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12
Light in Our World
The Spectrum of Light
The light our eyes see is only part of what is around us. Visible light, infrared
light, radio waves (including TV broadcasting and cell phones), microwaves,
and x-rays are all forms of electromagnetic radiation. They are actually
changing electric and magnetic elds. This radiation travels like waves in
water, spreading out from where it was created. These waves all travel at the
speed of light, but some are longer (higher wavelength) and some repeat faster
(higher frequency). Together they are called the electromagnetic spectrum:
The visible colors (red, orange, yellow, green, blue, and violet) have different
wavelengths. In the right conditions white light from the sun can be separated
according to wavelength, producing a rainbow of color. This happens with an
actual rainbow, and with prismatic lm.
Why is the sky blue? Some sunlight is scattered by tiny particles in the earth’s
atmosphere. The shorter wavelength blue light is scattered more than the
other colors, so the sky appears blue. At sunrise or sunset, longer wavelength
colors like red or
yellow are more
visible in the sky,
because sunlight
passes through more
of the atmosphere
before reaching your
eyes. In space, the
sky always appears
black because there
is no atmosphere or
scattering effect.
Infrared
Infrared light is invisible light given off by
anything warm. Infrared is used in remote
controls to control TVs and appliances.
Infrared is invisible, so it doesn’t disrupt
your view of the TV. Infrared doesn’t go
through walls, so it doesn’t interfere with
devices in other rooms.
The remote control sends a stream of
infrared light pulses to the TV, encoded
with the desired commands. The infrared
light is created using an infrared light
emitting diode (LED). Infrared detectors
convert the received light to electric current,
and decode the commands. The detectors
are tuned to focus on the infrared light,
and ignore visible light. This set contains
an infrared detector (U24), which can
be activated by a TV remote control; see
projects 18 and 25 for examples.
Infrared has other uses such as night vision
devices help to see people and animals in
the dark, by looking at the heat they give
off as infrared light. You probably saw this
in the movies.
Glow-in-the-dark
Some materials can absorb light, store it for a while, and slowly release
it back out. “Glow-in-the-dark” materials can be “charged” by bright light,
then will slowly emit light and “glow” for a while in a dark room. The glow
fan blade in this set has a glow powder mixed in the plastic.
It’s like a slow, delayed reection of the light.
Sound
Sound, like light, spreads out like waves from where it was made. Sound
is variations in air pressure. You “hear” sound when your ears feel these
air pressure variations. Sound has much longer wavelength than light,
which enables sound to travel around corners. Sound can also be
thought of as a wave of vibration, and can travel through water and solid
objects. Sound travels about 1,000 feet per second in air, and about
5,000 feet per second in water.
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