Download Use Figure 9 p. 499

Physics Teacher Notes - Grothaus
Ch. 17 and 18 – Reflection and Refraction
Reflection and Mirrors
Refraction and Lenses
***When waves interact with matter, they can be reflected, transmitted, or a combination of
both. Waves that are transmitted can be refracted.***
Reflection
When a wave reaches a boundary between two media, some or all of the wave
might bounce back into the first medium. This is called reflection.
This can be total or partial.
A metal surface is rigid to light waves that shine upon it. Light is totally reflected. The
light reflected has almost the same intensity – only a small amount of energy due to friction
of vibrating electrons. (loose outer electrons again!)
Waves do follow the laws of conservation of momentum and energy!
Mirrors –
began with smooth metal mirrors
then coating the back of a flat piece of glass with a thin sheet of metal (16th century)
then coating glass with silver (1857) – first time got a sharp, well defined image
today – evaporating aluminum or silver onto highly polished glass
used to see reflections, in lasers, telescopes and other precise optical
systems
Glass and water are not rigid to light waves. If you shine light perpendicular to water,
about 2% is reflected, and to glass, about 4% is reflected.
When there is light on the other side, you don’t notice this, but when it’s dark on the other
side, you see your reflection – it is still 4% reflected, but you notice it more when it’s dark.
We’ll come back to this. Let’s continue with mirrors for now
The Law of Reflection
Demonstration with a ball.
Note the angle that it hits the wall, and the angle at which it reflects.
(Think pool table)
The same thing happens with light.
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Physics Teacher Notes - Grothaus
Specifically:
Ch. 17 and 18 – Reflection and Refraction
i  l
angle of incidence = angle of reflection (from the normal)
The law of reflection states that the angle of incidence and the angle of reflection
are equal to each other
Note: the incident ray, the normal and the reflected ray all lie in the same plane.
Does the frequency of the light change when it is reflected? How can you tell?
Wave front – entire wave reflects from the normal wherever it hits the reflective surface
Smooth surfaces – smooth surface vs. rough surface
Mirrors have a smooth surface – specular reflection
Parallel rays reflect in parallel
When light hits a rough surface, light reacts differently.
Light bounces off in all directions randomly. This is how we see most objects.
When light is incident on a rough surface, it is reflected in many directions.
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Physics Teacher Notes - Grothaus
Ch. 17 and 18 – Reflection and Refraction
This is called diffuse reflection.
Each ray of light follows the law of reflection – just differently angled surfaces at each
point.
Light from this page is diffuse. Paper is rough.
“Smooth is relative” – the rule of thumb is if the “edges” are less than 1/8th the wavelength
of the light that hits it, it will be smooth. The longer the wavelength the more differences in
edges can be there and still act like it’s smooth. Paper is rough to visible light, but may be
smooth to radio waves.
(example – mountain bike with fat tires vs. racing bike with skinny tires)
A normal road is rough. Reflection is diffuse and some of it will return to your eyes.
When it is wet, however, there’s water on the surface that acts like a mirror
(smooth). Light from your headlights will go where?
Would your book be easier or harder to read if the pages were shinier?
p. 467: 1 – 5
Objects and Plane-Mirror Images
A “regular” mirror is a plane mirror (not plain)
Flat, smooth – specular reflection
Your book defines an object as either a luminous source of light rays (like a lightbulb), or an
illuminated source of light rays, such as a person.
A mirrored surface can reflect these light rays so that an image is visible.
In the picture below, the candle is the object. Light reflects diffusely from the object, so
let’s consider one spot – like the tip of the flame.
The combination of all the points, creates an image of the candle.
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Physics Teacher Notes - Grothaus
Ch. 17 and 18 – Reflection and Refraction
The light from the candle reflects from the mirror to the eye, but what it looks like is that the
image is behind the mirror – but it’s not!
This is called a virtual image – an image that appears to be in a place where light does
not really reach.
Plane mirrors only produce virtual images
Properties of Plane Mirror Images:
1. With a plane mirror, the image position is equal to the negative of the object position.
(if the mirror is set at the origin).
xi  xo The negative sign indicates that the image is behind the mirror
and is therefore virtual
2. With a plane mirror, image height is equal to object height
hi  ho
3. Image orientation – always right side up (not upside down)
However, there’s a difference in your image – what is it?
