BIEB 166 Lecture 20 (WI13)

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University of California - San Diego
Biol/Ecology, Behavior, & Evol
BIEB 166
James Nieh

Lecture 20 What is light? - Most energy animals communicate with is from the sun ○ Some of the animals use energy they produced themselves Energy characteristic - Light is a form of electromagnetic energy , with oscillating electric field and oscillating magnetic field at right angles to each other ○ An electromagnetic wave, produced by an oscillating electron ○ Electric field generating magnetic field, and the magnetic field generating magnetic field, etc - Fastest transmitting energy in the universe - EM radiation is generated when the motion of electrons in atoms and molecules is accelerated due to input energy - The vibrating negatively charged electron generates an electric field which induces a magnetic field. Coupled electric and magnetic oscillations move outward at the speed at which they mutually induce each other indefinitely = c Particle characteristics Quanta Travel in a vacuum Photons production - When electrons are accelerated by close approach to ions - As temperature rises, photon production increases - Some of the kinetic energy of the electron is absorbed by the ion, the electron has to enter a higher orbital state ○ Higher orbital state are higher energy ○ Inherently unstable ○ Decay ○ Release energy as a photon of light Atomic resonance modes - A molecule can absorb EMR if the energy of the wave is equal to the energy needed to push the molecule into a higher oscillatory state - Molecules have resonant frequencies at which they can absorb the corresponding frequencies of light ○ Generate photon of specific frequency ○ Laser - Photon coming in, exciting an ion - Electron move to a higher orbital state (unstable), decay down (fall back down to a lower energy state), and produce another photon - Only certain amount of energy can be absorbed What makes objects transparent? - They do not absorb photons - No material is transparent to all kinds of photon - Glass is largely transparent, doesn't allow photons to be absorbed or trapped Wave characteristics Frequency - Frequency = speed/wavelength - The frequency of light is extremely high, due to the speed of the light - Wavelength is very short Reflection, refraction, and diffraction 1. Positively and negatively interference with each other - In-phase and out of phase 2. Light is white because it combines all the other colors of light to produce white color 3. Refraction - Light in traveling through the prism is bent ○ From medium of different densities, the light will be both reflected and refracted - Refraction is the bending of a light ray as it passes into a medium with a different speed of light ○ Degree of refraction depends upon light frequency and change in the speed of light in the medium - Frequency dependent refraction ○ Short wavelengths refracted more strongly ○ Blue is spread more than the red 4. Why are diamonds so beautiful? - Diamond is extreme8y dense - C vacuum= 3.0 * 10 - C diamond 1.25 * 10 8 ○ Speed is cut to less than 50% ○ Light would be strongly refracted 5. Diffraction - Bending of light around an object or an edge/particle of similar wavelength size - When an object is similar in size to a light wavelength, then that light is bent around the object Light travels in a vacuum - Different from sound - Fastest in a vacuum (not necessary to interact with anything) Spreading loss and attenuation Polarization (light that has all the same E-field orientation) - Filtering of light such that the resultant electric field lines align along a single plane - Selectively blocking more and more polarized light - Polarization exist - There are certain materials that can filter out light, and that light only of a single axis can passes through Waves coming in from different directions + angles - A horizontal screen will block all the waves except the horizontal one - Another vertical screen will block the remaining wave - Polarizer Bees - Honeybees used polarized light patterns to determine the sun's position - Look up in the sky, see the polarized light - The atmosphere acts as a filter and filtering out certain axis of polarization - Insects/arthropods The natural spectrum of electromagnetic radiation - The spectrum is very large - EM radiation occurs over wide ranges in frequency and wa-14ength from largest radio waves (0.1 to 1000m) to smallest x-rays and cosmic rays (10 to 10 m) - Visible light is 10 scattering RAYLEIGH m - Gamma waves: higher energy, energy in the individual waves and photons that they can damage biological structures The transmission properties of long versus short EMR 1. Long wavelength EMR - Everything from near ultra-violet to radio waves ○ All of these sources of waves are easily absorbed 2. Short wavelength EMR - X-ray - High frequency ultra-violet radiation - Gamma waves - Pass through tissues without interacting with them - Breaking molecular and atomic bonds Four ways molecules can oscillate - Wave can stimulate a molecule or an atom - A molecule or an atom can also generate EMR if it's a photon or an ion 1. Translational movements - Molecules move to new location - Heat (mo
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