1. 1 Dr. Mike Nofziger 2014 L8- The Geometry of Light : “How does light get from point A to Z?” Basic, fundamental Law of Optics: ”Light travels in straight lines…” - from a source of light… - after reflection… - after scattering… Light travels in straight lines. We call these straight-line paths “rays”

2. 2 Dr. Mike Nofziger 2014 L8- The Geometry of Light : “How does light get from point A to Z?” Extramission theory of vision ≡ Rays of light are emitted by the eye, that go out to ‘touch and interact’ with an object, then return into the eye, causing the sensation of vision. Based on the idea that the eye contained some “internal fire” which then interacted with the “external fire” of visible light. One small problem: “How do we see in the dark?” Developed by various Greek philosophers: Empedocles (400 BCE), Plato (400 BCE), Euclid (300 BCE) Ptolemy (2 nd century CE), Galen (2 nd century) Aristotle (383-322 BCE) actually ‘got it right’ but Plato’s model persisted….. Persisted for another 1000 years! Origin of the term “evil eye”……….!! NOT A CORRECT THEORY!!

3. 3 Dr. Mike Nofziger 2014 L8- The Geometry of Light : “How does light get from point A to Z?” This woodcut from Descartes' 1644 Principles of Philosophy diagrams Descartes' theory of vision and its interaction with the pineal gland. Descartes believed that light rays impressed subtle particles into the eyes. The image was then transmitted to the pineal gland, which served as the nexus between mind and body. In this sketch the external stimulus is translated into an act of will (pointing) by the pineal gland. REFERENCE REFERENCE

4. 4 Dr. Mike Nofziger 2014 L8- The Geometry of Light : “How does light get from point A to Z?” Intromission theory of vision ≡ Rays of light are emitted or reflected by objects, which enter (and are brought to focus by) the eye, causing the sensation of vision. Our modern, correct understanding of vision. Credit is given to great Arab scientist Alhazen (Ibn al-Haytham): - he lived 965-1040 CE - he wrote The Book of Optics - methodical and experimentally-based theories of sight, reflection, refraction, lenses, mirrors, the structure of the eye, pinhole cameras…..etc.! - first to prove that light travels in straight lines. In his own words: Reference “…from each point of every colored body, illuminated by any light, issue light and color along every straight line that can be drawn from that point.”

5. 5 Dr. Mike Nofziger 2014 L8- The Geometry of Light : “How does light get from point A to Z?” Fig. 1.1 Superman’s x-ray vision comes from this eyes, but the rest of us can only see if light comes toward our eyes.

6. 6 Dr. Mike Nofziger 2014 L8- The Geometry of Light : “How does light get from point A to Z?” “Flowchart for Light” Sources? Objects? Detectors? Most objects reflect (or scatter) light from a light source. Examples—any solid object (clothes, trees, cars, etc. etc. etc.) ….. ? If the object is also the source, it must give off (emit) light in order to be seen. This type of source is said to be self-luminous. Examples—lightning, fireflies, candle flames, neon signs, LCD screens, ….. ? “If no light gets to your eyes, you don’t see anything!” ….. SO, how DOES light get to your eyes, in order for you to see an object? “You can see an object only if it reflects, scatters, or emits light.” SOURCE→OBJECT→DETECTOR

7. 7 Dr. Mike Nofziger 2014 L8- The Geometry of Light : “How does light get from point A to Z?” Reflection: ► Specular Reflection ≡ “one ray in, one ray out” ► Diffuse Reflection ≡ “one ray in, an infinite number of rays out” (scattering from rough surfaces may be thought of as “diffuse reflection”) Reference

8. 8 Dr. Mike Nofziger 2014 L8- The Geometry of Light : “How does light get from point A to Z?” “Why are safety stickers placed on windows?” Glass safety decals: Reference Product Details: Easy to Apply Decals Reduce the Risk of Injury Due to Accidental Collision With Storefront Panels and Doors Decals Have "Floating" Appearance With today's architecture incorporating more glass than ever, it's important to reduce the risk of people being injured by accidental collision with glass panels and doors. CRL Glass Safety Decals installed near eye level give a "floating" appearance that serves to warn pedestrians of the presence of a large glass panels and sliding doors. All Glass Safety Decals are a white color, and use a special dual-sided adhesive transfer material which when peeled off leaves only the decal, with no unsightly residue to spoil the 'all-glass' appearance. Our Cat. No. 105 Plastic Scraper will give you firm, even pressure as you apply Safety Decals to clean glass.

9. 9 Dr. Mike Nofziger 2014 L8- The Geometry of Light : “How does light get from point A to Z?” “Why are safety stickers placed on windows?” Glass safety decals: Reference Glass Door/ Window Safety Decals Another great new item available from Dream Baby. Tired of running into glass patio doors? This is the answer to that problem! Glass/Window decals helps prevent kids, visitors, and special needs persons from walking into glass doors and windows. Transparent static cling decals, visible from both sides. Easy to apply, removable for glass cleaning and reuse. This value pack contains 8 strips each 8.25" long x 2.75" high. Durable and re-usable making it great for day care centers, churches and personal home use.

