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Understanding Light Interactions: Reflection, Absorption, and Transmission

Explore how organisms tackle visual tasks such as foraging and mate selection, and learn about light interactions including reflection, absorption, and transmission. Understand the principles behind measuring and calculating light properties, such as transmission and absorption, in different environments like terrestrial and aquatic settings. Discover the significance of absorption in biological materials like retinal pigments and the role of light in powering life through photosynthesis.

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Understanding Light Interactions: Reflection, Absorption, and Transmission

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  1. Lecture #3 What you see is what you get 1/31/13

  2. Homework • Problems up on web site • Due next Tuesday • Questions??

  3. What are organisms’ visual tasks?

  4. Foraging

  5. Finding / choosing mates

  6. Avoiding predators

  7. Knowing when to stop

  8. What happens to light when we see?

  9. Today’s topics • Reflection • Absorption / Transmission • Measuring fR, fA, and fT • Follow the photon’s path • Spectral properties of light environments • Terrestrial • Aquatic • Energy of a photon

  10. Light interactions • Matter will interact with light in one of 4 ways • Reflected • Absorbed • Transmitted = Refracted • Scattered • For now we will deal with transparent materials so scattering will be negligible

  11. Light interactions • Photons are conserved • Light going in must go somewhere • Iincident = ITrans + IReflect + Iabsorb = I0 • Express as fraction of I0 • fT + fR + fA = 1 • fT=fraction transmitted • fR=fraction reflected • fA=fraction absorbed Iabsorb I0 Itrans Ireflect

  12. θ1 = θ2 = 0 1. Reflection at interface • Light will reflect at interface between materials with different indices of refraction • For light perpendicular to surface n=1.0 Water n=1.33

  13. Reflection at biological interfaces is usually pretty small: air / water • fR, fraction reflected θ1 = θ2 n=1.0 Water n=1.33

  14. 2. Absorption • Light will interact with molecules in material • It can excite molecules. If it matches electron resonance, then it will be absorbed • If not, it will be transmitted • We see what is not absorbed

  15. In the following, we assume… • Reflection is pretty small • Then fT + fR + fA = 1 and fR ≈ 0 so that • fT + fA = 1 What does that mean???

  16. Calculating transmission – solution of concentration, C • Beer’s law εdepends on what substance is C is concentration l is the pathlength I0 I, light transmitted through l

  17. Calculating transmission - solution • Beer’s law ε depends on what substance is C is concentration l is the pathlength I0 I0 I I Low concentration High concentration Less absorbed More absorbed More transmitted Less transmitted

  18. Calculating transmission - solution • Beer’s law ε depends on what substance is C is concentration l is the pathlength I0 I0 I I Short pathlength Longer pathlength Less absorbed More absorbed More transmitted Less transmitted

  19. Calculating transmission - pure substance, like water • Beer’s law α is attenuation coefficient I0 I l

  20. Units all cancel so take exponential of a unitless number • ε length-1 concentration-1 = L-1 molecules-1L3 = L2/molecule l length C concentration = molecule / L3 • L-1 • l L

  21. Transmission / absorption depend on wavelength

  22. 3. Measuring transmission / absorption Measure I0 - just beam flashlight Fiber optic Spectrometer

  23. Measuring transmission /absorption Measure I with object in beam flashlight Fiber optic Transmission = I / I0 fT + fR + fA = 1 For small fR fA = 1-fT Spectrometer

  24. For reflective objects Specular reflection

  25. For opaque objects light scatters in all directions Specular reflection Scattered Reflected light vs scattered light

  26. Scattering / reflection depend on wavelength • n depends on λ

  27. Measuring reflection / scattering Fiber optic Light source Spectrometer How can we measure I0?

  28. Measuring reflection / scattering Fiber optic Light source Spectrometer Measure I0 of light Use white target that reflects all wavelengths

  29. Measuring reflection / scattering Fiber optic Light source Spectrometer Measure I reflected from object fRorS = I / I0 fRorS + fA + fT = 1 where reflection and scattering depend on angle For small fT fRorS = 1 - fA

  30. Examples of absorption and reflection • The return of the spectrometer

  31. Why does absorption matter? • Retinal pigments absorb certain wavelengths • Biological materials • Photosynthesis uses light to power life • Wavelengths scattered depend on absorption • Colors of animals, food • Define our environment

  32. 4. The photon’s path - How do we see? Sensitivity • Light from a source, I • Reflected by object, R • Detected by eye, S • Q = I * R * S Intensity Reflectance Q = quanta of light detected

  33. What light illuminates an object? • Irradiance • Light flux on a surface - from all directions • Photons /s m2 Irradiance

  34. Depending on detector set up, we might measure irradiance or radiance • Irradiance • Light flux on a surface - from all directions • Photons /s m2 • Radiance • Light flux from a particular direction and angle • Photons /s m2sr Radiance Irradiance

  35. Light measurement • Many light meters measure watts / m2 • Watts are joules / s and so are related to photons / s • We’ll convert that in a minute • Some light meters measure lux • This is like watts / m2 but they take human sensitivity into account

  36. Lux meter (measures irradiance – all angles) Bright sunlight 20,000 lux

  37. Eyes respond to photons • Eye doesn’t care about watts • Chemical reactions in eye detect individual photons

  38. Energy of light source is given in watts 75 W light bulb 5 mW laser

  39. How many photons in a Watt • Watt is a measure of power = energy / time • 1 watt = 1 J/s • Convert watts to photons

  40. Energy of a photon – thank Planck • E = hf = h c / λ • h is Planck’s constant = 6.6256 x 10-34 Js • For 400 nm light: • E = (6.6256 x 10-34 Js) (2.998 x 108m/s) • 400 x 10-9 m • E = 4.96 x 10-19J per photon

  41. Energy of photon determines #photons/watt Red laser More photons per W at longer wavelength

  42. Red laser • Laser power is 3 mW at 650 nm • # photons/s = Power • energy per photon • = 0.003 W • 3.0x10-19J/photon • = 9.8 x 1015 photons / s

  43. 5. Natural light sources • Lots of variation in natural light • Light at high noon • Light at dawn, dusk • Light at midnight • Light in forest • Light at ocean surface • Light 100 m depth • Illuminant shapes what we can see

  44. Light environment : sky conditions

  45. Light environment : sky conditions

  46. Light environment : sky conditions

  47. Environment : Lighting conditions

  48. Light environment : Viewing angle

  49. Light environment : time of day

  50. Light environment : Time of day

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