Radiation - reading & notes
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How does heat energy travel through space? There is almost no matter between the Sun and the Earth.
Beyond Earth's atmosphere is a vacuum with no air, and no atoms. So thermal energy does not travel by
conduction or convection which require matter to transfer heat. Instead, thermal energy is transferred
from the Sun to Earth by radiation.
Radiation is the transfer of energy by electromagnetic waves. Some examples of electromagnetic waves
are visible light and infrared waves. Electromagnetic waves carry energy through solids, liquids, and gases.
But they can also carry energy through empty space. Energy transferred by radiation is often called radiant
energy. When you sit by a fireplace, you feel warm because heat is transferred by radiation from the fire to
your skin.
The diagram at right shows the whole range of
electromagnetic waves, called the electromagnetic
spectrum. The various types of electromagnetic waves
have different wavelengths and frequencies. Radio waves
and microwaves have lower frequencies and wavelengths,
while x-rays and ultraviolet waves have higher
wavelengths and frequencies. Visible light, are the
electromagnetic waves humans can see and fall within a
narrow range of the spectrum.
Examples of Electromagnetic radiation
Many of the electromagnetic waves above can heat up an
object when they strike the object, but the one most responsible for this is infrared radiation.
When you operate a heat lamp in the ceiling of a hotel bathroom, the lamp emits many forms of
electromagnetic radiation. The one that warms your skin the most is the infrared radiation that the lamp
emits. In fact, those bulbs are designed to emit heavily in the infrared range. When you feel the warmth
of a fire when sitting in front of a fireplace, most of that is caused by infrared energy waves given off by
the fire which strike your skin. That is why the front of you feels warm, but your back feels cold.
Remember, the hot gases coming off the fire are going up the chimney by convection. They are not
responsible for the warmth you feel.
All objects that possess heat energy give off infrared energy, even your body.
Night vision goggles pick up those infrared waves and use them to create a
visible image of the object. Hotter objects give off more infrared radiation
than cooler objects.
How does matter affect radiant energy?
When radiation strikes a material, three things can happen. The material
absorbs some of the energy. It also reflects some of the energy. And some of
the energy may be transmitted, or, passed through the material. The amount
of energy absorbed, reflected, or transmitted depends on the type of
material. Light-colored materials reflect more radiant energy, while darkcolored materials absorb more radiant energy. When material absorbs
radiant energy, the thermal energy of the material increases.
Radiation - notes
RADIATION: transfer of thermal energy by _________________________________________________

can travel through _______________
where there is no matter (vacuum)

not possible for the Sun to heat the Earth
by conduction and convection (both
require _________________ to transfer
heat)

electromagnetic waves - also called electromagnetic ____________________ or radiant energy

light-colored objects reflect more radiant energy, dark-colored objects _______________ more
radiant energy
Radiation Examples: We can see and feel some of the electromagnetic waves, while others we cannot.
1. Sun gives off __________________________________, visible light, and infrared waves
2. Infrared radiation from a fire - can be felt on skin
3. Shiny metal reflects radiant energy, and ___________ materials ____________________
 shiny metal slide reflects visible light into your eyes (you need sunglasses),
grass absorbs the visible light (doesn't hurt eyes to view, even on sunny days)
4. Dark color clothes absorb heat and light colors reflect.
 NYC is painting some building roofs white to reflect more of the radiant energy and keep the city
cooler. Most of the roofs are painted black so absorb the radiant energy and get hot. See
diagram below.