Transcript Light

4/20 do now – on a new sheet
• The diagram below shows two pulses traveling
toward each other in a uniform medium.
• Draw a diagram best represents the medium
when the pulses meet at point X.
Objectives
• Go over class work packets
• Electromagnetic spectrum
• Homework – castle learning
Light
1.
2.
3.
4.
5.
6.
7.
8.
9.
The Electromagnetic Spectrum
Visible Light
Speed of light
Ray diagrams
Reflection of light
Refraction of light
Speed of light and refraction
Absolute index of refraction
Snell’s Law
The Electromagnetic spectra
• Electromagnetic waves are TRANSVERSE waves which are
capable of traveling through a vacuum. Electromagnetic waves
are produced by a vibrating electric charges, they consist of
both an electric and a magnetic field at right angles to each
other.
• The alternating electric field creates alternating magnetic field,
and the alternating magnetic field creating alternating electric
field.
EM Wave Types
EM waves are classified according to the methods by which they
are generated or received.
Radio
Microwave
Infrared
Visible
Ultraviolet
X-ray
Gamma Ray
103
10-2
10-5
10-7
10-8
10-10
10-12
Wavelength (m)
About the size of…
Building
Human Honey Bee Pinpoint Protozoan
Molecules
Atoms
Atomic Nuclei
Freq (Hz)
104
Temperature of
bodies emitting
the wavelength
108
1012
1K
1015
100 K 10000 K
1016
1018
1020
10 million K
Electromagnetic Radio waves are not sound waves
Visible Light
• Visible Light, an electromagnetic, transverse wave, is the very
narrow band of wavelengths in the EM spectrum. Visible light
consists of wavelengths from approximately 700 nm to
approximately 400 nm. This narrow band of visible light is
affectionately known as ROYGBIV.
• Each individual wavelength within the spectrum of visible light
wavelengths is representative of a particular color.
Speed of Light
• All electromagnetic waves, including light, travel at
the same speed of 3 x 108 m/s in vacuum. The speed of
light in a vacuum is represented by the symbol c. The
speed of light in air is slightly less than it is in a
vacuum.
• The speed of light in a vacuum is the upper limit for
the speed of any material body. No object can travel
faster than c.
c  f 
Example #1
•
1.
2.
3.
4.
Radio waves and gamma rays traveling in
space have the same
frequency
wavelength
period
speed
Example #2
•
1.
2.
3.
4.
In a vacuum, all electromagnetic waves have
the same
wavelength
frequency
speed
amplitude
Example #3
•
Compared to the speed of microwaves in a
vacuum, the speed of x-rays in a vacuum is
1. less
2. greater
3. the same
Ray Diagrams
• Because it is not possible to see individual wave fronts in a
light wave, a ray is used to indicate the direction of wave
travel.
• A ray is a straight line that is draw at right angles to a wave
front and points in the direction of wave travel.
• Ray diagrams show only the direction of wave travel, not the
actual waves.
Light rays vocabularies
• An incident ray (I) is a ray that originates in a medium and
approaching a boundary.
• A reflected ray (R) is a ray the rebounded from a boundary.
• A refracted ray (R) is a ray that results from an incident ray
entering a second medium.
• At the point of incidence where the ray strikes the mirror, a line can
be drawn perpendicular to the surface of the boundary. This line is
known as the normal (N).
• Angle of incidence (θi) is the angle between
the incident ray and the normal.
• Angle of reflection (θr) is the angle between
the reflected ray and the normal.
• Angle of refraction (θr) is the angle between
the refracted ray and the normal.
Class work
• Worksheet 5.1.5 #1-2, 4, 6-12
4/21 Do now
The diagram represents shallow water waves of
wavelength λ passing through two small openings,
A and B, in a barrier. How do the length of path BP,
compare to the length of path AP? Be specific.
Reflection of Light - OBJECTIVES
1. The law of reflection
2. Distinguish between specular and diffuse
reflection of light.
3. Apply the law of reflection for flat mirrors
4. Describe the nature of image formed by flat
mirrors.
Review Light rays vocabularies
• An incident ray (I) is a ray that originates in a medium and
approaching a boundary.
• A reflected ray (R) is a ray the rebounded from a boundary.
