Post on 03-Jan-2016
1
Light
Chapters 36 – 39
2
Wave or Particle?
Newton -- particles.In the early 19th century, Young, Fresnel,
and others -- wave.In 1860 Maxwell -- electromagnetic wave.
3
Photoelectric effect
19th century -- Hertz -- shining light on a metal plate would make it emit electrons – producing an electric current.
Kinetic energy of the emitted electrons was independent of the intensity of the light.
Not wave theory
4
Photons
In 1905 Einstein proposed that light is quantized in small bundles called photons.
The energy of a photon depends on the frequency of the light, not the intensity.
5
Wave Particle Duality
In some situations, light behaves like a wave.
In other situations, it behaves like a particle.
This unit will only deal with light as a wave.
6
Light Basics
Light is an electromagnetic wave.
Doesn’t need a medium to travel
Travels at the speed of light
7
Speed of light
8
Speed of Light
Defined as 299 792 458 m/s
We use 3.00 x 108 m/s
Denoted as the letter c
9
Electromagnetic Spectrum
We detect a very small part of the electromagnetic spectrum as visible light.
10
Visible Spectrum
We can see wavelengths from about 400 to 700 nm.
Different wavelengths correspond to different colors.
11
Wave Front
Leading edge of a wave Shows the crests of the wave
12
Ray Model
A ray is an imaginary line along the direction of travel of a wave.
13
Reflection
14
Law of Reflection
The angle of reflection equals the angle of incidence.
Angles are measured from the normal, not the surface.
15
Example A ray reflects off 2 perpendicular mirrors. How does its final direction relate to its initial direction?
30°
30°
60°
60°
60°
60°
30°
anti-parallel
16
You tryRepeat example for 2 mirrors at a 60° angle.
90°
60°
30° 60°60°
anti-parallel
17
Speed of Light in MatterThe speed of light in a transparent material
such as air, water, or glass is less than the speed of light in a vacuum.
Each material has an index of refraction, n, where
Can n be greater than 1?Less than 1?Equal to 1?
v
cn
18
Refraction When light travels from one material into another,
its path is bent. If the second material has a higher index of
refraction, it is bent towards the normal.
19
Color
The index of refraction of a material has a slight dependence on wavelength.
Red (longer wavelength) is refracted less than violet (shorter wavelength).
20
Prisms
21
Rainbows
Rainbows are reflected light from water droplets in the air.
Different colors are refracted at different angles by the water, so they are separated.
22
Snell’s Law2211 sinsin nn
2sin33.145sin 322
23
You try
Light traveling in glass with n = 1.5 enters air with an angle of incidence of 30°.
What is the angle of refraction?
2211 sinsin nn
2sin130sin5.1 492
Did the light bend toward the normal or away from it?
24
Wavelength in new material
For any wave, the wavelength l and the frequency f are related by the equation
During refraction, the frequency does not change.
fv
25
Wavelength in new material
0fc fv
fn
c
fc
0 f
n
c
26
Wavelength in new material
n
cc
0
n0
27
Mirages
28
Mirages
29
Total Internal Reflection
30
Total Internal Reflection90sinsin 21 nn c
What if n2 is greater than n1?
1
2sinn
nc
31
Total Internal Reflection
33.11 waternn
00.12 airnn
1
2sinn
nc 33.1
00.1sin c
49c
32
Fiber Optics
33
q
q
Plane Mirrors
Image is same distance from mirror. Image is the same size. Image is upright.
mirror
objectimage
34
Real Images
An image is real if the light rays actually converge at that location.
A real image can be shown on a card or screen.
A real image is located in front of the mirror.
35
Virtual Images
An image is virtual if the light rays only appear to converge at that location.
A virtual image cannot be shown on a card or screen.
A virtual image is located behind the mirror.
36
Depth Inversion
The front and back of an object are reversed in a plane mirror.
This causes right and left to be reversed between the object and the image.
37
Spherical Mirrors
In spherical mirrors, the mirror is on the inner or outer surface of part of a hollow sphere.
38
Concave Mirrors
Reflect light from inner surface of sphere. Are “caved in”. Also called converging.
39
Mirrors
C is the center of the sphere. r is the radius of curvature. F is the focal point. f is the focal length.
Principal axisC F
r
f
2
rf
40
Finding the image 1
Image is inverted. Image is real.
Principal axisF
41
Concave Mirrors
To find the image Draw a ray parallel to the principal axis.
– It will reflect through the focal point. Draw a ray through the focal point.
– It will reflect parallel to the principal axis. The image is located at the intersection of the two
reflected rays. You can draw a ray to the center of the mirror. It
will reflect according to the law of reflection for flat surfaces.
42
Convex Mirrors
Reflect light from outer surface of sphere. Also called diverging.
43
Finding the image 2
Image is upright. Image is virtual.
FF
44
Convex MirrorsTo find the imageDraw a ray parallel to the principal axis.
– It will reflect as if it had come from the focal point.
Draw a ray through the focal point.– It will reflect parallel to the principal axis.– Extend the parallel reflected ray behind the
mirror.The image is located at the intersection of
the two reflected rays (or their extensions).
45
Mirror Practice 1
Image is upright or inverted.Image is real or virtual.
F
46
Mirror Practice 2
Image is upright or inverted.Image is real or virtual.
F
No Image!
47
Mirror Practice 3
Image is upright or inverted.Image is real or virtual.
