The Physics of Light

1. The Physics of Light

2. The Energy for Life • Radiant energy from the sun powers all life on earth • The amount of radiant energy that reaches the earth from the sun each day is the equivalent of about 1 million Hiroshima-sized atomic bombs (that’s a lot) • One third of this energy is radiated back into space • Most of the remainder is absorbed by the earth and converted to heat • Less than 1% is captured and utilized by the process of photosynthesis to provide the energy that is needed for all the activities of life on earth

3. Photons • Units of light energy • Familiarly known as “rays” or “waves” of electrons from the sun • When photons are absorbed they boost electrons of the absorbing atom to a higher energy level • A photon of light - produced whenever an electron in a higher-than-normal orbit falls back to its normal orbit • During the fall from high-energy to normal-energy the electron emits a photon – a packet of energy – w/ very specific characteristics  the photon has a frequency, or color that exactly matches the distance the electron falls

4. Energy Content of Photons The amt of energy is inversely related to the wavelength of light The shorter the wavelength, the greater the energy of each photon in light • Short wave-length light is photons of higher energy • Seen as violet • Long wave-length light is photons of lower energy • Seen as red

5. Electromagnetic Spectrum • The types of electromagnetic radiation are: • radio • infrared • visible light • ultraviolet • X rays • Gamma rays • Electromagnetic energy travels in rhythmic waves – like those created by dropping a pebble into a puddle of water http://fizyka.phys.put.poznan.pl/~pieransk/Physics%20Around%20Us/Physics%20around%20us.html

6. http://www.ualr.edu/~botany/emspectrum.gif

7. http://physics.uoregon.edu/~jimbrau/BrauImNew/Chap03/FG03_09.jpghttp://physics.uoregon.edu/~jimbrau/BrauImNew/Chap03/FG03_09.jpg

8. Light Terminology • Wavelength = which is the distance btwn any 2 corresponding points on successive waves – peak to peak or trough to trough • Frequency = the number of waves that pass a point in a space during any time interval – usually in one second • Measured in units of cycles (waves) per second or Hertz (Hz) http://library.thinkquest.org/03oct/02144/glossary/wavelength.html

9. http://geography.uoregon.edu/shinker/geog101/lectures/lec01/lec01_figs/wavelength-and-frequency-fig2-5.gifhttp://geography.uoregon.edu/shinker/geog101/lectures/lec01/lec01_figs/wavelength-and-frequency-fig2-5.gif

10. Ultraviolet Light • Sunlight contains abundant U.V. light • Ultraviolet light has a shorter wavelength than visible light and therefore has higher energy • UV waves are invisible to the human eye, some insects, like bumblebees, can see them • It was an important source of energy in the primitive earth • Ozone now shields much of the U.V. light • It’s UV light that causes sunburns!

11. Absorption Spectrum • A specific atom or molecule can absorb only certain photons of light • It depends on the electron energy levels available in the atom or molecule

12. Capturing Light Energy in Chemical Bonds • CAROTENOIDS • Carbon rings • Absorb photons over a wide range of energies • Not highly efficient • Examples are beta-carotene, vitamin A, and retinal • CHLOROPHYLLS • Porphyrin rings • Photons absorbed by chlorophyll excite electrons of magnesium atoms • Two types: • Chlorophyll a - absorbs in the violet-blue range (red is seen) • Chlorophyll b - absorbs in the red range (violet-blue is seen)

13. http://www.mbari.org/staff/ryjo/cosmos/Cabs.html

14. http://www.uic.edu/classes/bios/bios100/lecturesf04am/absorption-spectrum.jpghttp://www.uic.edu/classes/bios/bios100/lecturesf04am/absorption-spectrum.jpg