Waves and the EM Spectra College Chemistry

Waves Light travels similar to boat waves or a sine/cosine graph (up and down motion) Has amplitude, wavelength, frequency, and speed Waves transfer energy, NOT matter

Waves Amplitude – height of wave Brightness of wave (light) depends on amplitude Wavelength – distance between two crests or two troughs, one complete cycle Frequency – how fast the wave oscillates up and down Speed – light moves at a CONSTANT speed Speed of light: 3.00 x 108 m/s

Waves Speed, frequency, and wavelength are all interrelated and dependent on one another l = c/v l = wavelength c = speed of light (3.00 x 108 m/s) v = frequency (Hertz or Hz)

Example 7.1 Calculate the speed of a wave whose wavelength and frequency are 17.4 cm and 87.4 Hz, respectively. l = c/v 17.4 = c/87.4 Hz c = 1.52 x 103 cm/s

Light Light waves are a type of electromagnetic radiation, or EM Electromagnetic radiation – consists of electric and magnetic fields oscillating perpendicular to one another

Electromagnetic, EM, Spectrum All light waves fall onto the EM spectra!! This includes visible light that we see, and non-visible light that we do not see Visible light takes up less than 1” on the whiteboard of all light!!

EM Spectrum

Example 7.2 The wavelength of the green light from a traffic signal is centered at 522 nm. What is the frequency of this radiation? l = c/v 522 nm x (1 x 10-9 m / 1 nm) = 5.22 x 10-7 m 5.22 x 10-7 m = 3.00 x 108/v v = 5.75 x 1014 Hz WAVELENGTHS MUST ALWAYS BE IN METERS FOR THIS EQUATION! ANSWERS WILL BE IN METERS TOO!

Beginnings of Quantum Theory m- Max Planck Planck noticed the radiation (or energy) emitted by an object changes at different temperatures He suggested that there was a restricted on the amount of energy an object emits or absorbs – called these pieces of energy a quantum Quantum – fixed amount

Max Planck Think of a quantum as a staircase – you can be on one stair or another stair, but not in between This quantum of energy = 6.63 x 10-34 J/s All objects have to absorb/emit energy that are multiples of this number!!! We don’t notice it, because this is an extremely small number

Photoelectric Effect In the photoelectric effect, electrons are ejected from the surface of a metal when lights shines on it A minimum amount of energy is needed to release electrons This is how different colors from fireworks are made! Einstein proposed light consisted of quantas of energy - photon

Photoelectric Effect Photon – particles of light that each must possess energy E = hv E = energy v = frequency h = Planck’s constant (6.63 x 10-34 s) Planck’s theory did explain the photoelectric effect satisfactorily  but it did not explain the wavelike behavior of electrons…

Dual Nature of Energy Energy is a particle and behaves like a particle (collides with other particles) Energy is also a wave and behaves like a wave (travels at speed of light and has a frequency/wavelength) Check Out: http://www.youtube.com/watch?v=yDCCbxCJLlM

Example 7.3 Calculate the energy (in J) of a photon w/ a wavelength of 5.00 x 104 nm (IR) and We know E = hv AND v = c/l 1. 5.00 x 104 nm (1.0 x 10-9 m / 1 nm) = 5.00 x 10-5 m 2. v = 3.00 x 108 m/s / 5.00 x 10-5 m = 6 x 1012 Hz 3. E = (6.36 x 10-34 J*s)(6 x 1012 Hz) = 3.8 x 10-21 J

Meters to nanometers 1 m = 109 nm 1 nm = 10-9 m Memorize!