FF
48
Mirror Practice 4
Image is upright or inverted.Image is real or virtual.
FF
49
The Mirror Equation
Used to locate the image mathematically. do = object distance
di = image distance f = focal length r = radius of curvature
fdd io
111
rdd io
211
50
Mirror Conventions
do is always positive.
di is positive for real images. – Same side of mirror as object.
di is negative for virtual images. – Opposite side of mirror as the object.
51
Mirror Conventions
f is positive for a converging (concave) mirror.
f is negative for a diverging (convex) mirror.
52
Magnification
Ratio of the image size to the object size.
Same as the negative of the ratio of the image distance to the object distance.
m hi
ho
d i
do
53
Magnification
Negative magnification means the image is inverted.
Magnification between 1 and –1 means the image is smaller than the object.
54
Example
Scenario from mirror practice 4.
cm 10od cm 10f
fdd io
111
do is always positive.f is negative for a diverging (convex)
mirror.
cm 30 h
55
Example
Negative sign means virtual image
10
11
10
1
id
5
11
id5id
56
Example
Find the magnification of the image
m hi
ho
d i
do 10
5m
5.0m
Positive means upright image.Image is half as big as object.
5.1ih
57
Thin Lenses
Lenses are considered thin if their thickness is considerably smaller than their focal length.
Can be concave or convex, like mirrors.
Form images by refraction.
58
Convex Lenses
Convex lenses are converging.– Opposite of mirrors.
59
Finding the image 3
Image is inverted.Image is real.
F F
60
Convex lenses
To Find the image Draw a ray parallel to the axis.
– It will refract through the far focal point. Draw a ray through the near focal point.
– It will refract parallel to the axis. You can also draw a ray through the center of the
lens. It will continue straight through the lens without being bent.
The image is located at the intersection of the two refracted rays.
61
Concave Lenses
Concave lenses are diverging.– Opposite of mirrors.
62
Finding the image 4
Image is upright.Image is virtual.
F
63
Concave Lenses
To find the image Draw a ray parallel to the principal axis.
– It will be refracted as if it came from the focal point.– Extend this ray behind the lens.
Draw a ray through the center of the lens.– It will go straight through the lens.
The image forms at the intersection of the refracted rays (or their extensions).
64
Lens practice 1
F F
Image is upright or inverted.Image is real or virtual.
65
Lens practice 2
F F
Image is upright or inverted.Image is real or virtual.
No Image!
66
Lens practice 3
Image is upright or inverted.Image is real or virtual.
F
67
Lens practice 4
Image is upright or inverted.Image is real or virtual.
F
68
The Lens Equation
The same as the mirror equation.
Magnification is also the same as for mirrors.
69
Lens Conventions
do is always positive.
di is positive for real images. – Opposite side of lens as the object.
di is negative for virtual images. – Same side of lens as object.
f is positive for a converging lens.f is negative for a diverging lens.
70
The eye
Diagram on page 872Light is refracted at the cornea and the lens.A real image is formed on the retina at the
back of the eye.The optic nerve sends the data to the brain.Vision is best in a small central region.
71
The eye
For a clear vision, the image must be formed exactly at the retina.
This distance, di does not change.
In order to focus objects at varying do distances, the focal length of the lens must change.
The eye does this by bending the lens.
72
Near point
The shortest object distance you can see clearly.
Depends on the ability of your muscles to bend your lens.
Muscle flexibility decreases with age, so the near point increases.– Reading glasses needed
73
Myopic eyes
Near-sighted.The eye is too long, so the image forms in
front of the retina.Too much convergence – needs a diverging
lens to correct.
74
Hyperopic eye
Far-sightedThe eye is too short, so the image forms
behind the retina.Not enough convergence – needs a
converging lens to correct.
75
astigmatism
The cornea is not spherical.Cannot focus on horizontal and vertical
lines at the same time.Corrected with a cylindrical lens – curved
in one direction.
76
diopters
The power of a lens is measured in diopters.The power is the reciprocal of the focal
length in meters.How glasses are prescribed.The numbers on your contacts.
77
LASIK
Laser reshapes cornea to refract light differently.
Cornea must be sufficiently thick.Works best for near-sightedness and
astigmatism.Will not increase lens flexibility
– Reading glasses may still be needed.
78
cameras
The film is like the retina in your eye.The area of the lens is adjusted by the
aperture.Aperture size is described by the “f-
number”, which is the focal length divided by the diameter.
79
Interference
Principle of linear superposition:– When two or more waves overlap, the resultant
displacement at any point and at any instant may be found by adding the instantaneous displacements that would be produced at the point by the individual waves
– You just add them
80
81
Two source interference
Thomas Young
82
Two Source InterferenceFrom the diagram we can see that
L
ymm tan
If we use the fact that for very small angles, tan q is about sin q, we can say
L
ymm sin
83
Approximation
If we assume that the distance L from the slits to the screen is much larger than the spacing, d, between the slits, then we can say that the path length between the two rays r1 and r2 is
sin21 drr
84
Two Source interference
md m sin
In order to have constructive interference from the light from two adjacent slits, the path difference between their light rays must be a complete wavelength.
d
mLym
85
Diffraction GratingsConsist of a large number of equally spaced
lines or slits on a flat surface.N is the number of slits per unit length
(such as mm or cm)d is the distance between two adjacent slits.
Nd
1
86
Diffraction gratings
Use the same equations as two source interference.
However, the patterns produced are sharper and narrower