1 / 45

Outline Of Today’s Lecture

In this lecture, we will explore the facts of light, including its wave-like and particle-like properties. We will also discuss the evolutionary advantages of being sensitive to electromagnetic energy and how light plays a crucial role as a messenger in our world. Additionally, we will delve into the concept of polarization and demonstrate its effects using filters. Join us for an enlightening session on the fascinating world of light.

pearsona
Download Presentation

Outline Of Today’s Lecture

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Outline Of Today’s Lecture • “The Facts of Light” • “The Birds & The Bees” • Demo on “Smoked” Filters • The Human Eye: Physiological Optics

  2. Part 1 “The Facts Of Light”

  3. Part 1: “The Facts Of Light” • Light is the region of electromagnetic radiation spectrum that humans can typically see (~400 to ~700 nanometers). • Light has both wave-like and particle-like properties. • Let’s consider some different types of waves….

  4. Part 1: “The Facts Of Light” • Let’s “Do the Wave”! • Stadium-Style = Transverse Wave • Transverse Wave - A wave in which oscillations are perpendicular to the direction the wave travels. • Electromagnetic energy is a transverse wave.

  5. Part 1: “The Facts Of Light” • Let’s “Do the (other) Wave”! • Geek-Style = Longitudinal Wave • Longitudinal Wave - A wave in which oscillations are parallel to the direction the wave travels. • Acoustic energy is a longitudinal wave.

  6. Part 1: “The Facts Of Light” • Now, let’s consider the particle-like properties of light. • The smallest (particle or) unit of light is the photon. • Not all photons are equal!

  7. Part 1: “The Facts Of Light” • Some photons oscillate at relatively high rates. These are said to be of high energy. • High energy photons travel a short distance over the course of each oscillation, so they have short wavelengths. • Short-wave (i.e., high energy) light typically appears blue-ish to most humans. • On the other hand….

  8. Part 1: “The Facts Of Light” • Some photons oscillate at relatively low rates. These are said to be of low energy. • Low energy photons travel a long distance over the course of each oscillation, so they have long wavelengths. • Long-wave (i.e., low energy) light typically appears red-ish to most humans.

  9. Part 1: “The Facts Of Light” “The rays are NOT colored.” Sir Isaac Newton

  10. Part 1: “The Facts Of Light” “The red’s in the head.” Billy Wooten

  11. Part 1: “The Facts Of Light” • The four F’s of evolutionary survival (according to Terry Sejnowski) • Fighting • Fleeing • Feeding • Reproduction

  12. Part 1: “The Facts Of Light” • What are the evolutionary advantages of being sensitive to electromagnetic energy (versus some other energy form)? • Electromagnetic energy travels quickly, thereby conveying (almost) immediate information about distant objects, food, predators, potential mates, etc.. • Electromagnetic energy tends to move in straight lines, thereby preserving information about the shape of objects, food, predators, potential mates, etc..

  13. Part 1: “The Facts Of Light” • What are the evolutionary advantages of being sensitive to light, per se (versus some other part of the E.M. spectrum)? • Light is “bouncy”. Unlike longer-wave energy, which passes through many opaque objects, light can be reflected (‘bounced’) off of objects, making them visible. Light’s a better messenger. • On our planet, light is plentiful. By contrast, shorter-wave energy tends to be absorbed by our atmosphere (i.e., by oxygen and nitrogen), and is less plentiful.

  14. Part 1: “The Facts Of Light” • Summary of evolutionary advantages to using light as the “messenger”. • Plenty fast. • Plenty straight. • Plenty bouncy. • And just-plain-old, PLENTY!

  15. Part 1: “The Facts Of Light” • Regarding the evolutionary advantage to using light… • “The Eyes Have It! ” • Some eyes even more so than others….

  16. Part 2 “The Birds & the Bees” (Demo on Polarized Filters)

  17. Part 2: Polarized Filter Demo • Some eyes are sensitive to the manner in which light is polarized --that is-- to the plane of polarization (plane of oscillation). Bow-&-Arrow Demo • Some birds and insects have eyes that can pick-up polarization information. • Humans, other mammals, and other so-called “higher vertebrates” are blind to polarization information.

  18. Part 2: Polarized Filter Demo • Put on your polarized filters, and close your left eye. • Take one of the filters from a second pair, and rotate it in front of your right eye. • Repeat this with the other filter.

  19. Part 2: Polarized Filter Demo • What’s happening with the polarized filters? • On a given pair of filters, one filter blocks certain planes of polarization, and “passes” others. • The other filter does the same for a complimentary set of planes. • By properly orienting two polarized filters, you can block all the light.

