Farbe. Physics of Color

Transcription

1 Farbe Physics of Color

2 Light Basic Properties visible light is electromagnetic radiation in a particular region of the entire spectrum distinguishing criterion: its frequency ~ nm THz AM radio FM radio & TV mobile tel. microwaves infrared visible ultraviolet X-rays f λ = c λ (nm) wavelength frequency f (Hz) Werner Purgathofer 2

3 Light Spectrum normally, a ray of light contains many different waves with individual frequencies the associated distribution of wavelength intensities per wavelength is referred to as the spectrum of a given ray or light source nm Werner Purgathofer 3

4 Light Particles or Waves? light waves usually propagate according to the laws of geometric optics (as photons) however, certain properties can only be described by taking their wave nature into account parallel light photons Werner Purgathofer 4

5 Light Particles or Waves? light waves usually propagate according to the laws of geometric optics (as photons) however, certain properties can only be described by taking their wave nature into account parallel light waves Werner Purgathofer 5

6 Light Particles or Waves? a photon (from Greek φῶς, phos = light) is the elementary particle responsible for electromagnetic phenomena, the carrier of electromagnetic radiation photons have wave and particle properties ( wave-particle-duality ) energy of a photon: E = h f h Planck constant [Planck sches Wirkungsquantum] ( h = 4, evs ) f frequency (in Hertz) Werner Purgathofer 6

7 Wave Types longitudinal wave = compression wave vibrations parallel to direction of travel example: sound transverse wave = swinging wave vibrations perpendicular to direction of travel example: water, light Werner Purgathofer 7

8 Light Coherence light is a transverse wave its frequency alone is sometimes not sufficient to describe a given wave train temporal coherence: how monochromatic? spatial coherence: cross-correlation between points in a wave polarization coherence Werner Purgathofer 8

9 Light Polarization polarization describes the oscillation direction of a transverse wave. light is called polarized if all waves oscillate the same way. in 2 dimensions oscillation distribution can be linear, circular, elliptical Werner Purgathofer 9

10 Linear, Circular and Elliptical Polarization Werner Purgathofer 10

11 Circular Polarization Werner Purgathofer 11

12 Polarization Examples PiccoloNamek normal photo photo with polarization filter Werner Purgathofer 12

13 Polarization Examples Adrian Gonsalves/Flickr circular polarized glasses for 3D cinemas Werner Purgathofer 13

14 Polarization Examples AntiG reflections are often polarized: window seen without/with polarized sunglasses Werner Purgathofer 14

15 Polarization Examples Edmund Optics reflections are often polarized: photo without/with polarization filter Werner Purgathofer 15

16 Causes for Light Color for directly viewed emitters, only their characteristics have to be considered in all other cases, the interaction of light with the objects in a scene is light in at least partially responsible for diffuse absorption the perceived reflection color of an object specular reflection Werner Purgathofer 16 scattering and emission internal reflection transmitted light

17 Energy Levels & Visible Range a light frequency corresponds to a wavelength and has an energy level E = const/λ = h f h...planck constant [Plancksches Wirkungsquantum] f...frequency (waves/sec) AM radio FM radio & TV mobile tel. microwaves infrared visible ultraviolet ~ nm ~ THz X-rays (nm) wavelength frequency (Hz) Werner Purgathofer 17

18 Quantum Properties of Matter atoms & molecules have stable ground states by inserting energy (e.g. by heat, electricity or photons) they can reach an excited state several excitation levels are possible excited states are usually not stable: by emission of a quantum of energy (e.g. a photon or heat) the system returns to a lower energy state (decay) Werner Purgathofer 18

19 Example Hydrogen Atom ground state: single electron in lowest possible orbit (1s) excited state: electron moves to higher orbit (e.g. 2p) decay: atom emits photon to return to lower energy state energy absorption photon emission Werner Purgathofer 19

20 Multiple Decays ground state: electron in a stable orbit excited state: electron moves to higher orbit 2 decays: 1. atom emits photon to return to lower energy state 2. atom emits another photon to return to stable state photon energies characterize the matter (spectrum) energy absorption photon emission photon emission Werner Purgathofer 20

21 Quantum Properties of Matter quantum systems such as molecules can only exist in certain discrete states only certain transitions between these states are allowed the states correspond to energy levels E = const/λ = h f Werner Purgathofer 21

22 Laser-Light Light Amplification by Stimulation of Emitted Radiation produces temporal and spatial coherent light all photons are in phase is monochromatic main difference to ordinary light: it propagates in almost perfectly straight line intensity nm red laser spectrum example Wikipedia Werner Purgathofer nm

23 Light and Biology bond-dissociation energy [Bindungsenergie] of most molecules > 3 ev 750 nm wave has 1,65 ev 380 nm wave has 3,26 ev ( E = h f with h = 4, evs 750 nm 0, Hz 750 nm has 1,65 ev ) atmosphere absorbs higher energy waves rhodopsine and lodopsines have to be able to react chemically no vision beyond 780 nm possible Werner Purgathofer 23