Light The Wave Speed Equation Using the wave

Light & The Wave Speed Equation Using the wave speed equation to solve problems about electromagnetic waves

Wave Speed • Wave speed tells you how fast a wave is moving • Wave speed is measured in units meters per second (m/s)

The Wave Speed Equation • v – wave speed (m/s) • λ – wavelength (m) • f – frequency (Hz) v = λf (Wave speed = wavelength times frequency) • When solving problems about EM waves, the wave speed is always equal to the speed of light: v = c = 3 x 108 m/s

The Wave Speed Equation Practice: Red visible light has a wavelength of 6. 5 x 10 -7 m. What is the frequency of red light? Given in the problem. Not given in the problem. This is the speed of light, which is the value you use for ALL electromagnetic waves. Solution: G (givens): λ = 6. 5 x 10 -7 m v=c=3 x 108 m/s U (unknown): f = ? E (equation): v = λf S (substitute): 3 x 108 m/s = 6. 5 x 10 -7 m × f S (solve): see next slide

The Wave Speed Equation Practice: Red visible light has a wavelength of 6. 5 x 10 -7 m. What is the frequency of red light? Solution: S (substitute): 3 x 108 m/s = 6. 5 x 10 -7 m × f Divide both sides by 6. 5 x 10 m to cancel S (solve): 3 x 108 m/s = 6. 5 x 10 -7 m × f out this number on the right-hand side -7 -7 and get the frequency (f) on its own. 6. 5 x 10 Hz 6. 5 x 10 m 3 x 108 m/s = f On the left-hand side, group together your numbers and powers of ten. -7 6. 5 x 10 Hz Simplify each group separately. -7 When you divide powers of ten, you SUBTRACT their exponents. f = 0. 462 x 1015 Hz = 4. 62 x 1014 Hz Write your final answer in correct scientific notation.