Light: How light travels
Objectives:
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Recognise that light travels in straight lines
Use the idea that light travels in straight lines to explain how objects are seen
Use the idea that light travels in straight lines to explain why shadows have the same
shape as the objects that cast them
Plenary
Revise what pupils have already learnt about light with a demonstration of how light is
reflected from an object. Use a mirror and a laser pointer with a fine, bright beam. Darken the
room or, for visual appeal, perform the demonstration inside a tank filled with smoke. Pupils
should see that when the beam is pointed at the mirror, light is reflected from the surface.
How does light reach our eyes?
Using the demonstration as a reminder, pupils should be able to explain that (if light is not
travelling directly from source to eye), it is reflected from an object and then travels into the
eye. Emphasise that the light clearly travels in a straight line.
Pupils must note:
Light travels in a straight line. When an object reflects light into our eye, we can see that object.
Ask the pupils to illustrate this with a diagram which includes a source, an object and an
observer. Straight lines should point from source to object and from object to observer.
EXPERIMENT: Distribute copies of the periscope pattern. Pupils should cut along the solid
lines, fold along the dashed lines and glue squares of mirror board on the indicated tabs. To
make their periscopes more robust, glue the pattern to a piece of card. Give the pupils
missions to spy on other classes without being seen, then report back on what they saw. They
should discuss how the periscope might work and fill in the simple ray diagram.
Light moves in a straight horizontal line from the source, is reflected from the upper mirror
then travels in a straight vertical line to the lower mirror, where it is reflected and exits in a
straight horizontal line into the eye.
The shape of shadows
Using items from the classroom or making moving shadow puppets (using card, split pins
and straws), encourage pupils to form interesting shadows with light from a projector.
Ask what is happening to the light when a shadow is made. Pupils should already know that
shadows are made when light is blocked by an object. As a class, attempt to draw a simple
2D diagram of a sheet of paper casting a shadow
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Point out that you can tell what an object looks like from observing its shadow. Tear a hole in
the middle of the paper and demonstrate. Adapt the diagram:
Explain, referencing the diagram, that light is only blocked within the outline of the object.
Because light travels in straight lines, the resulting shadow will mimic the shape of the
object.
Pupils must note:
Shadows are the same shape as the objects which cast them because light travels in straight lines.
Colourful light (non-statutory)
Cast shadows using coloured objects, such as purple sunglasses or a glass of squash. Ask the
pupils to describe how the shadows are different from normal shadows. Pupils may create
their own stained glass inspired art by applying glass paint to clear sheets of acetate, then
shining light through them to create colourful shadows.
Using a Pyrex or glass prism in a dark room, split white light into the spectrum of colours.
Ask pupils where they have seen this before (rainbows, light split by gems). Introduce the
idea that these colours are hidden inside white light. Pupils may draw a diagram of the white
light being split into a rainbow.
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Possible extra-curricular questions
Why do straws in a glass of water look bent?
Light travels in straight lines, but when it passes from one medium to another (between water
and air), its movement is affected. The light is being bent, not the straw. This is a process
called refraction. It is caused by light travelling at different speeds in water and air. The
property of a material that tells us how fast light travels in it is called its refractive index.
How are rainbows formed?
Rainbows are formed by refraction (see above) of light in raindrops. White light is made up
of coloured light of different wavelengths. Depending on its wavelength, each colour is
refracted/bent by a different angle in the droplet. When the light emerges, the colours emerge
at different angles, causing a rainbow.
How are some shadows colourful?
When a colourful translucent object (such as a stained glass window or a glass of squash)
casts a shadow, the shadow may also be colourful. This is because only light of a certain
colour is being let through the object while the other colours are blocked.
Why do the colours of the rainbow add up to white?
In painting, all colours can be made with the primary colours (red, blue, yellow) and in
electronic displays, all colours can be made up with standard LED colours (red, green, blue).
However, when it comes to optics, different rules apply. Colours correspond with different
wavelengths/energies of light. Red light has the longest wavelengths/lowest energy and violet
light has the shorter wavelengths/highest energy. All other colours are in between, and
correspond to a different wavelength and energy. White light is a combination of every
possible colour.
© Education Umbrella 2015