  20. Profile of Light Reaching Retina No Filter 100 Number of Photons 0 Orientation A Orientation A + 90 degrees Plane of Polarization

  21. Profile of Light Reaching Retina A filter that passes Plane A, but blocks the perpendicular plane. 100 Number of Photons 0 Plane A Plane A + 90 degrees Plane of Polarization

  22. Profile of Light Reaching Retina A filter that blocks Plane A, but passes the perpendicular plane. 100 Number of Photons 0 Plane A Plane A + 90 degrees Plane of Polarization

  23. Profile of Light Reaching Retina Two properly aligned filters will block (nearly) all the light 100 Number of Photons 0 Plane A Plane A + 90 degrees Plane of Polarization

  24. Part 3 Demo on “Smoked” Filters

  25. Part 3: “Smoked” Filter Demo • Put a “smoked” filter in front of your right eye. • Take a smoked filter from a second pair, and rotate it in front of your right eye. • Does orientation matter in the same way that it did for polarized filters?

  26. Part 3: “Smoked” Filter Demo • What’s happening with the “smoked” filters? • On the first pair, your filter partially attenuates all planes of polarization equally. • Your second filter does the same. It attenuates whatever is remaining from the first filter. Again, all planes of polarization are equally attenuated. • Unlike polarized filters, orientation makes no difference for smoked filters.

  27. Profile of Light Reaching Retina No Filter 100 Number of Photons 0 Orientation A Orientation A + 90 degrees Plane of Polarization

  28. Profile of Light Reaching Retina A single smoked filter attenuates all planes equally 100 Number of Photons 0 Orientation A Orientation A + 90 degrees Plane of Polarization

  29. Profile of Light Reaching Retina A second smoked filter further attenuates what little light passes through the first filter. Again, all planes are reduced equally. 100 Number of Photons 0 Orientation A Orientation A + 90 degrees Plane of Polarization

  30. Part 3: “Smoked” Filter Demo • The point is …. • If I placed just one of the filters before your eye, you could NOT determine whether (A) the light was reduced equally across all planes of polarization, or (B) the reduction was polarization-plane specific. • This is because, as a mammal, you are “polarization blind”. • Not all animals are polarization blind. • In principle, sensitivity to the plane of polarization could guide migration patterns in birds.

  31. Learning Check • Be able to draw a luminance-by-polarization profile for polarized filters. • Be able to draw a luminance-by-polarization profile for “smoked” filters.

  32. Part 4 The Human Eye: Physiological Optics

  33. The Eye & Physiological Optics The eye is said to have two optical components: The Cornea & The Lens

  34. The Eye & Physiological Optics • Image formation on the retina depends on the cornea. • The cornea refracts (or “bends”) light. That is, it changes the direction of light entering the eye. • Because the cornea is convex, it makes incoming light rays converge to a focal point. • The refractive power of the cornea, or any other optical device, is measured in diopters (40 for the cornea).

  35. The Eye & Physiological Optics Diopter = 1 / focal length in meters Focal length is the distance from the optical device, to the focal point. Focal Point Focal Length

  36. The Eye & Physiological Optics • Image formation on the retina also depends on the crystalline lens. • Unlike the cornea, the crystalline lens can change shape, providing additional optical power of 10 to 30 diopters. This variability in the lens’ shape is called accommodation. • Together, the cornea and the lens provide refraction ranging from 50 and 70 diopters --at least in healthy, young eyes.

  37. The Eye & Physiological Optics Myopic Near-sighted The eye ball is “too long”, given the refraction. Retina Can be corrected with a concave “-” lens.

  38. The Eye & Physiological Optics Hyperopic Far-sighted The eye ball is “too short”, given the refraction. Retina Can be corrected with a convex “+” lens.

  39. The Eye & Physiological Optics Presbyopic “Old Sight” Caused by sclerosis (hardening) of crystalline lens, i.e., reduced refraction. Corrected with bifocals Reading is corrected with a strong convex lens. Distance can be corrected with a weaker convex lens.

  40. The Eye & Physiological Optics • Demo on lenses. • Two lenses will be passed around the classroom. • One is a +3 diopter (convex) lens. The positive value indicates an increase in refractive power. • One is a -3 diopter (concave) lens. The negative value indicates an decrease in refractive power. • Put one on top of the other, and they “cancel”.

  41. The Eye & Physiological Optics • How is image size measured on the retina? • Size (or retinal subtense) is expressed in degrees of visual angle. • If a 1-centimeter long stimulus is viewed from 57.29 centimeters away, the stimulus subtends 1 degree of visual angle on the retina. • One degree of visual angle corresponds ~300 micrometers (millionths of a meter) on the retina.

  42. The Eye & Physiological Optics Visual Angle = Arctan (stimulus size / viewing distance) The arctangent is the “inverse” of the tangent function. We use arctan when we know the lengths, but need to find the angle.

  43. The Eye & Physiological Optics • “Rule of thumb” - your thumb nail subtends approximately 1 degree of visual angle, when your arm is fully extended. • The full moon subtends approximately 0.5 degrees of visual angle. • What about the “blind spot”?

  44. The Eye & Physiological Optics • The “blind spot” is a perceptual phenomenon that arises from the optic disc (an anatomical structure on the nasal side of each retina). • It is vulgar to call the optic disk ‘the blind spot’. • The blind spot is approximately 6 degrees of visual angle in diameter, so the optic disk subtends about 1,800 micrometers on the retina. • The blind spot is “many, many moons” big! (approximately 12).

  45. The Eye & Physiological Optics Blind Spot Demo Hand-out

More Related