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Physics Teacher Notes - Grothaus
Chapter 27 – Light
Title: Light
***Light is the ONLY thing you see! All visible objects either emit or reflect light.***
We can only see about 0.1% of the entire electromagnetic spectrum.
We are “colorblind” to other waves
Contrary to popular belief, dogs are not colorblind – they just can’t
see all of the colors we can – only yellow, blue and violet
Bulls are colorblind! They can’t even see red….
Insects can see in ultraviolet as well as all the colors we see.
Using instruments, we can see lower – infrared detecting devices and radio
and microwave receivers
And higher – ultraviolet, X-ray, and gamma-ray detectors
Early Concepts of Light
Some early philosophers believed that light traveled from our eyes to the objects we look
at. “That’s why we don’t see anything until we look at it”.
Some thought light consisted of tiny particles, although some (Empedocles and Huygens)
thought it traveled in waves.
The particle theory – light seems to move in straight lines and doesn’t seem to spread out
like most waves do
The wave theory became the accepted theory in the 19th century.
In 1905, Einstein published a theory called the “photoelectric effect”. The theory states
that light consists of particles called photons. Photons are massless bundles of
concentrated electromagnetic energy.
Scientists now agree that light has a dual nature, part particle, part wave.
Conceptual Physics
Page 1 of 13
Chapter 27 – Light
Physics Teacher Notes - Grothaus
In this chapter, we will only discuss the wave nature of light.
The Speed of Light
The actual speed of light is 299,792,458 m/s, which we round to
300,000 km/s = 300,000,000 m/s
300,000,000 m/s = 187,000 mi/s
The astronauts traveling in the space shuttle go about 5 mi/s
The speed of light in a vacuum is a universal constant. c = 300,000 km/s
7.5 times around the Earth in one second
1.28 seconds for light reflected off the moon to get to the Earth
8 minutes from the sun to the Earth
4 years from the nearest star – Alpha Centauri
The distance light travels in one year is called one light-year
Alpha Centauri is 4 light-years away!
A light year is a distance measurement – not a time measurement!!!
d = vt = (3.0 10 8 m / s)(1year)(
365days 24hours 3600sec
9.461015 m
or 5.879 1012 miles
“All that we see, even our own reflection, is from the past.”
The diameter of our galaxy is 100,000 light-years
Some galaxies are 10 billion light-years from Earth
If one exploded 5 billion years ago…..
Interesting facts: Aristotle thought light was instantaneous
Galileo tried to measure by putting two people on hills about a mile apart – estimated that
light was 10 times the speed of sound.
Only for theorists: VSauce – Would headlights work at the speed of light?
watch on your own?
Conceptual Physics
Electromagnetic Waves
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Chapter 27 – Light
Physics Teacher Notes - Grothaus
Light is energy emitted by accelerating electric charge (often electrons in atoms).
This is a wave that’s partly electric and partly magnetic – electromagnetic
Light is only a very small portion of the entire electromagnetic spectrum.
Lower frequencies than red is called infrared (longer wavelengths)
Higher frequencies are called ultraviolet (shorter wavelengths)
Visible light
Gamma rays
Sound waves are mechanical waves – a vibration of material particles.
Light waves are electromagnetic waves and are not vibrations of material,
but are vibrations of pure energy
Sound is not part of the EM spectrum. It has its own spectrum, of which we
only hear a small part.
Light and Transparent Materials
Light is energy carried in an electromagnetic wave that is generated by vibration of
electric charges. When light hits an object, electrons in that object are forced into
vibration (think resonance).
How matter receives light depends on the frequency of the light and the natural
frequency of the electrons in that matter.
Visible light – 100 trillion times per second ( 10 Hz )
For something to move that fast, it has to have very little inertia –
Electrons have such little mass, they have very little inertia and will vibrate
See Figure 27.6 – electrons vibrate, but are “attached” to their nucleus
Natural frequency – last chapter
Conceptual Physics
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Physics Teacher Notes - Grothaus
Chapter 27 – Light
Electrons have a natural frequency as well. Materials respond more to vibrations
at some frequencies than others.
