PHYSICS - UNIT 2: LIGHT
Introductory Activities
Teacher Notes
These are a set of introductory activities. They are intended to stimulate the students' curiosity about
what light does and to express that curiosity in questions that they would like to find answers to. The
questions from the whole class and other physics classes are combined and submitted back to the
students. At the end of the unit, as an assessment exercise, they are expected to submit written
explanations to a specified number of the questions. The studnst choose which questions they answer.
The activities are designd to be short, taking about 5 - 10 minutes each and are done over two periods.
The students can move freely from one to the other as activities become free.
The tasks are designed as a worksheet that the students complete and hand in. It does not need to be
assessed, although some feedback to student answers and comments woudl be appropriate.
Student Worksheets (pages 2 - 5)
1.
Reflection of Light
2.
Large Curved Mirrors
3.
Making Light bend
4.
How deep is the pool?
5.
How can a magnifying glass be flat?
6.
Investigating a Slide Projector
7.
Investigating Bifocal spectacles
8.
Rainbow produced by a prism
9.
Colour Mixing using Light boxes
10. Do you see more of yourself in a mirror if you go closer?
Other possible activities (page 6)
There are many possible activities that could be included, the only limits are availability of equipment,
amount of bench space, and feasibility of the activity. Some grand ideas did not work in practice.
Context Questions (pages 7 & 8)
A compilation of the questions that previous students have produced is included. They are useful as a
reserve to augment the questions a particular class or year generates.
Photonics (pages 9 - 11)
A small selection of the practical activities from the Photonics workshop for the Unit 4 Detailed Study
that are relevant to Unit 2 have also be included.
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Physics Unit 2 : Light
What is “The Introductory Activity?”
The Introductory Activity is a set of short practical tasks designed to stimulate your interest in the
topic and to encourage you to come up with questions, called Context Questions, for which you
would like answers.
These "Context Questions", will form an important part of the work of this topic.
Typical Context Questions might be:How does a microscope work?,
How do telescopes differ?,
Why does light bend?,
How is a rainbow formed?
After the short practical tasks, you are asked to come up with about three questions of your own that
you would like answers to. You will then form a group of three with two others to compare your
questions and come up with a list of about six questions that the group thinks are worthwhile.
When you have done this, your group will combine with another group and produce a list of about
nine questions.
After this, the questions from these large groups will be combined for the whole class.
What's the purpose of the Context Questions?
The questions will guide our work during the topic. Answers to many of the questions will come up
during class work, experiments, problem solving and general reading.
At the end of the topic, as part of your Assessment you will need to submit answers to a selection of
the questions.
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Reflection of Light. How do flat or “plane” mirrors work?
Equipment: Mirror strip, pin board, sheet of paper, 4 pins and a protractor
Place a strip mirror in the middle of the page parallel to the short
edge. Now draw a line along the back of the mirror.
Place two pins in the page, so that they are in a line pointing
towards the mirror at an angle between 30 and 60 degrees, and a
few cms apart.
With your eye down at mirror level, look into the mirror and
move your head to locate the images of the two pins in the
mirror.
Move your head until the images of the pins in the mirror appear one behind the other.
Now place two extra pins in the page between your eye and the mirror so that these two pins line
up with the images of the first two pins.
Now you move the pins and mirror away and draw lines through each of the pairs of pins
towards the mirror line.
Where do your two lines cross?
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Draw a line at right angles to mirror line where the two lines cross and measure the two angles
between this line and your two pin lines.
How do the angles compare? ............................................................................................
Large Curved Mirrors
There are two large mirrors. One curves inwards and is called “Concave”, the other curves
outwards and is called “Convex”.
For each mirror, walk up to it from several metres away. Describe how your appearance or
“image” changes. (Refer to size, direction and distortion if any.)
Concave Mirror: ............................................................................................................................
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Convex Mirror: ..............................................................................................................................
Where have you seen Convex mirrors before?
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Stand about a metre in front of the convex mirror and then in front of a plane mirror and
compare how much of the room you can see in each of them. In which type of mirror can you
see more?
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Making Light bend
Equipment: Glass block, pin board, sheet of paper and 4 pins.
Place the glass block with the largest side down in the middle of the sheet of paper and draw a
line along each of the four edges.
Now place two pins in a line, a few cms apart, to meet a long edge at about 45 degrees.
Looking through the glass block from the other side, place two more pins in the paper, about a
few cms apart, so that all four pins appear to line up.
Now draw lines through each pair of pins to meet the nearest edge.
Connect up these two lines in the space where the glass block was by a straight line.
The line represents the path taken by the light as it travels from the head of the first pin through
the glass block to the head of the last pin, and then to your eye.
