PHYS 219 Spring semester Lecture 17: Light Rays and the Reflection of Light. Ron Reifenberger Birck Nanotechnology Center Purdue University

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1 PHYS 219 Spring semester 2016 Lecture 17: Light Rays and the Reflection of Light Ron Reifenberger Birck Nanotechnology Center Purdue University Lecture 17 1 A Few Issues What is the difference between TOTAL GRADE and RUNNING GRADE in Blackboard? Total Grade is the current grade out of everything this semester, including all items even though they may not have come due. Running Grade is the current grade just including assignments that have already come due. Running Grade will tend to be higher now, but at the end of the semester both grades will agree. 2 1

Optics - Historical Context Optics is arguably the oldest discipline in

$The most commonly observed optical phenomena are refraction.$ in Anatolia from polished copper in Mesopotamia and Egypt around 4000 to

$reflection and Refraction: lenses Nimrud lens from ancient Assyria dates$ $940-1000), an Iraqi mathematician, discovered law of refraction$ primitive optical instruments - microscope, telescope, etc. in the 1600s.

primitive optical instruments - microscope, telescope, etc. in the 1600s.

massive galaxy cluster Abell 2744 Image credits: NASA/ESA/STScI UGC 1810

2 Optics - Historical Context Optics is arguably the oldest discipline in physics What are the laws governing how light propagates? The most commonly observed optical phenomena are refraction. Reflection: mirrors from obsidian (volcanic glass) about 6,000 B.C. in Anatolia from polished copper in Mesopotamia and Egypt around 4000 to 3000 B.C. reflection and Refraction: lenses Nimrud lens from ancient Assyria dates to ~750 B.C. Ibn Sahl (c ), an Iraqi mathematician, discovered law of refraction Understanding the physics of reflection and refraction leads to primitive optical instruments - microscope, telescope, etc. in the 1600s. 3 Four Hundred Years Later telescopes and microscopes in the 2000s massive galaxy cluster Abell 2744 Image credits: NASA/ESA/STScI UGC 1810 is a spiral galaxy located 300 million light-years away Different organelles, including the nucleus and components of the cytoplasmic cytoskeleton, have been labelled with fluorescent dyes 4 Image Credits: Harvard Medical School. 2

Optics The study of light and how light interacts with matter Light itself is very

Particle-like behavior (photons) Quantum optics Where to begin?

light ray straight line and an arrow Arrows point in the direction of energy flow

3 Optics The study of light and how light interacts with matter Light itself is very enigmatic since it shows two separate and distinct faces Wave-like behavior Particle-like behavior (photons) Quantum optics Where to begin?? 5 Ray diagrams Representing how light travels from one location to another Define a light ray straight line and an arrow Arrows point in the direction of energy flow Diagrams that include light rays are called ray diagrams Rays are a mathematical model, not a physical entity Examples: 6 3

Which is Correct? Each point on an extended light source emits light in many different directions. This light can be represented by multiple rays diverging from that point.

4 Which is Correct? Each point on an extended light source emits light in many different directions. This light can be represented by multiple rays diverging from that point. Defines the ray model of light 1 click 7 Light Rays Explain Shadows and Semi-shadows A sharp shadow is called an umbra (Latin: umbra for shadow ). A shadow is a region behind the object where no light reaches. A semi-shadow is called a penumbra (Latin: paene "almost, nearly" plus umbra "shadow" A semi-shadow is a region where some light reaches and some does not. It appears as a fuzzy shadow From experiment, only point sources cast an umbra alone 8 4

History records that ~2000 years after completion, in the 6th

How did he do it? And what value did he obtain for H?

H S 2m 3m H 208m 2 H 2 08m139m 3 9 Two clicks Innovation: Pinhole

effect of an inverted image forming through a pinhole Before the

5 Example In ~2600 B.C., the Egyptians constructed the Great Pyramid of Giza. History records that ~2000 years after completion, in the 6th century B.C., Thales measured the height of the Great Pyramid. How did he do it? And what value did he obtain for H? S H S=208 m the Giza pyramid h=2 m the staff of Thales s=3 m h s H S 2m 3m H 208m 2 H 2 08m139m 3 9 Two clicks Innovation: Pinhole Camera Light tight box with a small hole: pinhole camera, also called a camera obscura. In 5 th century BC, Mozi ( 墨子 ) in ancient China mentioned the effect of an inverted image forming through a pinhole Before the invention of modern cameras with lenses, pinhole cameras were used to make photographs. CAMERA light tight box pinhole Inverted image from pinhole camera Viewing screen One click 10 5

Vermeer and the Camera Obscura Dutch artist Johannes Vermeer (1632-75) was revered for his

The painting is now declared too precious to be loaned. Interesting coincidence?

6 Vermeer and the Camera Obscura Dutch artist Johannes Vermeer ( ) was revered for his strikingly realistic 17th century paintings because they were able to capture light and detail like never before. Vermeer s masterpiece Girl with a Pearl Earring has been called the Mona Lisa of the North. The painting is now declared too precious to be loaned. Interesting coincidence? Antony van Leeuwenhoek who invented the optical microscope was the executor of Vermeer's will. 11 Reflection of light When viewed from above, light from a laser pointer reflects from a vertical mirror. Top View 12 6

Reflection of light: Definitions Incident light: the light striking the mirror Normal line: a line perpendicular to the surface where the incident light hits the mirror.

7 Reflection of light: Definitions Incident light: the light striking the mirror Normal line: a line perpendicular to the surface where the incident light hits the mirror. Forms the reference line for angle measurement. Angle of incidence: the angle between the incident beam and the normal line Angle of reflection: the angle between the reflected beam and the normal line 13 Law of reflection 14 7

8 The law of reflection is used to deflect light in many ways -θ Normal line -θ New reflected ray Original reflected ray Establish sign conventions for rotation: CW rotation is + CCW rotation is Initially, incident light rotated through (+2) Mirror rotates through ( θ) Reflected light now rotates through +2(-θ) Change in angular deflection: +2(-θ) - 2 = -2θ Light rotated CCW by 2θ 15 EXAMPLE Two plane mirrors meet at an angle φ. A single ray reflects successively from both mirrors. What is angle? How does depend on angle of incident beam? Normal lines incident ray reflected ray Assume φ < 90 and that only two reflections, one from each mirror, take place. 1. From ABC From ABD At pointd

9 Specular vs. diffuse reflection 17 Up Next Bending of Light (Refraction) 18 9

How Microscopes Work By Cindy Grigg

How Microscopes Work By Cindy Grigg By Cindy Grigg 1 Inventions often lead scientists to make new discoveries. One of the most important discoveries in life science was the microscope. A microscope is used for looking at things too small