MOTION MOUNTAIN

what is light? 101
Challenge 108 s
Ref. 61
F I G U R E 58 Some interference patterns: the interference that a playing guitar produces in laser
holography that show how the body vibrates, the interference produced by a good parabolic telescope
mirror of 27 cm diameter, a speckle laser pattern on a rough surface and the diffraction pattern
produced by two parallel narrow slits illuminated with green light and with white light respectively
(© Bernard Richardson, Cardiff University, Mel Bartels, Epzcaw and Dietrich Zawischa).
ment of light waves were performed between the seventeenth and the twentieth century.Forexample,in1800,WilliamHerscheldiscoveredinfraredlight
using a prism and
a thermometer. (Can you guess how?) In 1801, Johann Wilhelm Ritter (b. 1776 Samitz,
d. 1810 Munich) a more than colourful figure of natural Romanticism, discoveredultraviolet
light using silver chloride, AgCl, and again a prism. Modern cameras can image
infrared light, as shown beautifully in Figure 59.
At the end of the twentieth century a beautiful confirmation of the oscillations in light
waves became possible. Using quite sophisticated experiments, researchers measured the
oscillation frequency of light directly. They actually managed to count how often light
Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net