Backwards -- Front to back? Left to Right?
Physics Girl – Why is a mirror image flipped horizontally, etc.
https://www.youtube.com/watch?v=vBpxhfBlVLU
Science Theater:
See video: https://www.youtube.com/watch?v=RlSSy8K_Gos
If time – impossible things #1 – mirrors – not very good
https://www.youtube.com/watch?v=kHZZQrJ41z4
Why will a camera with automatic focus give poor results if you take a picture of yourself in a
mirror?
What happens when you move the mirror closer or farther away? Can you see more of
yourself? - try: https://www.youtube.com/watch?v=WEV8LNn0szI
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Physics Teacher Notes - Grothaus
Ch. 17 and 18 – Reflection and Refraction
What size of a mirror do you need to see yourself?
What do you think?
Try this with small mirrors!
Try this with small mirrors!
One way glass?
How do they make one-way glass?
Minute physics:
https://www.youtube.com/watch?v=VvqbjUtt3mM
How stuff works
http://science.howstuffworks.com/question421.htm
Vsauce – What color is a mirror – only if there’s time.
https://www.youtube.com/watch?v=-yrZpTHBEss
p. 470: 8, 9, 10
Reflection of Sound
Remember that sound is also a wave, and that all waves have the same properties.
Sound energy that is not reflected is absorbed or transmitted.
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Physics Teacher Notes - Grothaus
Ch. 17 and 18 – Reflection and Refraction
An echo is a reflected sound. More sound is reflected when the surface is rigid and
smooth than when it’s soft and irregular.
The study of sound is called acoustics. When designing a building, you want to
understand the reflective properties of the surfaces.
When the walls of a room or auditorium is too reflective – get reverberations and the
sound is garbled. (sound bouncing everywhere)
When they are too absorbent, the sound is lower – flat and lifeless. Need a balance!
Why does your voice sound fuller when you sing in the shower?
Concert halls often have grooves in the walls so that the sound is diffuse.
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Ch. 17 and 18 – Reflection and Refraction
Echo problems –
Example 1: While hiking through a canyon, Noah Formula lets out a scream. An echo
(reflection of the scream off a nearby canyon wall) is heard 0.82 seconds after the scream.
The speed of the sound wave in air is 342 m/s. Calculate the distance from Noah to the
nearby canyon wall.
Example 2: A radio wave sent into space strikes an asteroid and is reflected back to Earth 1
second after being emitted. How far away is the asteroid?
Answers to problems: 1. 140 meters,
Section 2 -
2. 1.5x10^8 m away
Curved Mirrors
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Physics Teacher Notes - Grothaus
Ch. 17 and 18 – Reflection and Refraction
The principles we’ve discussed only apply to plane mirrors. Curved mirrors are different.
The law of reflection still holds, but sizes and distances of object and image are no
longer equal.
Look at the difference between the convex side vs. the concave side of a mirror.
What do you see?
Anatomy of a curved mirror:
See the short video of the pencil
http://www.physicsclassroom.com/class/refln/Lesson-3/The-Anatomy-of-a-Curved-Mirror
animations: http://www.physicsclassroom.com/Class/refln/u13l3b.cfm#image
Concave vs. Convex mirrors:
https://www.youtube.com/watch?v=jtTBOMVMSYM
First minute is mirrors, the rest is refraction
https://www.youtube.com/watch?v=uQE659ICjqQ
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Ch. 17 and 18 – Reflection and Refraction
Chapter 18 – Refraction and Lenses
Refraction
Think of a toy car traveling down a ramp when it hits a piece of carpet. What’s going to
happen if it hits the carpet straight on? At an angle?
This happens because of the change in speed between the two media.
Same thing happens when a wave hits a different medium.
When a wave that is traveling at an angle changes its speed upon crossing a
boundary between two media, it bends. This is called refraction.
Remember this:
When a wave goes faster, it bends away from the normal.
When a wave goes slower, it bends toward the normal.