• A refracted ray (R) is a ray that results from an incident ray
entering a second medium.
• At the point of incidence where the ray strikes the mirror, a line can
be drawn perpendicular to the surface of the boundary. This line is
known as the normal (N).
• Angle of incidence (θi) is the angle between
the incident ray and the normal.
• Angle of reflection (θr) is the angle between
the reflected ray and the normal.
• Angle of refraction (θr) is the angle between
the refracted ray and the normal.
THE LAW OF REFLECTION
• The law of reflection states that when a ray of light reflects off a
surface, the angle of incidence is equal to the angle of reflection.
θi = θr
Once a normal to the surface at the point of incidence is drawn, the
angle of incidence can then be determined. The light ray will then
reflect according to the law of reflection. The Law of Reflection is
Always Observed (regardless of the orientation of the surface)
Example #1
• The diagram above shows two rays of light striking a plane
mirror. Which diagram below best represents the reflected
rays?
1.
2.
3.
4.
Example #2
•
The diagram represents a light ray being reflected from a
plane mirror. The angle between the incident ray and the
reflected ray is 70.°. What is the angle of incidence for this
ray?
1.
2.
3.
4.
20.°
35°
55°
70.°
Example #3
• Identify which angle is angle of incidence and
which angle is angle of reflection.
1. Incident angle
is ___
2. Reflected
angle is _____
A
B
C
D
Specular vs. Diffuse Reflection
• Specular reflection: Reflection off of SMOOTH
SURFACES such as mirrors or a calm body of water.
• Diffuse reflection: Reflection off of ROUGH SURFACES
such as clothing, paper, and the asphalt roadway.
• Each individual ray obeys the laws of reflection.
Why Does a Rough Surface Diffuses A Beam of Light?
• For each type of reflection, each individual ray follows the law of
reflection. However, the roughness of the material means that
each individual ray meets a surface which has a different
orientation. The normal line at the point of incidence is different
for different rays. Subsequently, when the individual rays reflect
off the rough surface according to the law of reflection, they
SCATTER in different directions. The result is that the rays of light
are incident upon the surface in a concentrated bundle and are
diffused upon reflection.
Image Formation in Plane Mirrors
• When light gives off from an object reaches the mirror and
reflects off the mirror according to the law of reflection, an
image is formed. Each one of these rays of light can be extended
backwards behind the mirror where they will all intersect at a
point (the image point). Any person who is positioned along the
line of one of these reflected rays can sight along the line and
view the image - a representation of the object.
Image characteristics in a plane mirror
1. An image has the same size as the object.
2. The image is as far behind the mirror as the
object is in front of the mirror.
3. The image has the same orientation as the object.
4. The image is laterally inverted. (left and right
reversal)
5. The image is virtual, no actual light meet at the
image position. Virtual image can not be captured
on a screen.
What Portion of a Mirror is Required?
• Ray diagrams can be used to determine what portion of
a plane mirror must be used in order to view an image.
•In order to view his image,
the man must look as low
as his feet, and as high as
the tip of his head. The man
only needs the portion of
mirror extending between
points X and Y in order to
view his entire image. All
to view an image of yourself in a plane other portions of the mirror
are useless to the task of
mirror, you will need an amount of
this man viewing his own
mirror equal to one-half of your
image.
height.
Reflection of light and Color of objects
• The color of the objects that we see are largely due to
the way those objects reflect the light to our eyes. The
color of an object is not actually within the object
itself. Rather, the color is in the light that shines upon
it and is ultimately reflected or transmitted to our eyes.
Class work
• Light, reflection, mirror packet –
– Light reflection #1-8
– Specular versus Diffuse Reflection #1-5, 7-11
4/22 do now
• If an incident ray of light makes an angle of
35o with the mirror surface, then what is the
angle of reflection?
homework work – castle learning
Refraction of Light - OBJECTIVES
• Recognize situations in which refraction will occur.
• Identify which direction light will bend when it
passes from once medium to another.
• Solve problems using Snell’s law
Refraction of Light Waves
• Refraction is the bending of a wave as it passes at an angle
from one medium into another.
• When a beam of light approaches a boundary at an angle, it
changes direction as it crosses the boundary separating two
medium.
