LIGHT
an
illuminating
topic!
Light
• We see light in 1 of 2 ways
–We see light emitted from
its source
–We see objects by
reflected light from a
source
Unique Properties of Light
• It’s a wave, a particle,
electromagnetic radiation;
travels in a straight line, can be
affected by gravity
• In this chapter we will assume
light travels in a perfectly
straight line “RAY MODEL”
RAY MODEL
• Path of light = RAY
• Reflection : Plane Mirror
• When light strike objects, 3 things
happen:
–Reflection: shiny surfaces
–Absorption: very dark objects
–Transmition: clear objects
Key Terms
Normal
Angle of
incidence
Angle of
reflection
Mirror
Always measure angle with normal
Law of reflection:
θi = θr
Diffuse Reflection
• Law of reflection at work
when surface is NOT smooth
overhead picture
•When you look at a mirror you see an
image (yourself, others, environment)
•Lefts and rights are switched
How Images are Formed
Plane Mirrors
object distance
do
di
so
si
the image appears
to be as far behind
the mirror as the
object is in front of
the mirror
image distance
Image Types
• Real: light passes through the
image and the image could
appear on paper, Film, or screen.
Curved mirrors and lenses form
these types (movie projector)
• Virtual: light seems to be coming
from there, light rays do not
actually pass through the image
How tall must a mirror be to
see all of yourself? Lets say
you are 1.6m tall
OVERHEAD
Text page 687
look at picture pg. 683
Is it right side up or upside
down? How can you tell?
solution pg. 688
OVERHEAD
Fig. 23-6
Fig. 23-4
Fig. 23-2
Spherical Mirrors
• Spherical mirrors are the most common
type of curved mirrors. They form “part”
of a sphere
• CONVEX: reflection occurs on outer
surface (convenience stores)
• CONCAVE: reflection occurs on inner
surface (shaving mirror)
Key Terms
C
B
F
A
Mirror
f
R radius
Principal
Axis
continued…
• Principal axis: (CA) straight line
perpendicular to curved surface
• Focal Point: (F) where rays
parallel to CA converge
• Focal Length: (f) distance from F
to center of mirror
• Center of Curvature: (C) this is
the actual radius of the mirror
• Radius: BC
–Cf = Bf
• The focal length is ½ the
radius of curvature
–f = r/2
–f = R/2
The Rules for Ray Diagrams
1. Rays parallel to principal axis
go through F.
2. Rays that go through F reflect
parallel to the principal axis.
3. Rays passing through C reflect
back through C
IMAGE
o’
I o
C
f
I’
Inverted
Enlarged
Real
DO HANDOUT!
Handout
• For handout #1 measure (mm): ho, hi,
do, di, focal length
• Now check and see if:
ho/hi = do/di
do/di = do-f/f
1/ do + 1/di = 1/f
Mag. Formula: M = hi/ho = -di/do
Index of Refraction
• This is the ratio of speeds for light ( or
any other EM wave for that matter)
• Light travels its fastest in a vacuum (outer
(outer space)
• As light passes through different
materials its speed changes (slows down)
• Speed of light in vacuum: 2.9979 x 108
m/s “C”
– Normally rounded to 3.00 x 108 m/s = C
continued…
• C is a special constant (speed limit of
the universe)
• When light goes through the air, water,
glass, salt, or diamonds, it slows down.
We use V to represent the slower
speeds
• n is the symbol for index of refraction
and the ratio looks like this: n = C/V
Table 23-1
• Shows n for some common mediums
*note n can NEVER be less than 1
Vacuum
1.00000
air
1.0003
water
1.33
ethyl alcohol
1.36
diamond
2.42
Snell’s Law
• Refraction
• Willebrord Snell in 1612 wrote this law:
– n1 sinθ1 = n2 sinθ2
• As light passes from 1 medium to
another, it refracts (bends). If the
refracted ray is in a slower moving
medium it bends toward the normal. If
the refracted ray is in a faster moving
medium it bends away from the normal.
look at pg. 698 example 23-8, 23-9
θ1
Air n1
θ2
Air n2
Water n2
θ2
Water n1
θ1
θ1 > θ2
θ2 > θ1
θ inci. > refr. θ
θ refr. > inci. θ
n2 > n1
n1 > n2
Overhead
Fig 23-18
Total Internal Reflection
• Light bends away from the normal
when it leaves a higher n to a lower n
(I.e. water into air)
• At a certain angle, refraction will be
90° to the normal. This is called the
critical angle
θc
Sinθc = n2/n1 sin 90° = n2/n1
-Comes from snells law
• Any angle less than θc will be
refracted and partially reflected
• For angles greater than θc No
refraction at all. all light is reflected.
Total Internal Reflection
TIR
application: fiber optics, prisms, endoscopes
Overhead
Fig 23-23
FIG 23-21
Thin Lenses
• Most important simple optical device
• Where do we find lenses?
– Eye glasses, cameras, magnifying
glasses, telescopes, binoculars,
microscopes and medical instruments
• The faces can be concave, convex or
plane
– Lens Types:
• CONVERGING (thicker at center)
• DIVERGING(thinner at center)
OVER HEAD
• Double convex
• Plano Convex
• Convex Meniscus
• Double Concave
• Plano Concave
• Concave Meniscus
3 Rules for Ray DiagramsLenses
1.Parallels to axis refract
thru F
2.Thru F, refracts parallel to
Axis
3.Rays thru center of lens
travels in a straight line
Overhead Rules
for ray
diagrams lenses
Fig. 23-29
23-23, 23-33
23-24
23- 36
The Lens Equation
1/do + 1/di = 1/f
SIGN CONVENTION
1. focal length is (+) converging lenses (-)
for diverging
2. Object distance (+) if on side of lens from
which light is originating; other wise (-)
3. Image distance is (+) if its on opposite
side of the lens from where light is
coming from, other wise (-)
di (+) for real di (-) for virtual
4. hi is (+) if upright
hi (-) if inverted
• Take ho to ALWAYS be positive
• Magnification formula remains the same
M = hi/ho = -di/ do
Example problems pg 708- 709