These natural vibration frequencies depend on how strongly the electrons are
attached to a nearby nucleus. “spring strengths”
Glass is transparent (trans-): Natural frequency is in the ultraviolet range.
Ultraviolet light – resonance occurs – amplitude of the waves increase – large
vibration between electrons and their nuclei – more “bumping around” –
energy gets lost as heat. Glass is not transparent to UV (no sunburn)
Visible light – lower frequency – electrons are forced into vibrations with smaller
amplitudes -- atoms hold the energy for much less time – less bumping
around -- energy gets through and is reemitted as light on the other side
(less loss as heat)
When light goes through – slight delay – speed of light while in the material is less
than c (always 3.0 10 8 m/s in vacuum)
In atmosphere, very, very small delay, but still rounded to 3.0 10 8 m/s
In water, light travels at 0.75c ( 2.25 10 8 m/s)
In glass, light travels at 0.67c ( 2.0110 8 m/s)
In a diamond, light travels at 0.40c ( 1.20 10 8 km/s)
Infrared (even lower frequencies), vibrate the entire structure of the glass
Increases the internal energy and makes it warmer
See Figure 27.8, p.539
Conceptual Physics
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Physics Teacher Notes - Grothaus
Chapter 27 – Light
Interesting fact: Black light is UV light. How can UV light get through the glass in a light
Quartz is transparent to UV light – black light bulbs are made of quartz
Translucent – light doesn’t go straight through – diffuse transmission (bathroom
windows, shower doors, etc. (book doesn’t cover this, but you should know it)
Opaque Materials
When light shines on an object: (1) bounce off (reflect), (2) transmitted through the material or (3) it can be
absorbed by the material (usually some combination of these)
Conceptual Physics
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Physics Teacher Notes - Grothaus
Chapter 27 – Light
Materials that don’t let light through are opaque. – in this case, there is usually a combination of light reflecting
off and being absorbed.
In opaque materials, any vibrations given by light to the atoms and molecules are turned into random
kinetic energy – they become warmer.
Metals have an outer layer of “loose” electrons, not attached to any particular atom. why they conduct
electricity and heat so well!
When light shines on it, those electrons vibrate and reemit the light. This is a reflection – shiny
Why do wet things look darker?
Why is a black tar road hotter than a pane of window glass in the sunlight?
Why do eyeglasses that turn to sunshades when exposed to the sun not work inside of a car?
Vsauce – What is the speed of dark? (13.5 minutes..)
A think beam of light can be called a ray. A beam of light can be thought of as made from a
bundle of rays
When light shines on an object, some of the rays are stopped (absorbed and/or reflected from the
object) and some continue on in a straight line path.
Interesting fact:
The X-ray images of teeth, bones, and the contents of your carry-on bag are shadows, made by a kind of light that goes through
most things, but is scattered more by dense materials such as metals.
Shadow dancers:
Conceptual Physics
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Chapter 27 – Light
Physics Teacher Notes - Grothaus
Eclipses: all about shadows – we’ll be doing that
Light travels in waves – transverse waves
You can tell because of polarization – see p. 542
Waves travel in one plane – polarized
Light sources usually aren’t polarized because they send light out in all directions.
When you send it through a polarizer, you “filter” out all of the other components and are left with
light waves that are only on one plane
See p. 543 – filter examples
Skipping rocks – only flat will skip. If it comes in vertically, it will just sink.
Light that reflects at glancing angles from nonmetal surfaces, such as glass, water or roads, vibrates
mainly in the plane of the reflecting surface.
This means that you want sunglasses to be in what orientation to keep out light that is reflecting off of the
Demonstration with filters – two filters
Conceptual Physics
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Physics Teacher Notes - Grothaus
Chapter 27 – Light
Polarized Light and 3-D Viewing
Vision in three dimensions depends on both eyes giving impressions at the same time from a slightly different
Try this: Which eye do you use more? (“discover!” on p. 544)
The combination of views in the eye-brain system gives depth.
