Light, Photon Energies, and Atomic Spectra

1. Light, Photon Energies, and Atomic Spectra

2. Questions this answers… • Why does FM sound better than AM radio waves? • Why am I able to tune in to AM radio stations hundeds of miles away, when FM stations fade in 30-50 miles (or sometimes less)? • Why is the sky blue? • Is “blue-ray” really better? • Why do they put that thing on me when I get an x-ray?

3. Electromagnetic Radiation • Electromagnetic radiation (radiant energy) is characterized by its: • wavelength (color):  (Greek letter lambda) • frequency (energy): (Greek letter nu)

4. Electromagnetic Radiation • They are related by the equation: • where c = 3.00 x 108 m/s (the speed of light in a vacuum)

5. Waves • Wavelength = distance between successive “crests” • Frequency = the # of crests passing a given point per second

6. Example: The frequency of violet light is 7.31 x 1014 Hz, and that of red light is 4.57 x 1014 Hz. Calculate the wavelength of each color. 1 Hz = sec-1 or 1/sec Violet:

7. Example: The frequency of violet light is 7.31 x 1014 Hz, and that of red light is 4.57 x 1014 Hz. Calculate the wavelength of each color. 1 Hz = sec-1 or 1/sec Red:

8. Light • When sunlight or white light is passed through a prism, it gives the continuous spectrum observed in a rainbow. • We can describe light as composed of particles, or PHOTONS. • Each photon of light has a particular amount of energy (a quantum). • The amt. of energy possessed by a photon depends on the color of the light.

9. Max Planck (1858-1947) In 1899 a German physicist named Max Planck solved the problem by making a daring assumption. Energy comes in chunks of some minimum size. Planck gave the name “Quantum” (meaning fixed amount) to the smallest quantity of energy

10. Photon Energy • The energy of a photon (single quanta) is given by this equation: • where h = 6.6262 x 10-34 J•s ν = frequency (Hz) Plank was awarded the 1918 Noble Prize.

11. Example: Calculate the energy, in joules, of an individual photon of violet and red light. Violet:

12. Example: Calculate the energy, in joules, of an individual photon of violet and red light. Red:

13. What does this have to do with electron arrangement in atoms? • When all electrons are in the lowest possible energy levels, an atom is said to be in its GROUND STATE. • When an atom absorbs energy so that its electrons are “boosted” to higher energy levels, the atom is said to be in an EXCITED STATE.

14. Bright Line Emission Spectrum • The light emitted by an element when its electrons return to a lower energy state can be viewed as a bright line emission spectrum.

15. Absorption Spectrum • The light absorbed by an element when white light is passed through a sample is illustrated by the absorption spectrum. • Note: The wavelengths of light that are absorbed by the gas show up as black lines, and are equal to the wavelengths of light given off in the emission spectrum. • Why? • Absence of color appears black. Only reflected colors of light are visible.

16. Bright Line Spectra Each element & compound emits a characteristic and unique set of wavelengths. As the temperature of the gas changes, its spectral lines change in intensity; some lines fade out, some appear. The pattern for each substance is the same as for dark line spectra.

17. Note the same pattern for a given element.

19. Light • Electronic energy is quantized (only certain values of electron energy are possible). • When an electron moves from a lower energy level to a higher energy level in an atom, energy of a characteristic frequency (wavelength) is absorbed. • When an electron falls from a higher energy level back to the lower energy level, then radiation of the same frequency (wavelength) is emitted. • The bright-line emission spectrum is unique to each element, just like a fingerprint is unique to each person.

20. Example: A green line of wavelength 486 nm is observed in the emission spectrum of hydrogen. Calculate the energy of one photon of this green light. Green:

21. Example: The green light associated with the aurora borealis is emitted by excited (high-energy) oxygen atoms at 557.7 nm. What is the frequency of this light? Green: