Learning session 3: Refraction
Major point 3.1: Light changes direction as it moves from one medium to another
Major point 3.2: The angle of refraction depends on the angle of incidence, but
these are not necessarily equal
Major point 3.3: Concave lenses cause light to diverge
Major point 3.4: Convex lenses cause light to converge
Here is a selection of glass shapes. We call them lenses. Select four of two different shapes and shine light
through these lens – compare and contrast what the different shapes of lens do to light and write down all
of your observations.
Discussion
Reference should be made to how thick or thin the lens is, that it is transparent, that concave curves
inwards and convex curves outwards. You may have samples that are bi-concave which mean both sides
curve inwards and bi-convex where both sides bulge outwards.
Let's explore how light behaves when it travels through these lenses.
The students should be encouraged to look at objects through the lenses, comparing their effects when the
lens is close or further away from their eye. Does it make it bigger? Smaller? Upright? Upside down? Have
the students hold a lens in front of a light source causing it to shine onto a piece of white card. Draw their
attention to the fact that the light bends as it travels through the lens. Have them experiment by moving
the card closer and further away from the card. For convex lenses a strong point of light will be produced
at a particular distance, the focal length, and this can be measured. Concave lenses on the other hand
behave differently and do not bring light to a sharp focus, but rather, cause it to diverge.
Experiment
Torches, lenses, concave lens, convex lens, white card, ruler, pencil.
Let’s share what we have discovered.
Light rays are bent (refracted) as they move through lenses. A convex lens will focus light to a focal point.
Objects will only be magnified by a convex lens if it lies within its focal length. Distant objects appear upside
down and reduced. A concave lens will spread light apart and distant objects will appear smaller.
So, now we know that lenses can change the direction of light and that one reason for this is the shape and
thickness of the transparent medium. Scientists have called the bending of light as it moves from one
medium to another refraction.
Now, carefully observe this demonstration. Can you work out what is happening?
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Demonstration
Experiment to demonstrate refraction. Resources needed, coin, glass water, cup, blu tak, non transparent
bowl. Push the blu tak onto the middle of the coin. Push the coin blu tak side down in the centre of the
bowl so it is secure and wont move when you pour the water into the bowl. Put water into your cup to pout
into bowl. Move backwards away from the bowl until you cannot see the coin. Very slowly pour water into
the bowl until the coin appears before your eyes. Pour a bit more water, very slowly and continue adding
water as the coin reveals itself. What are you seeing? When empty, the bowl stops you seeing the coin.
When the bowl is full the light bends over the edge so that you can see the coin.
How can the coin suddenly appear? I never moved it. Each of you can explore this effect by using a plastic
cup, some water and a straw. Experiment by holding the straw at different angles in a glass full of water
and observe. What do you notice? What do you think could be happening?
Experiment
Resources needed: straws, transparent cups, water. Students fill the cups with water and place the straw
into the cup. When they look it will appear that the straw is bent. If they hold the straw perpendicular with
the surface of the water there will not be any refraction. If they hold the straw at different angles entering
the water it will cause different angles of refraction.
Let’s share our results. What did you observe? What could be causing this effect?
Refraction is caused by light changing speed as it moves from one medium to another of differing density.
For example moving from air (little density) into water (greater density). The light slows down and this
causes it to bend. Water isn’t the only medium that slows light. Here is a prism. These types of prisms are
dense and usually made from glass or plastic. I wonder what would happen if we shone light through this
dense medium. Would it have the same effect as it did with the water and straw experiment? Would it be
able to bend light?
Experiment
Use torches, lasers, sunlight to shine through prisms and observe the effect.
Encourage the students to notice that the prism bends light. Try shining the light at different angles and
into different parts of the prism. They may also notice it results in a rainbow. At this stage do not explain
why that could happen, rather point out that is very interesting and I wonder why that happened and how.
If a laser pointer is used and it is found that no spectrum of colours forms, this too should be noted. It is a
modern day equivalent of an experiment conducted by Newton.
Discussion
So, we have discovered that light bends as it travels into media of different density and that this bending of
light is called refraction. I noticed when we were experimenting with prisms that some of you managed to
create a rainbow. I wonder how the prism created a rainbow from white light and that got me wondering
how can we see rainbows in the sky, or sometimes even from your garden hose? We will explore rainbows
and colour in the next session.
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Free choice activity
Free choice activity, including:
●
Developing a device that uses a lens or lenses in its design, possibly making a drawn plan first.
Some ideas could be a reading device such as glasses or maybe a style of microscope or telescope.
●
Researching and presenting a report that shows the development of the first lens
Lenses of one type or another have been used since antiquity, but the first known formal treatment of
refraction and the design of lenses was by the Persian mathematician Ibn Sahl. See
http://en.wikipedia.org/wiki/Ibn_Sahl
●
Finding out what the American President Benjamin Franklin did with lenses and reporting to the class.
Franklin is credited with the invention of bifocal spectacles. See http://en.wikipedia.org/wiki/Bifocals
●
Learning how to draw a refracted ray by watching this demonstration:
http://www.youtube.com/watch?v=7P-JhRzDz7w then having a go at creating your own refracted ray
drawings. Experiment with different shaped media
●
Watching this clip on light and lenses, http://www.youtube.com/watch?v=TroFa0HrA5Q, and reporting
back to the class anything that you found interesting.
●
Lenses made of water are used in third world countries as cheap glasses – research how these work to
correct a person’s sight.
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