315
Light 315-
Without light, life on Earth would
be impossible. Sunlight provides the
energy to make plants grow and keep
all living things alive. light itself is
a form of energy that travels as tiny
packets of electromagnetic energy
called photons. When photons
enter our eyes, they stimulate
special light-sensitive cells so that
we can see. other forms of energy
that travel as electromagnetic
waves include radio waves, x-rays,
and microwaves in microwave
ovens. Just as there is a spectrum
of colors in light, there is also
an electromagnetic spectrum.
in fact, light waves are also a type
of electromagnetic wave, and the
colors in light form a small part of
the electromagnetic spectrum. light
waves, and all other electromagnetic
waves, travel at 186,000 miles
(300,000 km) per second, which is
so fast that they could circle the world
almost eight times in a second. Nothing
in the universe can travel faster than light.iNcaNdEScENt light bulb
in the middle of an incandescent electric
light bulb is a tiny spiral of tungsten wire
called the filament. When an electric
current is sent through the filament, it
warms up so much that it glows white-
hot. it is the brightly glowing filament
that produces light.brightNESS of light
the farther you are from a light, the less bright it will
seem. this is because light spreads out in all directions
from its source. So when you are far away, the light is
spread over a wide area. Many stars, for instance,
are much brighter than our sun, but their
light is spread out over so vast an area
that by the time it reaches us, the
stars do not even seem as
bright as a candle.When things burn,
they give out light
as well as heat.Current flowing
through metal
filament makes it
glow white-hot.SourcES of light
Many different objects give
off light. the sun, traditional
electric light bulbs, and
fireworks are incandescent,
which means they glow
because they are hot. but not
all lights are hot. chemicals,
not heat, produce the glowing
spots on the bodies of some
deep-sea fish. all cool lights,
including fluorescent lights,
are called luminescent.Electrical contact is made
when the bulb terminal is
screwed into the socket.light aNd ShadoW
light travels in straight lines, so,
in most cases, it cannot go around
obstacles in its path. When light rays
hit a solid object, some bounce back
and some are absorbed by the object,
warming it up a little. the area behind
receives no light rays and is left in shadow.Shine a flashlight
on a wall and
watch the pool of
light grow larger
and dimmer as
you move the
flashlight
further away.A candle is a wide
source of light, so it
produces a fuzzy
shadow.Some deep-sea fish
have luminescent
stripes and spots
along their bodies
that give out light.Searchlights
give out very
intense light, often
produced by an
electric spark
between two
pieces of carbon.Nuclear reactions inside the center
of the sun produce intense heat
and light. All stars produce light
from nuclear reactions.A chemical coating
inside the bulb
converts the UV
light into visible
white light.The electronics
hidden inside the
white plastic base
control the light.Inside a glass tube,
an electric current
flows through a gas
(mercury vapor)
making it emit
invisible ultra-violet
(UV) light.The explosion
of gunpowder
inside a firework
produces a burst
of colored light.Candles and
lanterns give
out light.fluorEScENt light
Most modern light bulbs are fluorescent tubes like this one.
When an electric current is passed through the gas in the tube,
gas atoms emit invisible, ultraviolet light. the ultraviolet light
strikes phosphors—chemicals in the tube’s lining—and makes
them glow with a bright white light.US_315_Light_1.indd 315 09/02/16 3:56 pm