Light enters the medium at an angle
(obliquely)
Light does not change
directions when it goes straight
down
The Angle of Refraction
• The amount of refraction of a ray is measured by the angle of
refraction. It is the angle between a ray emerging from the
interface of two media and the normal to that interface at the
point where the ray emerges.
• Note: the angle of refraction and the angle of incidence are on
the opposite side of the normal.
θi is the angle of incidence - the angle
which the incident ray makes with the
normal line.
θr is the angle of refraction - the angle
which the refracted ray makes with the
normal line.
The amount of angle of refraction depends upon the properties of
the two media at the interface.
The Cause of Refraction
• Refraction caused by a change in both the speed and
wavelength of the wave.
• When light enters from denser to less dense medium (water to
air), it speeds up. Since the frequency doesn’t change, the light
has a longer wavelength.
• When light enters from less dense to denser medium (air to
water) it slows down and transforms into a wave with a shorter
wavelength.
• The only time that a wave can be transmitted across a boundary,
change its speed, and still not refract is when the light wave
approaches the boundary in a direction which is perpendicular
to it.
The Ray Model of Light explains refreaction
• The ray of wave is constructed in a direction perpendicular to the
wave fronts of the light wave, which is the light wave's direction.
The idea that the path of light can be represented by a ray is
known as the ray model of light.
All wave fronts
are in phase
Conditions of Refraction
• A light wave must enter the boundary at an angle (obliquely)
in order to bend. A light wave will not undergo refraction if it
approaches the boundary in a direction which is perpendicular
to it.
The Direction of Bending
The speed of a light wave is dependent upon the optical density of the
material through which it moves. Light travels faster in less optically
dense medium.
If a ray of light passes across the boundary
from a denser material into a less dense
material, such as from water to air, the light
ray will bend away from the normal line.
If a ray of light passes across the
boundary from a less dense material
into a denser material, such as from air
to water, the light ray will bend towards
the normal line.
Note: the incident ray and the refracted ray are on the opposite side of the
normal line.
Refraction and visual distortion
• Since refraction of light occurs when it crosses the boundary,
visual distortions often occur. These distortions occur when light
changes medium as it travels from the object to our eyes.
Atmospheric Refraction - mirages
On sunny, hot days, a non-uniform medium has been created by the heating of the
roadway and the air just above it. While light will travel in a straight line through a uniform
medium, it will refract when traveling through a non-uniform medium. If a driver looks
down at the roadway at a very low angle (that is, at a position nearly one hundred yards
away), light from objects above the roadway will follow a curved path to the driver's eye as
shown in the diagram.
Atmospheric Refraction – visibility of the
sun
• Because the density of Earth’s atmosphere increases gradually as Earth’s
surface is approached from space, sunlight entering the atmosphere
obliquely, as it does at from space, is gradually refracted to produce a
curved path. Your brain has learned to assume that light entering your
eyes has been traveling in straight lines. Thus, at sunset you “see” the
sun higher in the sky than it actually is. When you “see” the sun on the
horizon, it has already set.
Image formed by lenses is refraction
– http://www.freezeray.com/flashFiles/eyeDefects.h
tm
Dispersion – refraction of white light
• The separation of visible light into its different colors
is known as dispersion.
Wavelength affects index of refraction.
Index of red light is the smallest, it bends the least. While index of violet light is
greatest, it bends the most.
EXAMPLE #1
• The diagram shows a ray of light passing from air into
glass at an angle of incidence of 0°. Which statement
best describes the speed and direction of the light
ray as it passes into the glass?
a. Only speed changes.
b. Only direction changes.
c. Both speed and direction change.
d. Neither speed nor direction changes
EXAMPLE #2
• The diagram shows how an observer located at
point P on Earth can see the Sun when it is
below the observer's horizon. This observation
is possible because of the ability of the Earth's
atmosphere to
a. reflect light
b. diffract light
c. refract light
d. polarize light
EXAMPLE #3
• Which phenomenon of light accounts for
the formation of images by a lens?