Describe what happens to the light as it travels from air into the glass block and what happens
as it leaves the glass block into the air.
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How deep is the pool? - Apparent Depth
Equipment: Glass block, 2 small sheets of graph paper, ruler and calculator
When you look into a swimming pool, the water does not appear as deep as it really is. This
experiment explores this phenomenon.
Place the smallest face of the glass block on the sheet of graph paper. Now look down through
the glass block at the graph paper.
What do you notice?
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Now slide the other sheet of graph paper up the side of the block until the pattern inside the
block and the pattern outside the block match up. The raised sheet of paper indicates that the
glass made the bottom sheet appear closer to you. This is because light is slowed down in glass.
You can calculate how much the light has slowed down by measuring the distance from the top
of the block down to where you raised the graph paper and dividing this by the height of the
block.
Make your measurements and calculate how much you think the light is slowed down by the
glass.
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How can a magnifying glass be flat?
Equipment: Overhead projector lens, a convex and a concave lens and a page of text.
Place the flat overhead projector lens over the printed page and slowly lift it.
What do you notice?
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Now lift the convex lens (fatter in the middle) off the page, then the concave lens (thinner in the
middle). Which type of lens is the flat lens like?
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Look closely at the surface of the flat lens. what do you notice?
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Flat lenses are called Fresnel lenses. They are often found on the rear windows of vans. Why?
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Investigating a Slide Projector
Equipment: Slide projector, ruler and slide with lines 1 cm apart on it (Use the diffraction slide
from the Diffraction kit, the small black box).
Use the slide in the slide projector to measure and calculate how much the projector magnifies
the slide
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How would you move the projector to get greater magnification?
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The image is now out of focus. To get back into focus do you have to now move the lens closer
or further away from the slide. Which way do you move the lens?.
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Investigating Bifocal spectacles
Equipment: Light box, bifocal spectacles and a screen
Arrange the light box to produce parallel rays of light.
Shine the rays through different sections of the spectacles onto the screen.
Convex sections converge or bring the rays together, concave sections diverge or bend the rays
apart.
Draw the outline of the spectacle and the different sections and label them convex or concave.
Rainbow produced by a prism.
Equipment: Lightbox, prism and a screen.
Draw the path of two different colours from the light source, through the prism and onto the
screen, which colour is bent the most?
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9.
Colour Mixing using Light boxes, colour slides and a screen.
Equipment: Three Light boxes, colour slides and a screen.
Use the light boxes and the colour slides to find the effect of mixing the following colours:
Red+ Blue, Red + Green, Blue + Green
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10.
Do you see more of yourself in a mirror if you go closer?
Equipment: Long plane mirror place against a wall
Stand in front off the large plane mirror and walk up and back from the mirror. Now answer the
question.
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11. Telescope
If you have a large telescope bring it out and aim it at a distant object through the window.
Student Name : ...........................................
Context Questions:
From doing the tasks and your other interests, write down 2 or 3 questions for which you would like
answers.
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Some Context Questions
Why does a magnifying glass enlarge objects?
How does a telescope work?
What would be the effect of swapping the lenses in a telescope?
How does a Fresnel lens differ from a normal lens?
How does an optometrist make lenses
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Other activities that were less successful.
12. Investigating Telescopes
There are many different types of telescopes using different types of lenses.
a)
An early type was called the Keplerian telescope which consisted of two convex lenses. Look
through the telescope at a distant object and describe the image.
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b)
Galileo used a telescope using a concave lens as the eyepiece lens. Look through this telescope
and compare the image with that of the Keplerian telescope
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Investigating the Microscope
The lenses have been selected and arranged to make a microscope.
Look through the eyepiece lens at the far screen and describe what you see.
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Move the eyepiece lens towards the other lens called the “objective” lens and describe what you
see.
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These tend not to work because the diameter of the two lenses commonly found in schools is too
small to give a large enough image to fill enough of teh visual field to be easier to see.
Other activities depending on your equipment
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Laser communication
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Optical fibres
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Light Context Questions
These are the questions generated during the Introductory Activity. They have been grouped under
different aspects of light. As you learn more about light you will be able to answer more and more of
these questions.
By the the end of the unit you must submit answers to a selection of 12 questions. Your answer to
each question should be no more than a few lines with a diagram if necessary. Your answers will be
assessed and the grade will contribute to the overall assessment of your communication skills.
How is a laser different from a normal light
globe?
How do medical lasers differ from the school
laser?
How does a hologram work?
How can a laser beam be so narrow?
How can you show the path of a laser?
Optical Instruments
How does the slide projector enlarge the slide?
How does a periscope work?
Why are concave mirrors so scary?
How does a microscope work?
How does a bicycle reflector work?
How is a telescope different in construction
from a microscope?
