Download 2.03electromagneticspectrumvisiblenotes

An electromagnetic wave is a three–dimensional transverse wave produced through an
energy transfer from accelerating electric charges; it does not need a medium to travel
and has both a magnetic field and an electric field.
The known electromagnetic waves include radio waves, microwaves, infrared waves,
visible light waves, ultra violet waves, X–rays, and gamma rays. These electromagnetic
waves all have certain properties in common:
• They are made up of a magnetic field and an electric field.
• They are considered to have the properties of both waves and particles. This is
called a dual nature or the duality of light.
• They can travel without a medium (meaning they can even travel through outer
space).
• They all travel at the same speed, often called the speed of light. Nothing travels
faster.
Waves of visible light travel at the same speed as microwaves and X–rays, the difference
between them is their wavelength and frequency.
In 1873, James Clerk Maxwell built upon the work done by Michael Faraday and
developed equations to show that the speed of light matched the speed of
electromagnetic waves. Maxwell predicted that light is a type of electromagnetic wave
and can travel through air, water, or the vacuum of space, although he still believed that
light was only energy.
It is now known that light has properties of both waves and particles, and that all
electromagnetic waves travel through space at the speed of light.
Image giving information on Gamma rays.
Gamma Rays Description: Gamma rays are given off during nuclear reactions.
Source: Radioactive substances
Detection: Geiger counter
Image giving information on X–rays.
X–Rays Description: X–rays are primarily used in medicine to image people’s bones.
Source: High-speed collisions between electrons Image resembling a flash card giving
information on Ultraviolet light.
Detection: X–ray film
Image giving information on Ultraviolet light.
Ultraviolet Light Description: Ultraviolet light (UV rays) is given off by very hot objects,
such as the sun. They are the cause of sunburns.
Source: Sun; sun lamps
Detection: Special cameras can see ultraviolet light.
Image giving information on Visible light.
Visible Light Description: Visible light constitutes all the colors that humans see, ranging
from red to violet.
Detection: Human eye
Image giving information on Infrared Radiation.
Infrared Radiation Description: Infrared radiation is heat emitted from warm objects. It
is used to detect the location of objects, such as people, that emit heat.
Source: Anything that gives off heat
Detection: Special cameras can see infrared.
Image giving information on Microwaves.
Microwaves Description: Microwaves are commonly used to heat food. They are also
used in radar guns.
Source: Special vacuum tubes, like those in a microwave oven.
Detection: Instruments used in radar detect microwaves.
Image giving information on Radio Waves.
Radio Waves Description: Radio waves are low-energy waves used to transmit radio,
television, and cellular phone signals.
Source: Electric circuits
Detection: Radio, TV, cellular phone towers
low frequency = long wavelength
high frequency = short wavelength
In a mirror, light waves hit the silver surface behind the glass and bounce off in a new
direction.
Refraction occurs when light waves pass through a boundary between two mediums.
Let’s look specifically at light waves. This can be seen when you place a pencil in a glass
of water. In this case, light passes through the two mediums of air and water. We
observe that the pencil appears to be bent or broken. This apparent bending of the
pencil is due to the bending of light waves and is called refraction.
The angle of refraction is defined by the outgoing light ray as it passes through the
boundary between the air and water and continues through the water. The angle of
refraction is the angle between this light ray and the imaginary line perpendicular to the
boundary surface.
The angle of refraction is smaller than the angle of incidence if the speed of light is
slower in the second medium. This is the case as light passes from air to water. We see a
new direction for a light wave in water compared to air because it moves more slowly in
water.
Interestingly, if the angle of incidence is parallel to the boundary
surface, then the angle of refraction is also parallel. This is the case shown in the glass
on the right.
Color results from our perception of light of varying wavelength. White light contains
light waves of all wavelengths in the visible region. When we shine white light through a
prism, it disperses into the various colors of the rainbow, each color representing a
different wavelength of visible light. The absence of light is black. Black is a result of
absorption of all the colors and an absence of reflected light waves. White is a reflection
of all the colors and results from a combination of all the light waves. As a matter of a
fact, technically light waves do not have color at all. It is our psychologicaland
physiological response to those light waves that assign them a color.
Visible light is composed of three main colors: red, green, and blue. This can be seen in
the following interactive. In this case, you will be mixing light of different colors to see
how various combinations result in light of new color.
Isaac Newton
Isaac Newton published his theory of light in 1704. Newton thought that light was
composed of particles. However, other scientists at that time theorized that light was a
form of energy that traveled in waves and therefore needed a medium through which to
travel. These scientists could not explain how light could travel through a vacuum, which
means there are no particles of matter. The properties of light prompted much
experimentation.
A major advancement was made nearly 170 years later when James Clerk Maxwell
described light as having both electric and magnetic components. This proved to be an
important breakthrough in thinking regarding light. Maxwell calculated the speed of
various electromagnetic waves and found that this speed was a constant value and that
it matched the speed of light. From this he concluded that light is an electromagnetic
wave produced through the movement of tiny electric charges. Scientists now know
that, unlike mechanical waves that need a medium to travel in, electromagnetic waves
do not need such a medium. This explains why light can travel through the vacuum of
space. Maxwell’s observations, along with other observations by later scientists, have
shown that nothing can travel at speeds higher than the speed of light in a vacuum.
Scientists have also learned that the speed of light is a constant, no matter how quickly
the source or the observer is traveling.
As much as Maxwell’s work led to better understanding of light, there continued to be
debate in the scientific community regarding the structure of light. Was it a particle or a
wave? Some evidence supported its wave-like nature, but other evidence could only be
explained using the concept of light as a stream of particles. Albert Einstein developed
an answer to this question by concluding that light has a dual nature—light exists as
both a wave and as a stream of particles called photons. Einstein’s research paved the
way for the development of twentieth-century quantum physics.