Transcript Document
Lecture series for Conceptual Physics, 8th Ed. 1 Electromagnetic Waves p452 Shaking a stick in water produces waves. Shaking a charged stick in space produces electromagnetic waves. The moving stick is an oscillating current… which produces an oscillating magnetic field… which produces an oscillating current. ETC. 2 Electromagnetic Wave Velocity p453 Light has a constant speed because of the Law of Conservation of Energy. The oscillating E-field creates the oscillating B-field and so on… If the E-field oscillation were weaker, then the B-field would be weaker and then the next Efield would be weaker and then…and then. James Clark Maxwell calculated the speed that light would have to travel in order to insure this mutual self inductance. He solved one the the “mysteries of the universe”, light was 3 On a date with his fiancé that evening he remarked that he was the only person in the world who knew what starlight was. The electromagnetic Spectrum p454 4 More on the E-M Spectrum Our eyes are sensitive to a small portion of the EMSpec. called “visible light”. We have gadgets that produce and receive other EM waves. p455- “the universe is a dense sea of radiation in which occasional concentrates are suspended.” Recall that v = f Light is a wave, so now, c = f . Where c is the speed of light, 300,000 km/s… f is the frequency in Hertz… is the wavelength in meters. 5 Transparent Materials p456 Here’s a clever analogy: Sound waves make the next tuning fork oscillate if the tines oscillate at that frequency . The frequency of visible light is more than 1014 Hz. Electrons can vibrate at that frequency. Remember that this is just a model. In the ultraviolet range glass electrons resonant with UV waves and generate heat. In the IR range, molecules of glass resonate and produce heat. In the visible range glass electrons can absorb and re-emit photons without heating up. 6 More on Transparent Materials A photon enters the glass. It is absorbed and re-emitted straight ahead at intervals. In the glass light travels slower than in air, 0.67c. 7 Check out BQ p 458. Opaque Materials p 458 Most things are opaque. They absorb light without re-emission. They warm up. The free electrons in metal allow photons to be absorbed and re-emitted as a “bounce.” 8 Does 70% of the focusing. Relaxed does 30% for the “infinity” setting. Page 462 Lens also accommodates… The muscles around it squeeze it into a more spherical shape to bring close objects into focus. The “blind spot” is surprisingly apparent when you stare at the dot with your right eye from the right distance. Rods- black/white Cones- color Cones are more concentrated at the fovea. 9 Peripheral vision Iris changes to make pupil large or small. Size of pupil is affected by emotions as well as light intensity. Hence, a candle-lit meal. Here’s another thing about your vision: Both sides are equally bright. Don’t see it? Cover the middle. p465 AND, lastly… 10 The end