Transcript Reflection & Refraction (Chap. 28)

Chapter 28
Reflection &
Refraction
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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
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Diffuse reflection
Specular reflection
Check Yourself
What objects in this room radiate light?
Is the projector screen radiating visible light?
Is the screen diffuse or specular reflector?
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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.
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Pinhole Camera
Small pinhole allows only small amount of
light in, blocking overlapping diffuse rays
and forming image inside the camera.
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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
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Artist’s secret
drawing weapon?
Drawing Skill or Optical Aids?
Was the chandelier drawn by van Eyck using
optical aids, such as a camera obscura?
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Portrait of Giovanni Arnolfini and his Wife (1434)
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by Hubert
Transparent, Reflecting Surfaces
Water and glass are transparent but reflect
some of the light striking their surface.
Can see some of the riverbed
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through
Can see inside the cab through the right window.
Strong
reflection
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One Way Mirror
Discussion
Room (Bright)
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Bright Room
GLASS
One way mirror is just clear glass window.
Dark Room
Reflected light from bright room
masks transmitted light from the
darkened
observation room
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Observation
Room (Dark)
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.
Reflected
Light
Source
Incident
Demonstrates the
Law of Reflection
Mirror
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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 wasn’t there).
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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
MIRROR
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F
G
Mirror Image Reflection
To observer at point B, the light from point A
seems to come from point A’, within mirror.
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Plane (Flat) Mirror
Tracing light rays from original, to mirror, to
eye allows us to construct image.
HAIR
NOSE
CHIN
Image
Original
Mirror
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Demo: Pocket Mirror
See full image of your head in a plane mirror
that’s 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.
Velazquez, Venus at her Mirror
Utamaro, Woman Powdering Her Neck
Incorrect
Incorrect
Helmut Newton,
Bergstrom over Paris
Correct
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Non-Flat Mirrors
Image in a curved mirror is
distorted from original.
Determine image location
and size by ray tracing
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Demo: Convex Mirrors
Mirrored surface on a sphere or similar
rounded shape. Image is smaller & closer.
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MC Escher
Hand with
Reflecting
Sphere
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.
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Ray Tracing for Convex Mirror
Image in convex mirror is smaller & closer.
Image
Original
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Mirror
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Image in the Mirror #2
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Portrait of Giovanni Arnolfini and his Wife (1434)
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by Hubert
Demo: Concave Mirrors
Mirror is on the inner curved
surface of sphere or
similar rounded shape.
Image size and location
depends on object’s
distance from the mirror.
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Telescope
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.
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Focal Point
Ray Tracing for Concave Mirror
Image in concave mirror is larger when
original is close to the mirror.
Original
Mirror
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Image
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.
Image
Original
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
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Screen
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Reflecting Telescopes
Large, modern telescopes form images
using concave mirrors.
Magellan
Telescope
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Hubble’s
mirror
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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
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Actual
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.
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Demo: Refraction thru Block
Light is refracted entering the block and
refracted back on leaving the block.
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Check Yourself
Which path does light
ray take after
entering the wedge?
A
Which path does it take
after leaving the
wedge?
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D
B
C
WEDGE
E
F
Lenses
Curvature of a lens surface produces a
continuous, variable angular refraction.
Concave lens
shrinks its image
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Convex lens
magnifies its image
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Demo: Concave Lenses
Curved surface of a concave lens causes
light rays to diverge, shrinking images.
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Demo: Convex Lenses
Curved surface of a convex lens causes
light rays to converge, magnifying images
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Demo: Real Image of Convex Lens
Image formed by convex lens can be
observed on a screen.
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Total Internal Reflection
When refraction angle exceeds 90º the light
does not cross the surface.
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Demo: Total Internal Reflection
Prism demonstrates total internal reflection if
the angle of incidence is large enough.
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Looking up Underwater
Try this when you’re 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
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Fiber Optics
Total internal reflection
causes light to reflect
inside a solid glass tube.
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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
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