Properties of Light: Understanding Waves, Interactions, and Spectra - Prof. Gerald S. Wilk, Study notes of Biology

Download Properties of Light: Understanding Waves, Interactions, and Spectra - Prof. Gerald S. Wilk and more Study notes Biology in PDF only on Docsity! Properties of Light •  Waves, particles and EM spectrum •  Interaction with matter •  Absorption •  Reflection, refraction and scattering •  Polarization and diffraction •  Reading foci: pp 175-185, 191-199 not responsible for boxes 7.1 and 7.2 EM Wave •  EM waves are generated by vibrating electrons •  Composed of two perpendicular oscillating fields •  Can be characterized by its frequency, which is inversely related to wavelength (f = c / λ) •  Shares with sound the properties of spreading loss, attenuation, reflection, refraction, and diffraction, but can travel in vacuum Interaction with matter •  All molecules have resonant frequencies at which they trap particular EM wavelengths •  Long wavelength EM (radiowaves) is not absorbed by biological materials •  Short microwaves increase atomic motion, especially water, and create heat •  Infrared radiation also increases atomic motion of some molecules and is perceived as heat –  Pit vipers and vampire bats have IR sensors •  Shortest x-rays and cosmic rays destroy molecules •  Energy in visible light is absorbed without damage to cells –  Special molecules (visual pigments) make use of changes in electron orbital states Absorption by water depends on wavelength Characteristic attenuation length (m) 107° 10-8 Infrared Ultraviolet Radio waves 1 L 1 1 i L L 1 1 107!2 30-19 30-8 10° 107 107 1 102 104 ~—-10® Wavelength (m) Absorption vs propagation of light •  If light wave frequency = molecules resonant frequency –  Light is absorbed (propagation is stopped) –  Medium is opaque •  If light wave frequency ≠ molecules resonant frequency –  Light is reradiated and propagated forward through medium –  Medium is transparent •  In a solid or liquid, propagation is in straight line •  In a gas, propagation is less organized and scatter increases Refraction Light Refraction Light Refraction Through Glass and Water by Water Figure 1 Figure 3 http://micro.magnet.fsu.edu/optics/lightandcolor/refraction.html Large angle causes internal reflections {B) air water Fee es ee = Receptor = Bose setts cell Lenses focus light Image Formation with a Convex Lens Focal Distance Focal Object Point Convex Lens Focal" Figure 2 Plane http://micro.magnet.fsu.edu/optics/lightandcolor/lenses. html Scatter depends on wavelength Intensity of scattering = constant / wavelength4 Blue is scattered more than yellow Sun is yellow, sky is blue E2 Scattered waves Direct waves Earth Polarized Light Light Waves Vibrating Perpendicular to the Highway ie Waves ibratin Paralle to the Highway Figure 2 http://micro.magnet.fsu. edu/primer/java/scienceopticsw/polarizedlight/filters/index.html Measuring light intensity •  Irradiance is total amount of light incident on a surface –  Includes scattered light –  Measured with a 180° lens (photographic light meter) •  Radiance is light emitted from specific area –  Measured with tube over area of interest –  Must specify angle of measurement •  Both measurements can be made wavelength- specific by filtering out other wavelengths –  A series of measurements creates an irradiance or radiance spectrum (in wavelengths) Radiance spectra example (courtesy of Karen Carleton) Metriaclima zebra ‘gold’ Absorption depends on frequency 0 meters 10 100+ Figure 7.12 Irradiance frequency spectra in water at different depths. The intensity of longer wavelengths in the red region drops off very quick- ly with increasing depth because of selective absorption. Ultraviolet and violet wavelengths are also absorbed. The consequence is an increasingly monochromatic blue medium at greater depths. (From Waterman ol 1981, based on Lundgren and ’ Hejerslev 1971.) Wavelength (nm) Irradiance (mW / cm2m) Refraction causes rainbows Light Diffraction http://www.physics.uoguelph.ca/applets/Intro_physics/kisalev/java/slitdiffr/index.html Propagation of light through a slit demonstrates wave properties. Cancellation and addition of diffracted waves results in striped pattern in contrast to what would be expected by particles. Why light works for visual communication •  Can be absorbed without damage to cells •  Most abundant wavelengths •  Reflects off solid objects –  Higher and lower frequencies pass through or bend around •  Straight-line transmission without scatter –  Permits formation of spatial maps •  Refraction at the boundary of two media –  Permits focusing and image formation by an eye behind a lens •  Frequency dependent effects influence colors