A pair of photographs or movie frames, taken a short distance apart (about average eye spacing), can
be seen in 3-D when the left eye sees only the left view and the right eye sees only the right view
See if you can see figure 27.18 in 3D.
There are lots of different ways to show 3D, but all of them involve each eye seeing a different view – binocular
One of the methods involves showing two views through polarization filters onto a screen. One is shown
vertically, the other horizontally. Each eye on the glasses is differently polarized. Each eye sees a different
They used to do it with different colors: red/cyan
Problems for this unit:
Conceptual Physics
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Chapter 27 – Light
Physics Teacher Notes - Grothaus
Remember the formulas from the wave unit:
Symbols and units:
Hz 
Period T
(time unit is in the numerator)
(time unit is in the denominator)
frequency and period are reciprocals of each other
(v  
How do you put scientific notation numbers into a calculator?
Example 1:
If you are listening to a radio station broadcast at 101 MHz ( 10110 Hz ) , what is the wavelength of the
radio wave?
Remember that
v = c  3.0 10 m / s
When we are talking about wavelengths of visible light, we often talk about it in nanometers
1nm 109 m
for example, violet light has a wavelength of 450 nm
this means that the wavelength is 450 10 m
Example 2:
Some laser pointers emit light waves with a wavelength of 533 nm. What is the frequency of this light?
Optical Illusion website links
Strong optical illusion simulation: really cool:
Conceptual Physics
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Physics Teacher Notes - Grothaus
Chapter 27 – Light
Perfect explanation:
Optic illusions: a montage will be shown and discussed
Vsauce blow your mind:
Peripheral vision effect: Scroll down to the “turn stars into monsters” part. This blows them
optical illusion montage – some classic, some not
**Google optical illusions, images – find some good ones here
**Google specifically, “optical illusion vehicles” and “optical illusion chalk”
Vsauce moving illusions:
If time? Einstein
Veritasium: shadow illusions (not great)
OKGo – The Writing’s on the Wall
The Atomic Color Code – Atomic Spectra
When made to emit light, every element has its own characteristic color.
Each one is a blend of different frequencies of light.
Atoms have specific energy levels – rings around the nucleus
Conceptual Physics
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Physics Teacher Notes - Grothaus
Chapter 27 – Light
The closer to the nucleus, the less energy
Analogy: Book above a table (release sound energy), screen door
Remember that when energy hits an electron, it gets excited – goes up a level
Think of a rubber band – If you stretch it, it wants to go back to its original
shape – electrons do too.
After an excited atom emits light in the form of a photon of a particular
frequency, it returns to its normal state.
Light has a spectrum of different frequencies.
The energy transition of the electron is what determines the frequency of the
photon that is released.
Each photon carries an amount of energy that corresponds to its frequency.
Red light (from neon) carries a certain amount of energy.
A photon of twice the frequency has twice as much energy and is
found in the ultraviolet part of the spectrum.
When many atoms in a material are excited, many photons with many
different frequencies are emitted.
Measuring the frequencies of light in a spectrum is also measuring the relative
energy levels in the atom emitting that light.
The frequencies, or colors, of light emitted by elements are the “fingerprints”
of the elements.
We can do this by using an instrument called a spectroscope.
It’s an arrangement of a thin slit through which the light is viewed and a prism that
takes that light and separates it into its individual components and
magnified so that you can see it.
Or another type of spectroscope uses a diffraction grating.
Conceptual Physics
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Physics Teacher Notes - Grothaus
Chapter 27 – Light
Gas vs liquid or solid
When you look at the light from a glowing element, you don’t see a continuous
spectrum, you see a series of lines – a line spectrum
Each line corresponds to a frequency of light –
Each color line appears in the same position as that color in the continuous
Conceptual Physics
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Physics Teacher Notes - Grothaus
Chapter 27 – Light
Lab Why look at the sun:
Sun eruption:
Sun documentary:
Superhero physics – Superman laser beam
Calculating the speed of light with a microwave:
Conceptual Physics
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