If the telescope image is upside down, how do
you make it right side up?
How does the magnification produced by a
projector vary with the distance between the
object and the screen?
What’s the difference between a magnifying
glass and a telescope?
What makes a flat lens convex or concave?
What is the highest power a telescope can
magnify?
How do the grooves in a Fresnel lens work?
Why do convex mirrors reflect more of their
surroundings and make them smaller?
Why does a concave mirror show both upright
and inverted images?
How does a convex lens make objects larger?
How does a mirror work?
Why is bullet proof glass so strong?
Why does my image appear upside down when
looking into a concave mirror?
What is the difference if you stand in front of a
plane mirror and in front of a concave or a
convex mirror?
Optical Fibres
How does light reflect or bend down an optical
fibre?
From what material is the optical fibre made?
How does an optical fibre work?
Why can light go through the optical fibre?
Colour
What makes substances absorb colours of the
spectrum and reflect others?
When colours are absorbed, is heat released?
If so, do different colours release different
amounts of heat?
Why is the sky blue?
How come some colours bend more than
others?
How do we see colours?
How does a rainbow look from above?
What is colour?
Why so some colours reflect heat?
Why does colour in a rainbow appear in that
order?
Why is the spectrum formed when light passes
through a prism?
Why does violet bend more than red in a
prism?
Why does light split up into colours of a
spectrum when sone through a prism/
Laser Light
How is sound converted into light for
transmission by the laser?
Why can the laser beam penetrate the glass
fibre but not our bodies?
If the laser travelled through a thick piece of
glass, will the transmission be slower?
How does the laser light carry the signal?
How does the laser read CDs?
Properties of Light
If light bends towards the normal as it slows
down, what happens when it is accelerated past
the normal speed as it bends away?
Why is light emitted from combustion?
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Can you trap light?
What does light speed up when it leaves glass?
How does light bend?
How does a magnifying glass turn an object
upside down?
What type of materials can be used to slow
down light?
How can light travel through solids such as
glass and not other solids?
Does light lose energy as it goes through glass?
Are there different types of light?
Why is white light not spread into a spectrum
after passing through window glass
Why does light travel slower in a glass block?
Why are the images produced by the concave
and convex mirrors so different?
What, in theory, do light waves look like?
How does the light bend in the glass block to
make the paper appear closer?
How do we measure the speed of light?
If a car travelling at the speed of light and its
head lights were on would you see any light?
Can light be slowed? Is so, can light be slowed
by gravity?
What makes a substance more reflective than
another?
Why does light travel in a straight line?
Why can’t I see light in a dark room?
Will a shadow move faster than light?
How can I make light?
Will a shadow appear faster than light?
Does the sun have a shadow?
How does light carry signals?
Why does light slow down in different media?
Is it possible to see something without it
having a shadow?
How does a light globe emit light?
How can concentrated light burn through
objects?
Why does light bend?
Why is the sun luminous?
What would happen if something travelled
faster than the speed of light?
What objects don’t reflect light?
Human and Animal Vision
Do human eyes have convex or concave
lenses?
Why do cats’ eyes reflect?
Why can’t we see ultra-violet?
Does a shorted sighted person use concave or
convex lenses?
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1.
2.
Selected Activities from the 2010 VCE Photonics Workshop
Bottle Of Light
a) Light Makes Its Escape,
b) Modern Optical Fibre.
1. Bottle of Light
Fibre optics – total internal reflection – critical angle
In 1870, before members of the prestigious British Royal Society, John Tyndall showed how a light
beam could be guided in an arcing stream of water, Tyndall shined a bright light into a horizontal pipe
leading out of tank of water. Then, when the water was allowed to flow out and downward in an arc,
light rays traveled inside the water until they were broken up by the water striking a collection pan.
With the help of a drink bottle and a laser pointer you will duplicate this experiment.
What it shows:
A Beam of laser light can be trapped inside a stream of water by total internal reflection. This is the
aquatics equivalent of a fibre optic cable.
How it works:
A stream of water flows from a hole in the side of a drink bottle (figure 1). The critical angle of 49º is
such that total internal reflection will occur in the stream even when it is reduced to almost a trickle.
Imperfections in the stream (and scattering agents added) allow some of the light to escape, and the
effect is seen as a sparkling waterfall.
The water reservoir is a 2L drink bottle, and the stream emanates from a hole just above the base
section (about 8cm from the bottom). To ensure a smooth flow a hole larger than 5 mm is cut in the
plastic and a piece if clear tape stuck across it – the hole being made using a regular hole punch; this
avoids unclean edges that occur when the plastic is cut and the thin wall produces a more laminar
flow.
Figure 1 Total internal reflection within a stream of water
Direct the stream of water onto a white tile in the sink, so you can see if the water optical fibre really
works!
