Lecture #3

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