10. 10 Dr. Mike Nofziger 2014 L8- The Geometry of Light : “How does light get from point A to Z?” “Why do birds fly into really clean windows?” "When people think of bird mortality they usually think of cats, wind turbines, and pesticide poisoning, among other causes. In fact, in North America the largest number of birds are killed each year by glass! Windows in our homes and other buildings can be deadly to birds. Many people do not realize that birds are killed at the windows they look through unless they happen to be there when the bird hits the window...... Although the “incidental, accidental, or unintentional take” of migratory birds is a criminal violation of the Migratory Bird Treaty Act, the U.S. Fish and Wildlife Service does not arrest anyone when birds are killed at windows! This is a good thing, as windows kill lots of birds." Reference Reference

11. 11 Dr. Mike Nofziger 2014 L8- The Geometry of Light : “How does light get from point A to Z?” “Why do birds fly into really clean windows?” Acopian BirdSavers prevent birds from flying into windows. Very elegant solution to the major problem of bird window strike kills. Very simple design. Aesthetically pleasing. And it works! Reference

12. 12 Dr. Mike Nofziger 2014 L8- The Geometry of Light : “How does light get from point A to Z?” “Why are safety stickers placed on windows?” Glass safety decals: "Now You Can Protect Wild Birds From Striking Windows Millions of wild birds are killed each year flying into windows. Now you can save the lives of the beautiful birds you love." "WindowAlert is a window decal that may be applied to home and office windows. The decals contain a component which brilliantly reflects ultraviolet sunlight. This ultraviolet light is invisible to humans, but glows like a stoplight for birds. Birds have vision that is up to 12 times better than that of humans. WindowAlert decals help birds see windows and avoid striking the glass." Reference Reference

13. 13 Dr. Mike Nofziger 2014 L8- The Geometry of Light : “How does light get from point A to Z?” "Things like mirrors and windowpanes do not scatter much light (when they are CLEAN :) and are actually difficult to see." But when they are dirty..... Demo:

14. 14 Dr. Mike Nofziger 2014 L8- The Geometry of Light : “How does light get from point A to Z?” Fig. 1.2 The void of space looks dark at night even though lightbeams are crossing it. Reference "long exposure: painting the laser beam with a white card"

15. 15 Dr. Mike Nofziger 2014 L8- The Geometry of Light : “How does light get from point A to Z?” “Earthshine” ● NASA ● Leonardo de Vinci NASALeonardo de Vinci reference reference reference referencereference reference reference reference reference reference

16. 16 Dr. Mike Nofziger 2014 L8- The Geometry of Light : “How does light get from point A to Z?” Albedo (reflection coefficient) is the percent of total light that is reflected from a surface. Examples: ▪ Our Moon (average of ≈ 13%, dark areas ≈ 8-9%, bright areas ≈ 20-30%) ▪ The earth (average of ≈ 30%; snow ≈ 80-90%, clouds ≈ 50%, land ≈ 10-40%, oceans ≈ 10%) ▪ on a typical day, half the earth is covered by clouds Ref. ▪ clouds have greatest influence over the average albedo of the earthRef. ▪ Charcoal (≈ 4%; almost a perfect black) ▪ Bright white paper (≈ 95%; almost a perfect white) "The Man in the Moon, or the Rabbit.....??.....!! " The differences in reflectance, from different parts of the moon's surface, give rise to shadow effects that allow us to "see" different shapes in the moon: Maria are the dark areas, R ≈ 8-9%, Highlands are the bright areas R ≈ 20-30% ReferenceReference (click for larger image)

17. 17 Dr. Mike Nofziger 2014 L8- The Geometry of Light : “How does light get from point A to Z?” This photo was taken at f2.8 with a 40 second exposure time and ASA 1000 color print film on April 21, 1997, at about 9:20 PM. It looks like a daytime photo, except for the stars. The sky looked black to the eye. REFERENCE REFERENCE “Film Version” of Light Scattering

18. 18 Dr. Mike Nofziger 2014 L8- The Geometry of Light : “How does light get from point A to Z?” “Digital Version” of Light Scattering Summitt Photography

19. 19 Dr. Mike Nofziger 2014 L8- The Geometry of Light : “How does light get from point A to Z?” "Crepuscular Rays " - rays of sunlight that appear to radiate from a single point in the sky (where the sun is) - 3-dimensional shafts of sunlight..... or, are they..... - 'tubes' of shadowed air extending from the clouds? (your choice) - usually occur around the time of "crepuscular" hours (around dawn or dusk) - in fact, rays of sunlight are (almost) parallel - the apparent divergence of the rays is an illusion of perspective - THINK "railroad track" illusion

20. 20 Dr. Mike Nofziger 2014 L8- The Geometry of Light : “How does light get from point A to Z?” - but the rays really are parallel!: The following picture was taken from the International Space Station, looking down on the clouds (from above the crepuscular rays):

21. 21 Dr. Mike Nofziger 2014 L8- The Geometry of Light : “How does light get from point A to Z?” "Anti-Crepuscular Rays " - rays of sunlight that appear to converge to a single point in the sky (the anti-solar point) - usually occur around the time of "crepuscular" hours (around dawn or dusk) - the apparent convergence of the rays is an illusion of perspective - these rays are an extension of the crepuscular rays, across the entire sky - more 'rare' and not seen as often as crepuscular rays - THINK "railroad track" illusion, now looking towards the "opposite" horizon