Lesson 16: Basic Properties of Light and Matter
In this lesson, we will study light, including its properties and the different
forms of light. We will then look at what makes up matter, and finally, we will
see how light and matter interact.
Prior to Newton's experiments with light in the 1660s, it was already known that
passing white light through a prism created a rainbow of color called a
spectrum. The visible light seen in a spectrum, however, is only a tiny part of
the complete spectrum of light, which is usually called the electromagnetic
spectrum. The image in the figure shows the electromagnetic spectrum, which is
the complete spectrum of light. Another name for light is electromagnetic
radiation.
Light is a wave, which means it transmits energy without carrying material
along with it. A wave moves up through peaks, and down through troughs.
Wavelength is the distance between adjacent peaks. Frequency is the number of
times the wave moves up and down each second. It is measured in hertz, or the
number of cycles per second.
Wavelengths increase as we go from
gamma rays to
radio waves, while frequency and energy are
the opposite. The longer the wavelength, the lower the frequency, and vice-versa.
Light can interact with both electrically charged particles and magnetic fields,
meaning it is an electromagnetic wave. That is why it is called electromagnetic
radiation and the full spectrum of light is called the electromagnetic spectrum.
Light can behave both as a wave and as a particle. Particles of light are
called "photons," and are characterized by a wavelength and frequency. The
higher the frequency of the photon, the more energy the light carries.
All light travels at the same speed, which is about 300,000 kilometers per
second. So a radio wave moves just as quickly as visible light, which moves at
the same speed as a gamma ray. The difference in light is based on the
frequency and wavelength - the speed stays the same.
There are
many forms of light - radio waves, infrared, ultraviolet, x-rays, microwaves,
and gamma rays, with radio waves being on the left end of the electromagnetic
spectrum and having the longest wavelength, to gamma rays on the right extreme
with the shortest wavelength.
Visible light has a wavelength of 400 nanometers, or 400 billionths of a meter,
at the blue or violet end of the spectrum and 700 nanometers at the red end. If
the light has a wavelength a bit longer than red light, then it is infrared. If
it has a wavelength a little shorter than blue or violet light, then it is
ultraviolet.
Around the border between infrared and
radio waves is often called microwaves,
since the wavelengths range from micrometers to centimeters.
The light our eyes can see is inside of an extremely limited segment of the
electromagnetic spectrum. The reddest light we can see has a wavelength only
twice as long as the bluest blue, while the radio waves from a radio station
are a billion times longer than the X rays used in a doctor's office.
Ancient Greeks believed all matter came from fire, water, earth and
air. Some Greeks starting with Democritus believed that
these four elements were made up of tiny particles called atoms, which comes
from a Greek word meaning indivisible.
Over time, we have learned that there are actually more than 100 chemical
elements, each composed of a different type of atom, and that atoms themselves
are made up of particles called protons, neutrons, and electrons.
Electrical charge is the physical property that
describes how an object will interact with electromagnetic fields. Protons have
a positive electrical charge (written as "plus 1") and electrons have a
negative charge (minus 1). Neutrons are neutral with no charge.
At the center of the atom is the nucleus, which is made up of the protons and
neutrons. The electrons surround the nucleus. The atom is held together because
oppositely charged particles attract, so the negatively charged electrons are
attracted to the positive charge of the protons inside of the nucleus.
What makes each element unique is the number of protons in each of its atoms.
The number of protons in an element is called its atomic number. For example,
Carbon always has six protons, so its atomic number is six.
When you combine the number of neutrons and protons in
the nucleus of an atom, you get that
element's atomic mass number. Carbon usually has an atomic mass number of
twelve, since a typical atom has six protons and six neutrons, but the number
of neutrons can vary. Isotopes are versions of an element with a different
number of neutrons, so the most common isotope of carbon is carbon-12, with six
protons and six neutrons, but there are also carbon-13 and carbon-14, with
seven and eight neutrons, respectively.
Although there are
only 118 chemical elements that have been identified so far, the number of
material substances that exist is far greater because atoms from different
elements combine to form molecules. For example, water exists because two
hydrogen atoms bond with one oxygen atom.
For the study of astronomy,
understanding how light and matter interact is very important. Light and matter
interact in four ways.
The first way is emission. Matter can emit light;
for example, we see a light bulb emit visible light.
Matter can absorb light. An example of this is when you place your hand near a
light bulb. Your hand absorbs some of the light and this absorbed energy warms
your hand.
Some forms of matter transmit light,
or allow it to pass through. Examples are glass and air.
Finally, matter can reflect or scatter light when it causes light to
bounce off. If the light bounces all in the same general direction, it is
called reflection, and when it bounces off more randomly, we say that it
scatters.
When a material transmits light, it looks transparent, whereas when it absorbs
light, it is opaque. Many materials are partially transparent or partially
opaque.
The color of an object is based on what light it transmits or reflects. For
example, a leaf reflects green light but absorbs all other colors.