Discussion:
The index of refraction of water is 1.33, and thus the critical angle for a water/air interface is sin-1
(1/1.33) = 49 º. So long as the water stream does not bend at too sharp an angle, light traveling along
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the length of the stream strikes the water/air interface at an angle greater than 49 º with respect to the
normal to the interface and is thus totally internally reflected.
2a. Light makes its Escape
The light carrying portion of an optical fibre must be protected
In this activity you will observe how changing the optical density (refractive index) of the material
surrounding the optic fibre will affect the fibre's light-transmitting ability.
Materials required
 Laser pointer
 Plasticine
 Acrylic fibre
 Small piece of white card
 Petri dishes of water and paraffin oil
Method
OBSERVE LASER SAFETY PRECAUTIONS WHILE DOING THIS PRAC
Insert the fibre into the end of the laser pointer. Observe the amount of light transmitting along the
fibre by holding the card to the end of the fibre. Gently bend the fibre into a 'U' shape and observe the
amount of light transmitted by the fibre.
Slowly immerse the bent part of the fibre into the water in the petri dish. As you immerse the fibre in
the water, observe the amount of light coming out the end of the fibre using the card.
Can you see light escaping from the fibre?
Where does the light go? Repeat in the paraffin dish. Turn the
laser off. Wipe the fibre with the paper towel provided.
Results
As the bottom of the fibre is immersed, the amount of light coming out the far end of the fibre
decreases. You should be able to see light escaping from the fibre by looking at the bottom of the
dish.
Discussion
The decrease in light from the fibre end is caused by the change in optical density outside the fibre
when it is dipped in water. The optical density of water is closer to that of the fibre than the optical
density of air; therefore, it doesn't trap light as well. When the light in the fibre encounters the water,
some of it escapes and travels to the bottom of the pan. The U-shape in the optic fibre increases the
amount of light escaping when it is immersed in water.
The correct term for optical density as we have applied it really is "refractive index" or index of
refraction. The refractive indices of the three materials that you worked with in this experiment are
shown in the table below. You might now ask: What good are optical fibres if their ability to transmit
light can be affected by conditions around them? If this were actually the case, they would not be very
useful. Most fibre optics used for commercial applications are manufactured with a coating around
the central light-carrying portion so that external conditions do not affect them. This coating is called
"cladding" while the central "light-carrying" portion is called the "core". A fibre's cladding always has
a lower refractive index than the core.
Water: 1.33
acrylic (plastic) 1.45
air: 1.00
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2b. Modern Optical fibre
Commercial fibre is very pure and has a protective ‘cladding’
The acrylic fibre used in previous experiments carries light from one end to the other, but it doesn't
really do a very good job. To transmit light long distances, commercial optical fibres must be
composed of ultra pure transparent materials. For example, some commercial optical fibre material is
so pure that the light lost when traveling through a one hundred kilometer length is less than 10
percent of the light which 'entered the fibre. In the illustration is a basic optical fibre, with concentric
layers of core and cladding. The fibre you will use in this experiment contains a central "light
carrying" core and a very thin (10
transparent. You probably won't be able to distinguish it from the core.)
Materials Required
 Laser pointer
 Plasticine
 Optic fibre
 Small piece of white card
 Petri dish full of water and paraffin oil
OBSERVE LASER SAFETY PRECAUTIONS WHILE DOING THIS PRAC
We are going to repeat the last experiment, but using proper optic fibre, rather than just a length of
plastic fibre. Insert the optic fibre into the end of the laser pointer. Observe the amount of light
transmitting along the fibre by holding the card to the end of the fibre. Gently bend the fibre into a 'U'
shape and observe the amount of light transmitted by the fibre.
Slowly immerse the bent part of the fibre into the water in the trough. As you immerse the fibre in the
water, observe the amount of light coming out the end of the fibre using the card. Can you see light
escaping from the fibre? Why not? Repeat in the petri dish full of paraffin. Turn the laser off.
Results
As the bottom of the optic fibre is immersed, the amount of light coming out the far end of the fibre
stays the same. You should not be able to see light escaping from the fibre when looking at the
bottom of the trough.
Discussion
Light is transmitted from one end of the fibre to the other because light is being 'guided by the central
fibre core and trapped inside by the outer cladding layer. Light intensity doesn't change when you dip
the fibre in the water because the refractive index (optical density) immediately surrounding the
central core doesn't change as it did in previous experiment. The cladding layer remains constant and
acts as an optical shield between the fibre core and the water.
The fibre you just finished experimenting with is made of plastic. It is one of the two most commonly
used materials in commercial optical fibres. The other material is glass – commonly called ‘silica’ in
the technical community.
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