Try it: https://phet.colorado.edu/en/simulation/bending-light
Refraction of Light
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Physics Teacher Notes - Grothaus
Ch. 17 and 18 – Reflection and Refraction
Changes in the speed of light as it passes from one medium to another, or
variations in the temperatures and densities of the same medium, cause
refraction.
Remember that when it goes faster, it bends away from the normal and when it goes
slower, it bends towards the normal.
Demonstrations – glass and straw, fat root beer mug
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Ch. 17 and 18 – Reflection and Refraction
Archer fish – can somehow spit up at bugs that are outside the water and hit them every time.
https://www.youtube.com/watch?v=4T1SQtavaUM
Disappearing object https://www.youtube.com/watch?v=KyWgnFm3ebc
funny pictures:
http://www.lolhome.com/funny-picture-9979533160.html
http://www.geekfill.com/2013/07/31/water-refraction/
https://www.google.com/search?q=funny+water+refraction+images&espv=2&biw=128
0&bih=701&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwjEwq6HvvbLAhUIwm
MKHR1XAOUQsAQIGg&dpr=1#imgdii=i-2tlMZnWJy5gM%3A%3Bi2tlMZnWJy5gM%3A%3BmMxVs7yiEixi8M%3A&imgrc=i-2tlMZnWJy5gM%3A
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Ch. 17 and 18 – Reflection and Refraction
Physics Teacher Notes - Grothaus
Refraction also happens with sound waves. Remember that sound travels faster in warm air
and more slowly in cool air. The difference between sound and light refraction is that sound
bending is gradual. There isn’t a sudden boundary like with light traveling from one medium
to another.
Snell’s Law of Refraction:
The index of refraction (n): determines the angle of refraction of light as it crosses the
boundary between two mediums.
Index of refraction in a vacuum is 1.00
See table 1 in book p. 493
(use these numbers)
Index of refraction of a medium is equal to the speed of light in a vacuum divided by the
speed of light in the medium:
n
c
v
Can n ever be less than 1?
Snell’s law of refraction is stated as follows:
n1 sin1  n2 sin2
figure above comes from: http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/refr.html
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Physics Teacher Notes - Grothaus
Ch. 17 and 18 – Reflection and Refraction
The picture above shows what happens when it goes into and then out of a parallel surface like
a pane of glass. Towards the normal when it hits the glass, then bends away from the normal
in the same proportion when it leaves. The ray of light going in and the one going out will be
parallel.
Solve a problem: (get a calculator)
Review sin and sin inverse. (geometry students have done some trig by now???)
Example 1 p. 494:
Do p. 494: 1 - 5 - Homework
When wave speed changes, what else changes? Frequency? Wavelength?
http://www.tedmontgomery.com/remarks%5C08.1-12/optics/index.html
f
v

The speed changes and the wavelength changes accordingly. The frequency
stays the same. As velocity goes down, the wavelength also decreases so that the ratio
remains the same.
Atmospheric Refraction
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Ch. 17 and 18 – Reflection and Refraction
A mirage is caused by the refraction of light in Earth’s atmosphere.
On a hot day, the air down low by the pavement (or desert sand, etc.) is hotter than the
air above it. Changes the index of refraction. Goes slower in the hot air close to
the ground.
This bending of light, is called atmospheric refraction – causes mirages.
http://www.slideshare.net/RamdasDhoni/ppt-on-atmospheric-refraction
Twinkling stars: caused by refraction through the atmosphere.
Called astronomical scintillation (or stellar scintillation).
Not seen from space or on a moon or planet with no atmosphere
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Ch. 17 and 18 – Reflection and Refraction
More twinkling the closer to the horizon
Dispersion in a Prism ****
http://www.cyberphysics.co.uk/topics/light/dispersion.htm
Look at a prism – when the light hits the material, violet goes slower and red goes faster
The natural frequency of the material in the prism is in the ultraviolet range. This means that
the violet light will slow down more (more vibrations and so slower light). The red doesn’t
slow down as much because the molecules are not vibrating any more than usual.
Since different frequencies of light travel at different speeds in transparent
materials, they will refract differently and bend at different angles.
The separation of light into colors arranged according to their frequencies is called
dispersion.