A. reflection
B. refraction
C. dispersion
D. Polarization
Class work
• Packet – light, refraction and lenses: Light
Refraction #1-8
• Worksheet 5.2.2- #1-5, 8-11
Lab - diffraction
Set up hair holder, a paper screen, laser pointer as shown. (a sample is set up in the
classroom)
Examine the pattern on
the screen, it should
appear similar to the
image on the right.
4/23 do now
• A ray of light strikes a plane mirror at an angle
of incidence equal to 35°. What is the angle
between the incident ray and the reflected
ray? [draw a picture to show your work]
Optical Density and Light Speed
• An electromagnetic wave (i.e., a light wave) is produced by a
vibrating electric charge. As the wave moves through the
vacuum of empty space, it travels at a speed of c (3 x 108 m/s).
• When light wave moves through a medium that is not vacuum,
its speed slows down due to the collision with the particles in
the medium.
• the speed of the wave depends upon the optical density of that
material. The optical density of a medium is not the same as its
physical density.
Optical Density and the Index of
Refraction
• One indicator of the optical density of a material is the absolute
index of refraction value of the material.
• Absolute index of refraction, n, is the ratio of the speed of light
in a vacuum, c, to the speed of light in a material medium, v.
n=c/v
A vacuum is given an n value of 1.0.
The absolute index of refraction has no
units.
The greater the value of n, the denser the
medium and the slower light travels in the
medium, the shorter the wavelength.
The product of the absolute index of
refraction of a material and the speed
of light in that material is 3.00 x 108
m/s, the speed of light in vacuum.
n∙v = c
Check your reference table
• Absolute indices of refraction:
• In what material the light travels slowest?
diamond
• In what material the light travels fastest?
air
Index of refraction and ratio of wavelength
n2 v1 1
 
n1 v2 2
examples
• Packet – Light, Reflection and Mirrors:
“Direction of Bending” #1-10
Snell’s law
• The general relationship governs the refraction of light as it
passes from one medium to another of different optical density
is known as Snell’s Law
n1/n2 = sinθ2/ sinθ1
n1sinθ1 = n2sinθ2
• Angles θ1 and θ2 are the angles of incidence and refraction
respectively, and n1 and n2 are the absolute indices of the
incident and refractive media, respectively.
• If θ1 is zero, θ2 will be zero, which means when light enters
perpendicularly to the boundary, it is not changing direction.
• Snell’s law can be rearranged in this way sinθ1/sinθ2 = n2/n1
• The ratio n2/n1 is called the relative index of refraction for the
two media.
Using Snell's Law to Predict An Angle Value
• Use Snell's law, a protractor, and the index of refraction values
to complete the following diagrams. Measure θi, calculate θr,
and draw in the refracted ray with the calculated angle of
refraction.
45o
60o
32o
35o
Examples
1. A ray of light in air is approaching the boundary with water at
an angle of 52 degrees. Determine the angle of refraction of
the light ray.
2. A ray of light in air is approaching the a layer of crown glass
at an angle of 42.0o. Determine the angle of refraction of the
light ray upon entering the crown glass and upon leaving the
crown glass.
An important
concept
• When light approaches a layer
which has the shape of a
parallelogram that is bounded
on both sides by the same
material, then the angle at
which the light enters the
material is equal to the angle at
which light exits the layer.
Class work
• Light, Reflection, and Mirror Packet – Snell’s
law #1-4
4/24 do now
• A person observes a fireworks display from a
safe distance of 0.750 kilometer. Assuming
that sound travels at 340. meters per second
in air, what is the time between the person
seeing and hearing a fireworks explosion?
TOTAL INTERNAL REFLECTION
The complete reflection of light at the boundary of two transparent
media; this effect occurs when the angle of incidence exceeds the
critical angle.
Critical angle
• The minimum angle of incidence for which total internal
reflection occurs.
• Since the maximum possible angle of refraction is 90o, the
corresponding incident angle is critical angle.
• This particular value for the angle of incidence could be
calculated using Snell's Law:
n1sinθ1 = n2sinθ2
n1sinθcritical = n2sin90o
Example
• A laser beam is shining from water into air, what is the critical
angle of water?
• Given: (ni = 1.33, nr = 1.00, θr = 90o,
• Unknown: θi = ?
