Learning session 1: Properties of light
In the following lesson plan, descriptions of classroom activities appear in normal text, material spoken by
the teacher is italicised, and explanatory notes for the teacher are shown against a yellow background, as is
this paragraph.
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Major point 1.1: Light is intangible
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Major point 1.2: Light travels in straight lines
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Major point 1.3: Light travels very quickly, but not infinitely so
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Major point 1.4: Light interacts with matter by being absorbed, transmitted, or reflected.
The teacher is found by the students on their arrival shining a torch or laser pointer around the room with a
puzzled expression. After a time, someone will ask why.
I’m trying to figure out how something I do over here makes a bright spot appear over there. Any ideas?
[See note 1.1]
This is an appeal by the teacher for help, engaging the students. A typical response from a student would
be "It's just a beam of light from the torch", accompanied by a look of incredulity, but most will soon realise
that there is more to light than meets the eye. A student will typically respond with something like: "It's a
beam of light from the torch!"
What's a beam of light? I can't feel anything coming out of the torch, and I can't grab hold of it. I can only
see a spot of light appear over there. But when I turn the torch on and off here it appears or disappears over
there at the same time. It will show up on a wall, but not on a window, but sometimes when I'm pointing it
at a window the spot turns up somewhere else completely! I can't get it to go around corners and it won't
go through my hand. If I point the torch through this cellophane, the spot on the wall is the colour of the
cellophane! How does it know? What's going on here?
Discussion
The teacher should encourage a general discussion of the properties of light that such a simple experiment
demonstrates, and these should be listed. Entries would include that somehow an action at one place has
an effect at a distance, that that can be blocked, that the effect depends on the type of substance involved.
One idea that should not be raised at this stage if possible is the idea that light actually travels from place
to place. While it certainly does, this conclusion can be reached on essentially logical grounds and the
process of doing this is instructive for the students. If it should arise, no great harm will have been done
however.
Development of a list of properties of light from the Major points above : speed, light can travel very fast; It
reflects off some objects; Light travels through some objects; light can disperse and make a rainbow; light
can cause interference with other light sources; Refraction light can bend as it goes through some objects.
I think we need to investigate this further and I have some resources we could use, but before we start we
should record our questions and include any predictions we may have.
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Here the students should be encouraged to adopt elements of the scientific method: hypothesis formation,
experimental design, experimentation, result recording, and analysis. Resources provided should include a
torch and possibly also a low power laser pointer (with a warning given about not looking into the beam),
mirrors, glass, paper; cardboard, coloured cellophane, string, stopwatch, scales, and an empty box with a
lid; while other materials from the classroom may also be used.
Experimentation
The teacher should move around the room and discuss the experiments and students predictions, focusing
student attention on observation and interpretation. "What do you think will happen?" "Why do you think
this is happening?" "Would you get the same result if ..?" "Do you get the same result when you repeat the
experiment?" "Does the experiment lead you to new questions?"
Students should be led to verify that a light beam illuminates all points along its path, and so perhaps
something exists within that beam. That this path always seems to be a straight line can also be raised and
suggested as a subject for experiment. One method would be to run a string from the torch to the
illuminated spot, which, only when pulled tight, is completely illuminated. Another would be to try to pass
the beam through three cardboard plates with holes in them and to find that this was only possible when
the holes were in a straight line. Attempts to trap light in a box and weigh it can be encouraged, including
comparing the weight of a box containing a torch when it is on, and when it is off. Attempts can be made to
see if there is a delay between turning a torch on and the appearance of the distant illuminated spot using
a stopwatch. These of course will "fail", but are instructive.
The types of interaction of light with matter can be explored: reflection, transmission, possibly with colour
changes, and absorption.
Let's share our findings. What can we conclude from these results and do we have any new questions?
The teacher should let each student/group present their findings. They should share their questions and
predictions and which resources they used. They could talk about the experimental method they chose and
why. Were there any other factors that affected their results? They would also include their results and
conclusions and any new questions that arose. The new questions should be documented and kept.