Look again:
https://phet.colorado.edu/en/simulation/bending-light
The Rainbow ****
To see a rainbow, you have to have sun in one part of the sky and falling rain on the
opposite side.
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Ch. 17 and 18 – Reflection and Refraction
See figure 29.27.
Rainbows can only be seen early and late in the day. When the sun is higher in the sky
(above 42 degrees above the horizon), the bow would be below the horizon.
A rainbow is actually a circle but the ground gets in the way. Sometimes you can see the
whole circle from an airplane!!
https://en.wikipedia.org/wiki/Glory_(optical_phenomenon)
https://www.google.com/search?q=glory+rainbows&espv=2&biw=1280&bih=701&source=ln
ms&tbm=isch&sa=X&ved=0ahUKEwjdzP-DwvbLAhUS32MKHf7NDTgQ_AUIBygC
A rainbow isn’t in a particular place. It moves with you. The person standing next to you
is technically seeing a different rainbow than you are.
This is not a rainbow – minute physics
https://www.youtube.com/watch?v=9udYi7exojk
Double rainbows: Double reflection within raindrops. - actually a reflection of the
primary bow. (images)
What color is on the outside of a rainbow?
What color is on the outside of a secondary rainbow?
Is it brighter or dimmer?
Use Figure 9 p. 499
Rainbow Lighting Point out that the sky is brighter inside the arc of a primary rainbow. This is because
many light rays from the Sun reflect at angles of less than 42°. Light of different wavelengths creates the
bright area within the rainbow. Little light is reflected beyond 42°. This creates a darkened area, known as
Alexander’s Dark Band, between the primary and secondary rainbows.
Total Internal Reflection
See this video for a great demonstration of this phenomenon:
http://video.mit.edu/watch/mit-physics-demo-refraction-a-total-internal-reflection-12044/
The critical angle for water to air is 48 degrees. This is the angle that the angle of
incidence that results in the refracted ray is 90 degrees – so the light will no longer
refract out, but will reflect in.
Total internal reflection is TOTAL. Mirrors only reflect 90 to 95 percent of incident light.
This is why prisms are normally used in optical instruments instead of mirrors.
The critical angle for diamond is 24.6 degrees. Such a low critical angle results in a lot of
internal reflection which is why diamonds are so shiny!
Optical fibers are transparent fibers that send light from one place to another.
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Ch. 17 and 18 – Reflection and Refraction
Because of the higher frequencies of light than electric currents, a pair of glass fibers as
thin as a human hair can carry 1300 simultaneous telephone conversations, while
a conventional copper cable can carry only 24.
Total Internal Reflection in Water
Estimated Time 5 minutes
Materials 1-L plastic soda bottle, laser pointer, plastic tub, water
Procedure Punch a small hole in the side of the soda bottle. Place the bottle at the edge of a table so that
the water will flow into the tub, then fill the bottle with water. Darken the room and shine the laser pointer
through the bottle from the side opposite the hole, aiming at the hole. Students should be able to view
total internal reflection of the red laser light through the water stream.
Other resources:
http://www.uq.edu.au/_School_Science_Lessons/UNPh28.html
http://www.ekshiksha.org.in/eContent-Show.do?documentId=61
Section 2: Convex and Concave Lenses:
***Lenses change the path of light.***
Converging and Diverging Lenses
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Physics Teacher Notes - Grothaus
Ch. 17 and 18 – Reflection and Refraction
A lens is a piece of transparent material, such as glass, that refracts light.
A lens forms an image by bending rays of light that pass through it.
Remember:
Concave mirror – converging mirror – mostly upside-down, real image (unless you get
really close)
Convex mirror – diverging mirror – right side up, smaller, virtual image (right side mirrors
– “objects are closer than they appear”)
Shapes of lenses -
When it thicker on the ends, the image diverges, so it’s called a diverging lens. A
diverging lens is also called a concave lens. This causes the
rays of light to look like they come from a single point, which is
also called a focal point.
A concave lens – diverging lens – always virtual, right-side up, smaller
View finder in cameras
Not going to look at these
When it is thicker in the middle, the image converges to a point, so it’s called a
converging lens. A converging lens is also called a convex lens. It takes
parallel rays of light and meets at a single point called a focal point.
