Light: Electromagnetic Waves Flashcards
Maxwell’s Predictions
In 1865 _____________ provided a mathematical theory that showed a close
relationship between electric and magnetic phenomena. His theory predicted that
electric and magnetic fields can move through space as waves. The theory he
developed is based on the following:
- Electric field lines originate on ___________ and terminate on __________.
- Magnetic field lines always form __________.
- A varying magnetic field induces an ___________.
- Magnetic fields are generated by ___________ (or currents) or by a ____________.
The waves sent out by the oscillating charges are ____________ and ____________s, and so they are called ______________, traveling through empty space with
a speed of about 300 000 000 m/s.
In 1887,___________ was the first to generate and detect electromagnetic waves in a
laboratory setting.
James Clerk Maxwell
positive charges, negative charges
closed loops
electric field
moving charges, varying electric fields
fluctuating electric, magnetic field, electromagnetic waves
Heinrich Hertz
Production of Electromagnetic Waves
_______________ are radiated by any circuit carrying an alternating current. The fundamental mechanism responsible for this radiation is the acceleration of a ___________. Whenever a charged particle undergoes an acceleration, it must radiate ________.
An alternating voltage applied to the wires of an antenna forces an electric charge in the antenna to ________.
- This is a common technique for accelerating charged particles and is the source of the radio waves emitted by the broadcast antenna of a radio station. As the charges continue to oscillate between the rods, the electric field moves away from the antenna at the ____________.
Because the oscillating charges create a current in the rods, a ________ is also generated. The magnetic field lines circle the antenna and are perpendicular to the electric field at all points. Both fields are __________ to the direction of motion of the wave. Hence, we see that an electromagnetic wave is a __________.
At great distance from the antenna, the strengths of the electric and magnetic fields become ________.
Electromagnetic waves, charged particle, energy,
oscillate
speed of light
magnetic field, transverse wave
perpendicular
very weak
Properties of Electromagnetic Waves
Electromagnetic waves travel with the _________. In fact, it can be shown that the speed of an electromagnetic wave is related to the permeability and permittivity of the medium through which it travels. For free space it is where e is speed of light, po = 4π- 107 Ns2/C2 the permeability constant of vacuum, and
= 8.85-10-12 C/Nm2 is the permittivity of vacuum.
It can be shown also that the magnitude of the electric to the magnetic field in an electromagnetic wave ____________
speed of light
equals the speed of light
Electromagnetic waves carry both _______ and __________ as they travel through space.
energy, momentum
The Spectrum of Electromagnetic Waves
All electromagnetic waves travel through vacuum with a ________. Hence, their
frequency, f, and wavelength, are related by the expression
speed of c
____________ are the result of charges accelerating through conduction wires.
They are used in radio and television communication systems.
RADIO WAVES
___________, have wavelengths ranging between about ___ mm and __ cm, and
are generated by electronic devices. They are well suited for the radar systems
used in aircraft navigation. Microwave ovens are an interesting domestic
application.
MICROWAVES
1, 30
____________ (sometimes called heat waves), produced by hot bodies and
molecules, have wavelengths ranging from about _ mm to the longest
wavelength of visible light, ____ nm.
They are readily absorbed by most materials.
The infrared energy absorbed by a substance appears as _____. This is because
the energy ________ the atoms of the object, increasing their vibrational or
translational motion, and the result is a temperature rise. Physical therapy and
infrared photography are some practical applications.
INFRARED WAVES
1mm 700nm
heat, agitates
the most familiar form of electromagnetic waves, may be
defined as the part of the spectrum that is detected by a human eye.
Light is produced by the rearrangement of electrons in atoms and molecules. The
wavelength of visible light are classified as colors ranging from violet, _____ nm, to
red, ______. The eye’s sensitivity is a function of wavelength and is greatest at a
wavelength of about ______ (yellow-green).
VISIBLE LIGHT
400, 700 nm
560 nm
_________ covers wavelengths ranging from about __ nm to
___ nm. The Sun is an important source of ultraviolet light (which is the main
cause of suntans).
Most of the ultraviolet light from the Sun is absorbed by atoms
in the upper atmosphere, or stratosphere. This is fortunate, because UV light in
large quantities has harmful effects on humans. One important constituent of the
stratosphere is ozone from reactions of oxygen with ultraviolet radiation. This
ozone shield convert lethal high-energy ultraviolet radiation to _____, which warms
the stratosphere.
ULTRAVIOLET LIGHT (UV)
400, 0.6
heat
_______ are electromagnetic waves with wavelengths from about ____ nm to ___ The most common source of x-rays is the acceleration of _____________ bombarding a metal target. X-rays are used as a diagnostic tool in
medicine and as a treatment for certain forms of cancer. Because x-rays___________ or __________ living tissues and organisms, care must be taken to avoid
unnecessary exposure and overexposure.
X rays
10nm, 0.1pm
damage or destroy
_________ are emitted by radioactive nuclei. They are highly penetrating
and cause serious damage when absorbed by living tissues. Those working near
such radiation must be protected by garments containing heavily absorbing
materials, such as layers of _______.
GAMMA RAYS
lead
The Nature of Light
Until the beginning of the 19th century, _____ was considered to be a stream of particles, emitted by a light source that stimulated the sence of sight on entering the eye. That was proposed by ________.
During Newton’s lifetime ____________ proposed another theory-wave theory of
light. The wave theory did not receive immediate acceptance because there were not clear experimental evidence and also due to Newton’s great reputation as a scientist.
The first clear demonstration of the wave nature of light was provided in1801 by ___________, who showed that light exhibits interference behavior. That is,
for example, at certain points in the vicinity of two sources, light waves can combine and cancel each other by destructive interference.
The most important development concerning the theory of light was the work of ________ , who in 1865 predicted that light was a form of electromagnetic wave. Although his theory explained most known properties of light, some subsequent experiments could not be explained by the assumption that light was a wave. The most striking of these was the ____________ (clean metal surfaces emit charges when exposed to ultraviolet light).
In 1905 ________ formulated theory of light quanta and explained the photoelectric effect.
He concluded that light is composed of ____________ (photons) with energy ___________ to the frequency of the electromagnetic wave, E-hf, where his
Planck’s constant.
Thus, light must have a ________. That is, in some cases light acts as a wave and in others as a particle, but _____ acts as both in the same experiments.
light, Newton
Christian Huygens
Thomas Young
Maxwell
photoelectric effect
Einstein
corpuscles
proportional
dual nature
never
The Speed of Light
Light travels so fast that early attempts to measure its speed were unsuccessful. - The first known successful estimate of the speed of light was made in 1675 by ________.
His technique involved astronomical observations of one of the moons of Jupiter, Io. He estimated the speed of light to be about 210 000 km/s.
Later, in 1849, _____ arrived at a value of 310 000 m/s. A recent value of the speed of light in a vacuum, obtained using a _________ , is 299 792 458 m/s.
Ole Roemer
Fizeau
laser technique
Geometric Optics
Explanations of such phenomena can be done by geometrical analysis of light rays. That part of optics is often called _________.
First property of light, inside geometric optics, can be understood based on common experience: light travels in a _________ path until it encounters a boundary between two different materials. When light strikes a boundary it either is reflected from the boundary, passes into the material on the other side of the boundary, or partially does both.
We use the ____________ to represent beams of light.
geometric optics
straight line
ray approximation