Solve: n1sinθ1 = n2sinθ2
1.33sinθi = (1.00)sin90o
θi = 48.7o
When the angles of incidence is greater than 48.6o (the critical angle), all of the
energy (the total energy) carried by the incident wave to the boundary stays
within the water (internal to the original medium) and undergoes reflection off
the boundary.
Two Requirements for Total Internal
Reflection
•
Total internal reflection (TIR) is the phenomenon
which involves the reflection of all the incident
light off the boundary. TIR only takes place when
both of the following two conditions are met:
1. the light is in the denser medium and approaching
the less dense medium.
2. the angle of incidence is greater than the so-called
critical angle.
TIR and the Sparkle of Diamonds
• Relatively speaking, the critical angle for the diamond-air
boundary is an extremely small number. This property about the
diamond-air boundary plays an important role in the brilliance of a
diamond gemstone. Having a small critical angle, light has the
tendency to become "trapped" inside of a diamond once it enters.
A light ray will typically undergo TIR several times before finally
refracting out of the diamond.
More examples of TIR
A prism in an optical instrument will allow light to undergo
TIR whereas a mirror allows light to both reflect and
refract. So for a prism, 100 percent of the light is reflected.
But for a mirror, only about 95 percent of the light is
reflected.
Rainbows – Refraction and TIR
Class work
• Light, Reflection, and Mirror Packet – total
internal reflection
• The whole packet is due on Monday, except
Image Formation and Characteristics (front
and back) and Light Reflection #9-10
Lab – law of reflection
Purpose: Verify the law of reflection: the angle of incidence and the angle of
reflection are equal.
Material: Plane mirror, Pins, Plane paper, Cardboard, Straightedge, protractor,
pencil
Procedure:
– Follow procedure on the lab paper
Data section:
– The Data Section should include a data table with labeled column
headings (and units)
Conclusion/discussion of results:
1. What is the relationship between the incident and reflected angles?
2. Does the light reflect off the front surface or back surface of the mirror? Is
there any evidence of this from your experiment?
Procedure:
1. Place the mirror on a piece of paper. Draw a line to indicate
the front of the mirror.
2. Stick two pins to indicate the path of the ray going from the
pin to the mirror; stick two more pins – make sure they line
up with the images of previous two pins – this line indicate
the reflected rays. Now you should have all four pins in front
of the mirror.
3. Remove the pins and carefully trace the path of the incident
beam and the reflected beam on the paper.
4. Using a protractor, draw a normal line for the place where the
incident ray from the pins hit the mirror.
5. Now measure both the incident and reflected angles.
• Each person in your lab team must do this for themselves.
Combine your data on your data chart.
Data table
name
angle of
incidence (o)
angle of
reflection (o)
Lab 36 – finding index of refraction
Purpose (5 pt): Determine the index of refraction of an unknown
material
Material (5 pt): Rectangular prism with unknown index of refraction,
Pins, Pencil, Plain paper, Cardboard, Straightedge, Protractor
Data section (20 pt):
– should contain colomns of measured and calculated data. The
rows and columns should be labeled; units should be identified.
Work should be shown for one calculation; the work should be
labeled and easy to follow.
Conclusion/discussion of results (10 pt): Calculate the index of
refraction using Snell’s Law. Record your values for n in the data
table and calculate the average value of n.
1. Trace the prism on the white paper which is
placed on top of the card board.
2. Line up two pins obliquely to the prism, the
beam will refract into the prism and then the
beam will refract again into the air.
3. Use pins to mark the points of the refracted ray
by lining them up with the images of the pins
inside the prism. Trace incident ray to the prism
and refracted ray out of the prism. Connect two
points at the intersections with the prism.
Normal
Procedure: For each person in the group, trace the prism on a blank sheet of paper.
θi
Surface 1
θri
Surface 2
θi
3. Each person in the group must draw two diagrams to indicate incident rays and
refracted rays.
4. Measure the angles of incident and refracted rays. Read the angles to the nearest
1/10th of a degree.
5. Record each person’s data in the group on the data table. Every one in your group
should have a data table.
6. Calculate the index of refraction using Snell’s Law. Record your values for n in the
data table and calculate the average value of n.
Data table
trials
θi
θr
n
1
2
3
4
5
6
Average n is
nisinθi = nrsinθr
______________________