"First I prepared a tube of lead, at the ends of which I fitted two glass lenses, both plane on one side while on the other side one was spherically convex and the other concave. Then placing my eye near the concave lens I perceived objects satisfactorily large and near, for they appeared three times closer and nine times larger than when seen with the naked eye alone." Galileo, The Starry Messenger, 1610

Assignment: Reading: Explorations, pp. 98-112. Sign up for a time slot for Observing Lab #2. Problem Set #4 is due on 29 September at 1:00 pm

In Class: Question to Ponder A "middle C" on a piano has a frequency of 256 Hz. If the speed of sound is 330 m/s, what is the wavelength of these sound waves? a) 84480 m b) 1.29 m c) 0.78 m d) 330 m e) 74 m So What is a Wave, Anyway? A disturbance in the medium The disturbance propagates at a well-defined speed, which is determined by the properties of the medium. Wavelength is peak-to-peak (or valley-to-valley) distance. Units are any length units. Frequency is how often (e.g., how many times per second), a peak (or valley) passes by a particular point. Units are "per second," called "Hz." Wavelength, frequency, and speed are related as follows: wavelength x frequency = speed Young Measures the Wavelength of Visible Light The pattern of interference that he sees in his two-slit experiment depends on both the wavelength of the light and the spacing between the two slits. Thus, he can measure the wavelengths of visible light. He finds that wavelength corrresponds to colour, and that wavelength increases from that of blue light, through the colors of the rainbow to red light. Blue light has a wavelength of about 400 nm (1 nm, or 1 "nano-meter" is one billionth of a meter (i.e., 1 nm = 1 x 10-9 m). Red light has a wavelength of about 700 nm. These are incredibly small wavelengths, so it's no big surprise that no one notices the wave effects of light before Young. Different Kinds of Light If light is a wave, why can't it have any old wavelength? I mean why does it have to be between 400 nm and 700 nm? Light can have any old wavelength: (Source: MicroWorlds -- Lawrence Berkeley Labs) Light waves range in wavelength from the very short (i.e., X-rays and gamma rays) thourgh the optical, into the infrared, microwave, and radio and TV. All are forms of light, and all move at the speed of light. From the wavelengths, we can get the frequencies of light: For example, green visible light has a wavelength of 500 nm Since wavelength x frequency = speed, frequency = speed/wavelength = 3 x 108 m/s / 500 x 10-9 m = 5 x 1014 1/s = = 5 x 1014 Hz This is incredibly frequent! 1014 waves per second. No wonder people didn't recognize light as a wave. Catching Light with Telescopes We need telescopes to help us collect the faint light from distant celestial objects so that we can analyze the light and learn something about the composition and state of the material. Telescopes help with both magnification and sensitivity. Though magnifacation (eg., making thisngs appear bigger) is good, it's useless unless the resolution (i.e., "clearness") of the view is comparably good. The aperture, or diameter of the telescope determines both the sensitivity and resolution of the telescope -- bigger is better. Types of telescopes Reflectors vs. refractors: Largest optical telescopes are now reflectors, because mirrors are easy to support from behind (lenses can only be supported from the edges, and really big lenses are so heavy that they sag under their own weight). Really large mirrors are now constructed from mutiple segments of mirror aligned very carefully.