LIGHT
A. Light & Energy
Does sunlight contain energy?
Sunlight is made up of a wide spectrum of wavelengths. Light contains energy and therefor can do ‘work’ by
producing electricity when it falls on solar panels (silicon) or produce photosynthesis in plants or move things such as
the vanes in a radiometer or change the colour of photographic paper or human skin.
During photosynthesis the plant uses light energy, carbon dioxide and water to produce glucose, starch and cellulose.
This is probably one of the most important chemical reactions in our world as it provides the food we need to eat.
Luminescence & LEDs
Max Planck and Einstein said that light of different colours have different energies. They indicated that a UV light
source will have a higher energy value than red light. This can easily be shown on a glow-in-the-dark screen
(phosphorescent screen) that can be prepared using photo luminescent pigment and white glue (1:3). Coloured LEDs
can then be used to demonstrate the energy differences.
B. Producing Light
Ways to produce light
In an incandescent bulb a glow wire is heated so it glows white hot and this is the source of light. We call it
thermoluminescence.
When two silica rocks are rubbed together they produce a light glow. Known as triboluminescence.
Certain insects and fishes can produce a light in the dark, eg fire flies, glow worms. This is known as
bioluminescence.
When certain chemicals are mixed, they produce a glowing liquid, known as chemiluminescence.
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Fluorescence & Phosphorescence
The UV component of the sun or a UV torch emits energetic ultraviolet light. The energy in the light can make
minerals, scorpions and other fluorescent materials fluoresce and phosphoresce in the dark. There are many uses
for these properties, to name a few: to detect counterfeit money, to characterize minerals and identify fingerprints.
It is also used extensively in the food industry to check if people follow hygienic guidelines.
Ultra Violet (UV) light causes colour changes
UV light or heat can make materials change colours reversibly. The chemical structure of the compounds are
changed temporarily by the light or heat and changes back when the light or heat source is removed. We call this
photo-chromism and thermo-chromism. Modern colour changing thermometers are examples of these.
C. Properties of light
Bending light : X-Ray Glasses
A diffraction grating is simply a screen made up of very narrow slits (500 slits/mm) - think of miniature window
blinds. When visible light of wavelength between 300 to 780 nm pass through these, the light is bend around the
obstructions (1000 nm wide) and produce a specific light (diffraction) pattern. Toy shop X-ray glasses use the narrow
slits found in feathers to create an “X Ray” image of your fingers when held against a light source. This makes a
great classroom project.
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Ghost pebbles, expanding cubes & spheres
Light changes its speed when it passes into a less dense / denser optical medium. This causes the light rays to bend
(refract). One way to observe this is to use a glass prism.
Fully gelled super absorbent polymers become invisible in water as they contain up to 95% water, exhibiting the
same optical density as water. Submerged in water the refraction is minimized.
Breaking light into its constituent colours:
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- Rainbow glasses & Peepholes
Rainbow glasses & peepholes have diffraction grating lenses that disperse (separate) white light into its constituent
rainbow colours. This is similar to the job water droplets do after a rain storm.
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- A super sized spectrum on an OHP
A large spectrum can easily be produced in a darkened classroom using a
diffraction grating in stead of a prism.
(See Light & Colour downloads on our website for detailed instructions).
Each wavelength (colour) is diffracted at a different angle. Red light (longer
wavelength) is diffracted most while short wavelengths such as violet are
diffracted least. Note that this is in reverse order with a prism – highlighting
the difference between diffraction and refraction.
To do: Addition to form white light: Recombine the spectrum colours using a
large fresnel lens between the grating and the screen. The colours should
converge to form a white spot.
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3D Anaglyphic Glasses
The anaglyphic 3D technique is probably the easiest technique for creating
stereo (3D) images. To view a photo in stereo the left eye must see ONLY the
left eye view and the right eye must see only the RIGHT eye view. The viewer
are therefor fed two images of the same object (usually) in red and cyan. The
images are slightly offset to produce two different perspectives. Each eye only
sees one of the images due to the coloured lenses and this creates a 3D
stereoscopic effect in the mind. (See Downloads on website for more
information).
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Polarizing Glasses
When two polarizing filters are placed atop each other, they can be transparent or block the light. By rotating one
of the filters, the transmitted light passing through the filters may be turned ‘on’ and ‘off’.
Polarized light (such as those produced on a computer screen) will be blocked by a polarizing filter orientated at
right angles to the light orientation. By placing transparent objects between two cross polarized filters it is possible
to identify materials that rotate polarized light. 3D polarizing glasses are cheap sources of polarized filters.
To do:
• Sandwich a plastic poly ethylene bag between two filters and stretch it;
• View a computer screen or LCD watch with a rotating filter;
• Use cello-tape to create patterns on a clear plastic sheet. Sandwich in between two filters and rotate one.
• Squeeze a piece of plexiglass (eg. clear plastic fork) between two filters to see the stress lines in the Plexiglass.
Persistence of Vision
The RGB Ball & Strobe Ring:
This is a fun way to demonstrate that white light is made up of the three primary colours: red, green & blue. Each
flickering LED imprints a colour on the retina but due to persistence of vision the eye-brain system retains the
image of each colour for a fraction of a second so the next colour is imprinted on top of it. The eye then produces a
combined perception of white light. Spinning the ball ‘separates’ the three imprints on the retina and three colours
are observed.
Visual AC current indicator
Power station generators produce alternating current (AC) at 50 Hz. This simply means that the current goes
through 50 cycles per second. One cycle is depicted below. Where the ‘wave’ crosses the line, the current is
momentarily off, and this happens twice per cycle or 100 times per second.
Now - how can we see this? By using a bi-colour LED and connecting it to 9 or 12 volts ac and an appropriate
resistor, you can demonstrate the change in current direction as well as the intermittent switching.
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Light Modulation: The Photo Phone Kit
Light is electromagnetic radiation and therefore can carry audio signals. On
19th February, 1880, Alexander Graham Bell and Sumner Tainter, prof. Bell’s
laboratory assistant, became the first people to transmit their voices over a
beam of light. Bell called his invention the photo phone and said it was
fundamentally a greater invention than the telephone.
In our set-up we use a torch with modulated light beam. When the iPod is
turned on, it adds a weak but fluctuating electrical signal to the constant
current from the torch’s battery. So the torch LEDs now flicker in sync with
the output from the iPod. The stronger battery current of the torch is
‘superimposed’ with the weaker signal from the iPod. These fluctuations are
picked up by the solar panel and are turned into electrical pulses that are transformed back into sound by the
speaker. This process is known as intensity or amplitude modulation (AM).
A DIY Laser Show
Download file from our website under Downloads for instructions.
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