Chapter 6 Electromagnetic Radiation wavelength Visible light Amplitude

Chapter 6: Electromagnetic Radiation wavelength Visible light Amplitude wavelength Node Ultraviolet radiation

Short wavelength --> high frequency high energy Long wavelength --> small frequency low energy

The electromagnetic spectrum.

Which has the longest wavelength? 1. 2. 3. 4. Infrared Ultraviolet X-rays Radio waves

Rank the following in order of increasing frequency: microwaves radiowaves X-rays blue light red light UV light IR light

• Waves have a frequency • Use the Greek letter “nu”, units are “cycles per sec” , for frequency, and = c • All radiation: • • c = velocity of light = 3. 00 x 108 m/sec • Long wavelength small frequency • Short wavelength high frequency

The Wave Nature of Light • The product of the frequency, (waves/sec) and the wavelength, l (m/wave) would give the speed of the wave in m/s. – In a vacuum, the speed of light, c, is 3. 00 x 108 m/s. Therefore, – So, given the frequency of light, its wavelength can be calculated, or vice versa.

The Wave Nature of Light • What is the wavelength of yellow light with a frequency of 5. 09 x 1014 s-1? (Note: s-1, commonly referred to as Hertz (Hz) is defined as “cycles or waves per second”. ) – If c = , then rearranging, we obtain = c/

The Wave Nature of Light • What is the frequency of violet light with a wavelength of 408 nm? – If c = , then rearranging, we obtain = c/.

What is the wavelength of WONY? What is the wavelength of cell phone radiation? Frequency = 850 MHz What is the wavelength of a microwave oven? Frequency = 2. 45 GHz

Quantization of Energy Light acts as if it consists of particles called PHOTONS, with discrete energy. Energy of radiation is proportional to frequency E = h • h = Planck’s constant = 6. 6262 x 10 -34 J • s

E = h • Relationships:

Short wavelength light has: 1. High frequency and low energy 2. High frequency and high energy 3. Low frequency and low energy 4. Low frequency and high energy

Rank the following in order of increasing photon energy: microwaves radiowaves X-rays blue light red light UV light IR light

Energy of Radiation What is the frequency of UV light with a wavelength of 230 nm? What is the energy of 1 photon of UV light with wavelength = 230 nm?

What is the energy of a photon of 525 nm light? 1. 2. 3. 4. 3. 79 x 10 -19 J 4. 83 x 10 -22 J 3. 67 x 1020 J 8. 43 x 1023 J

Radio Wave Energy • What is the energy of a photon corresponding to radio waves of frequency 1. 255 x 10 6 s-1?

What is the energy of a mole of 230 nm photons? Can this light break C-C bonds with an energy of 346 k. J/mol?

Does 1200 nm light have enough energy to break C-C bonds?

Where does light come from? • Excited solids emit a continuous spectrum of light • Excited gas-phase atoms emit only specific wavelengths of light (“lines”)

Light emitted by solids

Light emitted by hydrogen gas

The Bohr Model of Hydrogen Atom • Light absorbed or emitted is from electrons moving between energy levels • Only certain energies are observed • Therefore, only certain energy levels exist – This is the Quanitization of energy levels

Emission spectra of gaseous atoms • Excited atoms emit light of only certain wavelengths • The wavelengths of emitted light depend on the element.

Line spectra of atoms

Energy Adsorption/Emission

For H, the energy levels correspond to: Constant = 2. 18 x 10 -18 J Energy level diagram:

Each line corresponds to a transition: Example: n=3 n = 2

Explanation of line spectra Balmer series