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Ch. 17 and 18 – Reflection and Refraction
Key terminology that we need to know:
Principal axis
Focal point
Focal plane
Focal length
There’s a focal point on both sides because light can come in from
either side. They are the same distance away from the center of the
lens.
The film in a camera is in the focal plane behind the lens in the camera.
Online optics lab
What I want to know (from the back) – homework –
Looking at the five different areas, what is the difference in the images?
For each one:
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Physics Teacher Notes - Grothaus
Ch. 17 and 18 – Reflection and Refraction
I: bigger, smaller, or same size
II: Right side up (virtual) or Upside down (real)
III: Same side as object, opposite side AND
Closer, further or same distance from the center of the lens
Image Formation Summarized
A converging lens forms either a real or a virtual image. A diverging lens
always forms a virtual image.
Some Common Optical Instruments
Optical instruments that use lenses include the camera, the telescope (and
binoculars), and the compound microscope
The Eye
The human eye is similar to a camera. See the following slide show, slide # 20 for a
comparison: http://www.slideshare.net/FahdKhan1/converging-lens
http://www.familyconnect.org/parentsite.aspx?FolderID=22&TopicID=371&DocumentID=384
0
There are two special spots.
The fovea a small region in the center of your field of vision where your vision is
most distinct. More detail can be seen here.
The blind spot is a spot where the optic nerve connects. This spot has no rods or
cones so you can’t see at that spot.
Demo in the book – Figure 30.17
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Ch. 17 and 18 – Reflection and Refraction
The pupil looks black, but “red eye” with flash photography…. The light from the
flash reflects back from the blood on the retina.
With animals that need good night vision, there is an extra layer behind the retina
that reflects the light back to the retina. It gives it a “second chance” to pick
up the light. This is called the tapetum lucidum. This is what makes some
animals’ eyes look like they glow. “eye shine” . This can be different colors
in different animals. http://en.wikipedia.org/wiki/Tapetum_lucidum
Three common problems are farsightedness, nearsightedness and astigmatism.
A farsighted person can see far, but has trouble seeing close up.
Eyeball is too short or the cornea is too flat. The image forms behind the
retina. You need lenses (glasses) that are converging which focuses
the image forward, on the retina.
A nearsighted person can see close, but has trouble seeing far.
The eyeball is too long or the surface of the cornea is too curved. You need
lenses (glasses) that are diverging which focuses the image back, on the
retina.
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Ch. 17 and 18 – Reflection and Refraction
An astigmatism is when the cornea is more curved in one direction than the other
– the eyeball is uneven. You need a cylindrical lens that has more
curvature in one direction.
http://www.nlm.nih.gov/medlineplus/ency/imagepages/19511.htm
http://hyperphysics.phy-astr.gsu.edu/hbase/vision/eyedef.html
Why are stars star-shaped?
https://www.youtube.com/watch?v=VVAKFJ8VVp4
Any bending of a wave that isn’t caused by reflection or refraction is called diffraction.
http://phet.colorado.edu/en/simulation/wave-interference
When you look at this simulation with “slits”, it will show you the interference
patterns when there is a slit. When the slit is wide, the effect is small. When the
slit narrows, the diffraction is more pronounced.
Diffraction also occurs to some degree for all shadows because there is a barrier that
keeps light from going straight through. There will be some distortion. This can be
best seen with monochromatic (single color) light.
The extent of diffraction depends on the relative size of the wavelength compared
with the size of the obstruction that casts the shadow.
See figure 31.9 – small obstructions (compared with the wavelength) – the wave tends to
“fill in” behind it. Shadows are dim and fuzzy or there is no shadow at all.
When the object and the wavelength are closer in size, there is less “filling in” and sharpe
shadows are formed.
Example: FM radio waves have a much shorter wavelength than AM radio waves. The
FM radio waves don’t “fill in” as well as AM waves. That’s why you might have
bad reception with FM, but can hear the AM stations very well.
They call TV and FM waves, “line of sight” waves. Obstacles between the
transmission tower and the antennas will cause interference and thus, reception
problems.
AM radio waves fill in behind the obstacles, so less reception problems.
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