Chapter 28 Reflection

1
Chapter 28Reflection Refraction
2
Specular and Diffuse Reflection

• Reflection from most surfaces is diffuse, with
  incoming light scattered in all directions.
• Reflection from mirror surfaces is specular, with
  light reflecting in one direction.

Mirror
Specular reflection
Diffuse reflection
3
Check Yourself

• What objects in this room radiate light?
• Is the projector screen radiating visible light?
• Is the screen diffuse or specular reflector?

4
Diffuse Surface is Not a Mirror

• Light rays diffusely scatter in every direction
  from every spot on the surface.

Look at this spot on a diffuse surface
See a light ray from the top of your head
reflected from this spot. Also see light ray
from the tip of your nose reflected from this
spot.
5
Pinhole Camera

• Small pinhole allows only small amount of light
  in, blocking overlapping diffuse rays and forming
  image inside the camera.

6
Camera Obscura

• The camera obscura (room darkened) dates to
  ancient times it was first detailed in the
  writings of Leonardo da Vinci. 
• A room is completely sealed from light except for
  a coin-sized hole in one wall. An image of the
  outside world appears projected, upside down and
  reversed right-to-left, onto a wall opposite the
  opening.
• A portable camera obscura, equipped with a mirror
  at a 45-degree angle, reflects the image upward
  and focuses it on a viewing screen.

Diffuse light
Artists secret drawing weapon?
7
Drawing Skill or Optical Aids?
Was the chandelier drawn by van Eyck using
optical aids, such as a camera obscura?
Portrait of Giovanni Arnolfini and his Wife
(1434) by Hubert and Jan van EYCK
8
Transparent, Reflecting Surfaces

• Water and glass are transparent but reflect some
  of the light striking their surface.

Can see inside the cab through the right
window. Strong reflection of the sun on the left
window.
Can see some of the riverbed through the water
surface.
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One Way Mirror

• One way mirror is just clear glass window.

GLASS
Observation Room (Dark)
Discussion Room (Bright)
Reflected light from bright room masks
transmitted light from the darkened observation
room
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Law of Reflection

• Angle of incidence equals angle of reflection

SPECULAR REFLECTION
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Demo Blackboard Optics

• Special blackboard has mounted light source and
  mirrors.

Light Source
Demonstrates the Law of Reflection
Mirror
12
Ray Tracing

• Light ray from point A is reflected at point B
  and arrives at point B.

Notice the relationship between point B (where
the ray is reflected to) and point B (where the
ray would go if the mirror wasnt there).
13
Demo Who Do You See?

• Which person in the front row sees the guy with
  the hat (person F) in the mirror?

A
B
C
D
E
G
F
MIRROR
14
Mirror Image Reflection

• To observer at point B, the light from point A
  seems to come from point A, within mirror.

15
Plane (Flat) Mirror

• Tracing light rays from original, to mirror, to
  eye allows us to construct image.

HAIR
NOSE
CHIN
Original
Mirror
16
Demo Pocket Mirror

• See full image of your head in a plane mirror
  thats half the size of your head.

Mirror
Image
Original
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Image in the Mirror
Image is seen an equal distance from original
within a plane mirror.
Utamaro, Woman Powdering Her Neck
Velazquez, Venus at her Mirror
Incorrect
Incorrect
Helmut Newton, Bergstrom over Paris
Correct
18
Non-Flat Mirrors

• Image in a curved mirror is distorted from
  original.
• Determine image location and size by ray tracing

19
Demo Convex Mirrors

• Mirrored surface on a sphere or similar rounded
  shape. Image is smaller closer.

MC Escher Hand with Reflecting Sphere
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Demo Blackboard Convex Mirror

• Can use optics blackboard to trace diverging rays
  from convex mirror.
• Law of reflection still holds but mirror surface
  is curved.

21
Ray Tracing for Convex Mirror

• Image in convex mirror is smaller closer.

Image
Original
Mirror
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Image in the Mirror 2
Portrait of Giovanni Arnolfini and his Wife
(1434) by Hubert and Jan van EYCK
23
Demo Concave Mirrors

• Mirror is on the inner curved surface of sphere
  or similar rounded shape.
• Image size and location depends on objects
  distance from the mirror.

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Demo Blackboard Concave Mirror

• Use optics blackboard to trace convergence of
  rays from concave mirror.
• Notice that rays focus to a point (the focal
  point) then diverge, as with convex mirror.

Focal Point
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Ray Tracing for Concave Mirror

• Image in concave mirror is larger when original
  is close to the mirror.

Image
Mirror
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Ray Tracing for Concave Mirror

• Image in a concave mirror is inverted if the
  original is far away from the mirror.
• Image is formed in front of the mirror, not
  within it.

Mirror
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Demo Forming a Real Image

• Locate the image formed in front of a concave
  mirror using a strong light source and a screen.

Candle
Candle
Image
Image
Screen
Screen
28
Reflecting Telescopes

• Large, modern telescopes form images using
  concave mirrors.

Magellan Telescope
Hubbles mirror
29
Refraction

• Light rays bend (refract) passing from water to
  air, making objects appear to be shallower and
  closer to the observer.

Observer sees image
Image
Image
Actual
Actual
30
Law of Refraction

• Light passing from one material to another is
  refracted by a fractional angle that depends on
  the optical density of each material.
• Angle is smaller in the denser material.

31
Demo Refraction thru Block

• Light is refracted entering the block and
  refracted back on leaving the block.

32
Check Yourself

• Which path does light ray take after entering the
  wedge?
• Which path does it take after leaving the wedge?

A
B
C
WEDGE
33
Lenses

• Curvature of a lens surface produces a
  continuous, variable angular refraction.

Concave lens shrinks its image
Convex lens magnifies its image
34
Demo Concave Lenses

• Curved surface of a concave lens causes light
  rays to diverge, shrinking images.

35
Demo Convex Lenses

• Curved surface of a convex lens causes light rays
  to converge, magnifying images

36
Demo Real Image of Convex Lens

• Image formed by convex lens can be observed on a
  screen.

37
Total Internal Reflection

• When refraction angle exceeds 90º the light does
  not cross the surface.

Refracted
Reflected
Reflected
38
Demo Total Internal Reflection

• Prism demonstrates total internal reflection if
  the angle of incidence is large enough.

39
Looking up Underwater

• Try this when youre in the pool this summer

Looking straight up you see the sky but outside
the 96 cone surface is like a mirror
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Demo Total Internal Reflection

• Past the critical angle all the light is
  internally reflected.

Just below critical angle
41
Fiber Optics

• Total internal reflection causes light to reflect
  inside a solid glass tube.

42
The Big Picture

• The whole purpose of this course was to open your
  eyes to the universe.

The most incomprehensible thing about the world
is that it is at all comprehensible. Albert
Einstein
Camera obscura next to the Cliff House, San
Francisco