I noticed that some of you tried to measure how fast light was with the stopwatch, but no-one seemed to
have any luck. What could this mean?
Discussion
[Two main hypotheses should arise, but these may need to be prompted: that there is indeed something
produced by a light source that travels in a straight line to its destination (Empedocles), or that something
happens instantaneously at all points between the source and destination, causing light to arise from the
space there (Aristotle). Logic should lead to the conclusion that something must be produced and travel,
after all, if nothing did, how would some parts of space 'know' to become part of a light beam? Either light
is travelling, or something else even more bizarre is! This idea that light travels leads to two further
concepts: if something is travelling, it must be something quite different from matter as it has no
perceptible mass and is not affected by gravity since it travels in a straight line and we don't find pools of
light on the floor; and further, that if it travels at all, it must have a speed (Empedocles again), yet it
couldn't be measured so it must be very quick indeed. These last ideas are presented to the students a little
later.
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Lots of scientists have tried to measure the speed of light incluiding Galileo, Rømer and even Einstein. Finally
in 1975 it was measured to be around 300 000 km per second – that is about a billion kilometre per hour! In
space distances are so great the unit of distance is the light year – how far light travels in a year….can you
work this out?
Calculation of how far light can travel in a year
The accepted figure for the speed of light in a vacuum is c = 299,792,458 m / s. This distance travelled in a
year is therefore:
299,792,458 x 60 x 60 x 24 x 365.25 = 9,460,730,472,580,800 m
That may seem like a stupidly large number of no use to anybody, but it is really very useful, especially to
astronomers. That's because stars and galaxies are so far apart that the "light-year" makes a handy unit.
The light from our Sun, our closest star takes 8 minutes to reach us….just think 7 minutes ago the sun could
have died and we won’t know for another minute. Don’t worry the sun is not ‘projected’ to die for another
100 million years and that is a completely new Science unit.
Light is weird as we only ever see it taking the shortest distance from point A to B – from this we say that
light travels in straight lines.
We did see it bounce off some substances, pass through some, sometimes changing colour along the way,
and get completely absorbed by others.
During this unit we will explore these curious properties of light further but for now you may want to choose
an activity to be done in a group or independently. You can do more than one if you like. Here are some
ideas, but if you want to try something else, let me know.
Whenever students are asked to research information on the computer you need to make sure they are
equipped with the necessary skills. The information at this link is be very useful and student-friendly in this
regard: http://www.kidscomputerlab.org/index.php/research-skills/
Free choice activity, possibilities including:
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Given that light travels at 299,792,458 meters per second, calculate how many kilometres light can
travel in an hour, a day, a week, and a month. How far could a beam of light have travelled since you
were born? Would a person with a huge telescope on a planet orbiting around Van Maanen's star, one
of our close neighbours only 14 light-years away, have been able to see you on the day you were born?
There are 1000 m in a kilometre. There are 60 seconds in a minute; 60 minutes in an hour; 24 hours in a
day, and use 365.25 as the number of days in a year. The last part prompts the student to realise that in the
cosmic sense, light is slow.
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Research how Galileo began to measure the speed of light? Look carefully at the scientific method he
used and refine/improve upon it. Present a poster of how you would have done his experiment back in
the day.
There is abundant information at wikipedia, http://en.wikipedia.org/wiki/Galileo_Galilei , but another good
source is http://www.sciencekids.co.nz/sciencefacts/scientists/galileogalilei.html and
http://galileo.phys.virginia.edu/classes/109N/lectures/spedlite.html
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Using the resources given earlier – come up with a question that you wish to do an experiment upon; write
a hypothesis and then do an experiment to test your hypothesis – remember to work like a real scientist
and record all of your ideas, thoughts and results.
Resources
Laser pointers, torches, mirrors, coloured cellophane glass, paper, cardboard, string, stopwatch, scales,
empty boxes, other common classroom resources, computer access, library.
References
http://en.wikipedia.org/wiki/Speed_of_light
http://www.colorado.edu/physics/2000/waves_particles/lightspeed_evidence.html
http://en.wikipedia.org/wiki/Fizeau-Foucault_apparatus
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