Chapter 25.1

LIGHT 25.1
Chapter Twenty-Five: Light
25.1 Properties of Light
25.2 Color and Vision
25.3 Optics
Chapter 25.1 Learning Goals
Describe the properties of light.
Explain the relationship between
energy and the colors of light.
Describe waves included in the
electromagnetic spectrum in terms of
energy, frequency, and wavelength.
Investigation 25A
Key Question:
 What happens when you mix different
colors of light?
25.1 Properties of light
Light travels fast over
long distances and carries
energy and information.
Light travels in straight
lines, but can be bent by
lenses or reflected by
mirrors heat and warmth.
Light has color and can
be bright or dim.
25.1 The electromagnetic
Light, like sound and heat, is a form of
The visible light we see is part of the
electromagnetic spectrum.
25.1 Properties of light
You see book pages because light in the
room reflects from the page to your
Your eyes and brain use the information
carried by the light to make a mental
25.1 Light is produced by atoms
Most light is produced by atoms.
When you put some energy into the
atom, it excites the atom’s electrons.
Light is produced when the electron
releases this energy.
25.1 Incandescent light
Making light with heat is
called incandescence.
Atoms in the filament
convert electrical energy
to heat and then to light.
Incandescent bulbs
are inefficient, but
their waste heat can
be useful.
25.1 Fluorescent light
To make light,
fluorescent bulbs use
high-voltage electricity
to energize atoms of
gas in the bulb.
These atoms release
the electrical energy
directly as light (not
heat), in a process
called fluorescence.
25.1 Color and energy
When all the colors of the rainbow are
combined, we see light without any
We call the combination of all colors
white light.
25.1 Color and energy
Compare the hot, blue flame from a gas
stove to the orange flame of a match.
The light from a gas flame is blue (high
energy) and the light from a match is
red-orange (low energy).
25.1 The speed of light
The speed at which light travels through
air is about 300 million meters per second.
The speed of light is so important in
physics that it is given its own symbol, a
lower case “c”.
25.1 Speed of light
The speed at which electromagnetic waves
travel through air is about 300 million
meters per second.
The speed of light is so fast that when
lightning strikes a few miles away, we hear
the thunder after we see the lightning.
25.1 Wavelength and
Frequency of Light
Because the wavelength
of light is so small,
scientists measure it in
One nanometer (nm) is
one billionth of a meter
(0.000000001 m).
25.1 What kind of wave is light?
A sound wave is a oscillation of air.
A water wave is an oscillation of the
surface of water.
An oscillation of electricity or
magnetism creates electromagnetic
25.1 Electromagnetic waves
If you could shake
the magnet up and
down 450 trillion
times per second, you
would make waves of
red light with a
frequency of about
450 THz.
25.1 Electromagnetic spectrum
The entire range of electromagnetic waves,
including all possible frequencies, is called
the electromagnetic spectrum.
This spectrum includes visible light and
invisible waves:
radio wave
infrared light
ultraviolet light
gamma rays