Bubble Film Projector
What to do: Press the button to dip the frame into the washing up
liquid and let it flip back to an upright position.
What happens: You can watch the screen to see the bands of colour
produced when light reflects from the thin film of liquid in the frame.
HOW IT WORKS
1. When the frame is dipped into the detergent solution and released,
a thin film is left across the circular opening. This is just the same as
dipping a cup upside down into a washing up bowl. When you lift it
out, there is a thin film of water and detergent across the open end of
the cup, with pretty colours reflected from the film.
Bands of colour move down slowly
2. Because the film of liquid is vertical,
it begins to run down, so gets thinner and
thinner at the top (and thicker at the bottom)
until it bursts. The colours on the film
gradually move downwards - the projector
inverts the picture, so the colours on the
screen move upwards.
3. The colours occur because white light is made up of a whole
rainbow of different colours, and only some of these colours are
reflected by a film of any given thickness. As the thickness changes,
the colour of the reflected light changes.
4. Light consists of fast-moving electromagnetic waves and the different
colours in white light all have different wavelengths. The waves are
very tiny - the longest are those of red light and are about
0.0007mm long! The thin film starts off about ten times as thick as
this, and thins down to much less than a wavelength before it breaks
soap film
(magnified)
Most light goes
straight through
white light
2 beams of reflected light
Detergent molecules
line the surface
5% reflected
5% reflected
red
orange
yellow
green
blue
violet
= white light
0.0007mm
red
white light
yellow
violet
5. Light from the bulb bounces off both the
front and the back of the film, so the
reflected light consists of two overlapping
beams. One of these beams has been
through the film and back again so has
travelled further. Even though this is not
very much further, it is very important in
terms of light wavelengths.
soap film
A.
6. Wherever two sets of identical waves overlap (A.), they will
reinforce one another and make a bigger wave if they are in step
(i.e. if crest overlaps crest, and trough overlaps trough).
If they overlap out of step (B.), they cancel out, because the crest
of one wave fills up the trough of the other.
Both waves in step make
a stronger wave
Some of the wave is
reflected at back
This is why any particular thickness of film only reflects some
colours (i.e. wavelengths) of light - the waves reflected at front and
back must be leaving the film in step, because the extra trip
across the film and back again amounts to a whole number of
wavelengths. (Actually it is a whole number of wavelengths plus half a
wavelength, because the front face reflection inverts those waves, but the
back face reflection doesn't invert the waves reflected there). Other colours
are not reflected because their waves leave the film out of step!
This sounds a little odd - in fact, the waves whose wavelength
prevents them from being reflected by that thickness of film simply
pass straight through.
The wavelength of a different
colour goes straight through
B.
Wave is not reflected
7. So the light reflected from any part of the film is not the complete spectrum (or rainbow) of white
light, because one or more component colours will be missing. For example,
where the red light waves are not reflected, the film will appear a pretty
greeny-blue. As the film drains down, the coloured bands move
down as well, but you can see the coloured bands moving up the
screen, because the projector inverts the picture.
8. Just before the film bursts, there is no light of any colour reflected.
This is because the film is much thinner than a wavelength of light
(of any colour) and the waves reflected at the front face of the film are
always "inverted", i.e. a crest is reflected as a trough and vice-versa.
This means that for very thin films, there can be no light reflected because the inverted waves
reflected from the front of the film fill up the waves reflected from the back surface.
Film too thin to
reflect any colour
No yellow so film
looks mauve here
No red so film looks
greeny-blue here
DID YOU KNOW?
• Bubbles get their colours in the same way
as the film in the exhibit. The same sort of
colours may be seen in thin films of oil on
water. Pearls and mother of pearl get their
colours in a similar way, because they are
made up of thin layers of transparent
material.
• Camera lenses are coated with very thin
films of transparent minerals to prevent as
much light as possible being reflected.
STUART STREET
CARDIFF
CF10 5BW
• Pure water will not form a thin film; the molecules bind together very
tightly and break any film into droplets. Soap or detergent must be
added to line the surfaces to make a stable film. The thinnest film is
then about 1% of the wavelength of yellow light! At this thickness,
there is no water in the film; it is just two layers of detergent
molecules.
THINGS YOU CAN TRY YOURSELF.
• Do the washing up and dip a glass or a cup into the washing up water.
When you bring it out, there will be a: detergent film across the open end.
Look at the reflection of a light in the film and watch the bands of colours:
move downwards.
T 029 20 475 475
F 029 20 482 517
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