electromagnetic waves  transfer of energy by a disturbance
		• Young’s experiment  wave nature •  = wavelength = length between crests/valleys •  = frequency = number of crests passing per unit time •                        = cycles per sec (Hz) = 1/P • P = period = time for wave to repeat itself   = c (speed of light - celeritas) = 3 x 1010 cm/sec       = 300, 000 km/s visible light:  4000    8000 Ångstrom (1Å = 10-8 cm) For a star, let B= brightness = power/area = power/4d2 If L = total output of star, L = Bx4d2  and B = L/4d2