE INTERFERENCE - If light is a wave, then places where there's constructive interference (big amplitude waves) are bright, and places where there destructive interference (zero amplitude waves) are dark. - again, it's really impossible to make interference happen if light is particulate - what would you see behind two slits? -- two points of light. It really wasn't until the 1820's that people confronted this evidence that light was wavelike - probably partly because of Newton's stature science is funny that way The above two-slit experiment was conducted by Thomas Young in 1829 and showed that light really needed to be a wave. take two slits: - illuminate them - should see a pattern of light and dark "fringes" --> doesn't work - so light isn't a wave - that's what others said - but Th. Young thought otherwise - if the slits are too large, or too far apart - no interference - what means too large, or too far apart - compared to the wavelength of the wave - when he tried with smaller slits more closely spaced - he saw the pattern of dark and light Young discovered not only that light was a wave but that the wavelength of visible light is really small. Furthermore, the colors of light correspond to specific wavelengths - the angle at which constructive interference occurs is dependent on the wavelength of the light - so different wavelength light will produce a wider or narrower fringe pattern. - Thomas Young used the fringe spacing, and the distance between his slits to measure the wavelength of light -- very small; less than a millionth of a meter ranges from 400nm to 700 nm (nm = 10^-9) blue to red Because the angle of constructive interference depends on wavelength, a two-slit interference pattern of white light looks like a rainbow - diffraction gratings (which probably should be called interference gratings) are glass with thousands of tiny lines ruled on them - each pair of lines creates a tiny slit, and light from adjacent slits interferes to produce Young's pattern. take a look So, light must be a wave - diffraction, interference colors correspond to wavelengths of light - really small - blue = 400 nm - red = 700 nm --> what about longer wavelengths or shorter wavelengths? - what's so special about 400-700 nm? - absolutely nothing (except we can't see other wavelengths of light) - light can have any wavelength at all - long wavelength light --> radio waves -- meters in size - short wavelength light --> x-ray -- atomic sizes - light is nothing more than a wave - visible light is a wave of special wavelengths that we can see - all light waves move at the same speed - radio waves, xrays, visible light - it's only their wavelength (and frequency, of course) that differ. - we'll see next time that different wavelengths of light can give us different information about the universe, mainly because of the different ways light is produced.