CHAPTER 1314 Reflection and Refraction of Light

1
CHAPTER 13-14Reflection and Refraction of Light
A is A Something cannot be itself and something
else at the same time. Aristotle Light exhibits
wave-like properties when studied under certain
conditions. Light exhibits particle-like
properties when studied under certain
conditions. Light does not exhibit wave-like and
particle-like properties simultaneously.
2
Properties of Light
Speed In a vacuum (c) 2.997924574x108m/s
c 3.00x108m/s
In other mediums Speed of light is less than c.
Nothing can travel faster than the speed of light.
Direction Light travels in a straight line path
until it encounters a boundary between two
different mediums.
3
Ray Model of Light
Reflection Light rays reflect (bounce) off the
surface of a new medium that it encounters in a
very predictable fashion.
The Normal is a line perpendicular to the
surface of the second medium.
?1 ?1
Angle of Incidence (?1) Angle of Reflection
(?1) Angles are measured from the Normal.
4
?I ?r Always!! This law is obviously true in
some situations and not so obviously true in
other situations.
Light rays reflect from the smooth surface in
only one direction because all the Normal lines
are parallel to each other. All ?i are the same ?
All ?r are the same. Light rays reflect in many
directions from a rough surface because each ray
encounters the surface with a Normal not parallel
to the other Normals. However, for each ray
?i ?r
5
Flat (Plane) Mirrors
Terms Plane Mirror Flat Mirror Object Physica
l thing placed in front of the mirror Object
Distance (do) Distance of object from the front
of the mirror Image The form of the object
takes in the mirror Image Distance
(di) Distance of image from the front of the
mirror Virtual Image Located behind mirror. Rays
appear to come from it. Real Image Located
in front of mirror. Rays converge to form real
images.
6
Flat Mirror (cont)
Image Real or Virtual? Upright or
Inverted? Magnification?
Virtual Upright None (M1)
Image Distance (di) and Object Distance (do)?
Same
Ray Diagrams Follow the Law of
Reflection. Follow Snells Law (with
lenses). Rays can be traced forward or backward.
7
Spherical Mirrors
Principle Axis Line formed by point C and F. It
runs to the center of the mirror and is
perpendicular to the surface at the mirrors
center. Center of Curvature (C) A point
equidistant from all points on the mirrors
surface. Distance from C to mirror equals
radius of curvature of the sphere (from
which the mirror is formed) Focal Point
(F) A point where all rays parallel to the
Principle Axis converge after striking the
mirror. (F.5C)
NOTE Relatively flat or small spherical mirrors
are an approximation of a parabolic mirror.
8
With regard to a concave spherical mirror,
describe the 3 general locations of interest
where an object might be placed. 1. between
mirror and focal point 2. between focal point
and center of curvature 3. outside of center of
curvature
Ray Diagrams (Case 2) The purpose of ray diagrams
is to find the location of an image and determine
its characteristics. Rules 1. Rays originate at
the tip of object in all directions. 2. The ray
parallel to the principle axis reflects back
through the focal point (focal point
definition). 3. The ray going directly through
the focal point reflects back parallel with
principle axis (rays can be traced forward or
backward).
9
Ray Diagram (Case 1)
Notes1. Three rays of interest can be drawn. 2.
Two rays determine a point. Three rays rarely
come to a point even when they should. 3. Use
only two rays to find the tip of the image (Use
Rays 1 and 2.)
The rays diverge in front of the mirror, but
converge behind the mirror. Result Virtual
Image Upright Magnified
Read the book and draw the 3rd case yourself.
10
Convex Mirrors
Rays of Interest 1. Ray parallel to Principle
Axis 2. Ray reflected perpendicularly off the
surface (appear to come from center of
curvature).
Objects in mirror are closer than they appear.
11
Mirror Equation (for Concave and Convex Mirrors)
f ½ c
do ALWAYS di if image is
in front of mirror (real image) di - if
image is behind the mirror (virtual image) f
and c if they are in front of mirror
(concave) f and c - if they are behind
the mirror (convex) M if
upright
12
Ray Model of Light
Refraction The tendency for light ray to bend
when traveling from one medium into another
medium. Examples Rays traveling
from air to water air to
glass water to glass
Law of Reflection ?1 ?1 v1 speed of light in
air (still approximately 3.0x108m/s) v2 speed
of light in glass
v1 ? v2
?1 Incident Angle ?1 Reflected Angle ?2
Angle of Refraction
13
Light takes the quickest path between two
points. Only 1 Medium ? Straight Line Path 2
Mediums Encountered ? Bent Line Path The bent
line path allows light to travel relatively more
distance in the medium in which it travels faster
and less distance in the slower medium.
Draw how you would travel if you were a lifeguard
at point A trying to quickly reach a person at
point B.
14
Light bends toward the Normal when passing from
fast medium to a slow mediumjust like the
lifeguard. Light bends away from the Normal when
passing from a slow to a fast mediumjust like
you in the ocean if you noticed someone stealing
from your possessions on shore.
15
Law of Refraction
Index of Refraction is another physical property
of a substance (medium)
n ? 1 because c ? v
Always!
However nair ? 1.00 (to 3 sig.figs.)
f1 f2 Waves dont pile up at the boundary. v1
? v2 ?1 ? ?2 The wavelength changes at the
boundary.
16
Law of Refraction (Snells Law)
17
Example (Snells Law)
Find the angle (relative to the Normal) of the
ray in the water.
Strategy Draw the Normal and measure ?1 relative
to the normal. Look up n1 and n2 n1 1.00
(air) n2 1.33 (water) Plug into Snells Law and
Solve for ?2 1.00 sin60 1.33 sin?2
sin?2 .651 ?2 sin-1(.651)
?2 41?
18
Example (Snells Law and Critical Angles)
Find the critical angle where light travels
parallel to the surface of the water upon leaving
the water.
Apply Snells Law 1.33 sin?C 1.00 sin90
sin?C .75 ?C sin-1(.75)
?C 49?
Question What happens to the light leaving the
water if the incident angle is greater than ?C?
19
Total Internal Reflection
Where will the red ray travel?
Apply Snells Law 1.33 sin60 1.00 sin?2
sin?2 1.15 ?2 sin-1(1.15) ERROR
The red ray is reflected internally as if the
air/water surface acted like a mirror. The law of
reflection is followed ?1 ?2
20
Fiber Optics(Total Internal Reflection
Application)
Core Transparent material about the diameter
of a piece of spaghetti. Light travels
through the core. High or Low value for ncore??
High
Cladding Encases the core. Light is not
supposed to travel through cladding. High or
Low value for ncladding??
Low
Jacket Protective Coating
21
Lenses
Flat Lens is really a contradiction in
terms. Lenses are not flat! Thin implies no
offset from the original path.
22
Converging Lenses
Lens Factors More Curvature Larger Index of
Refraction Immerse glass (n1.5) lens in water
(n1.33)
Affect on Focal Length Shortens focal
length Shortens focal length Lengthens focal
length
23
Lenses
Terms Thin lens Thin compared to focal
length Converging lenses Parallel rays entering
the lens converge at a focal point behind the
lens.
Diverging lenses Parallel rays entering a lens
diverge away from each other behind the lens.
They appear to all come from a focal
point in front of the mirror.
24
Converging Lenses
Ray Diagrams Purpose Determine the location and
characteristics of an image formed by a
lens. Rules 1. Rays parallel to the principle
axis strike the lens and bend towards the focal
point behind the lens. (Definition of focal
point) 2. Rays passing through the focal point
in front of the lens exit from the lens
parallel to the principle axis. 3. Rays
going through the center of the lens (at any
angle) continue straight through the thin
lens.
Example (Objects outside focal point)
25
Converging Lenses Location of Interest

• Object is between lens and f.
• Object is between f and 2f.
• Object is outside of 2f.

Summary
do ? f 2f ? do ? f do ? 2f
Front of lens Behind lens Behind lens
Virtual Real Real
Positive Positive Negative
Yes No No
26
Diverging Lenses Ray Diagram Rules 1. Rays
parallel to principle axis diverge through the
lens and appear to originate at focal point in
front of lens. 2. Rays striking center of lens
pass through lens without bending. 3. Ray
directed towards focal point behind the lens
emerges from lens parallel to principle axis.
27
Lens Equation
Converging Lens (Sign Convention) f
positive do positive if object in front of
lens (real object) di positive if image is in
back of lens (real image) ho positive
if object is upright hi positive if image is
upright Diverging Lens f negative do
positive di positive if image is in back of
the lens (not typical)