The Wave Nature of Light
Electrons hold the key to understanding why substances behave as they do. When atoms react it is their outer pars, their electrons, that interact.
We refer to the arrangements of electrons in atoms as their electronic structure.
• Number of electrons
• Where they can be found
• The energies they possess
Be warned!: electrons to not behave like anything we are familiar with in the macroscopic world
The Wave Nature of Light
Much of our present understanding of the electronic structure of atoms has come from analysis of the light emitted or absorbed by substances
Electromagnetic radiation
• Carries energy through space (also known as radiant energy)
• Includes visible light, dental x-rays, radio waves, heat radiation from a fireplace
• Share certain fundamental characteristics
• All move through a vacuum at 3.00 x 108 m/s ("speed of light")
• Have "wave-like" characteristics
• The number of complete wavelengths, or cycles, that pass a given point in 1 second is the frequency of the wave
(frequency=cycles/second)
Electromagnetic radiation has both electric and magnetic properties. The wave-like property of electromagnetic radiation is due to the periodic oscillations of these components.
We can assign a frequency and a wavelength to electromagnetic radiation Because all electromagnetic radiation moves at the same speed (speed of light) wavelength and frequency are re
lated
• If the wavelength is long, there will be fewer cycles passing a given point per second, thus the frequency will be low
• If the wavelength is short, there will be more cycles passing a given point per second, and the frequency will be high
• Thus, there is an invers e relationship between wavelength and frequency
• (frequency [nu] * wavelength[lambda]) is a constant (c)
________________________________________
What is the speed of a wave?
Imagine you are on the beach watching the ocean waves go by, and you want to know the speed of the waves. There is an island offshore with a palm tree that will serve as a convenient frame of reference. You count the number of waves that pass by the tree in one minute:
In this case, two peaks (two wavelengths) pass by the tree in one minute. Thus, the frequency is 2 wavelengths/minute. If we measure the distance between the peaks (i.e. the wavelength) we can determine the speed of the wave:
Speed of the wave = (distance between peaks) * (frequency)
= (wavelength) * (frequency)
The unit of length chosen to describe a particular wavelength is typically dependent on the type of electromagnetic radiation
Unit Symbol Length (m) Type of Radiation
Angstrom Å 10-10 X-ray
Nanometer nm 10-9 UV, visible
Micrometer m 10-6 Infrared
Millimeter mm 10-3 Infrared
Centimeter cm 10-2 Microwave
Meter m 1 TV, radio
The range of EM wavelengths is dramatic
• The wavelengths of gamma-rays (<0.1 Å) are similar to the diameter of atomic nuclei
• The wavelengths of some radio waves can be larger than a football field
Frequency
• Frequency is expressed in cycles per second, also known as hertz (Hz)
• Usually the dimension 'cycles' is omitted and frequencies thus have the dimension of s-1
Sodium vapor lamps are sometimes used for public lighting. They give off a yellowish light with a wavelength of 589 nm. What is the frequency of this radiation?
frequency*wavelength = speed of light
frequency = speed of light/wavelength
= (3.00x108 m/s)/(589x10-9m)
= 5.09 x 1014 s-1
= 5.09 x 1014 cycles per second or 5.09 x 1014 hertz
Speed of light
Main article: Speed of light
The speed of light in a vacuum is presently defined to be exactly 299,792,458 m/s (approximately 186,282 miles per second). This definition of the speed of light means that the metre is now defined in terms of the speed of light. Light always travels at a constant speed, even between particles of a substance through which it is shining. Photons excite the adjoining particles that in turn transfer the energy to the neighbor. This may appear to slow the beam down through its trajectory in realtime. The time lost between entry and exit accounts to the displacement of energy through the substance between each particle that is excited.
Different physicists have attempted to measure the speed of light throughout history. Galileo attempted to measure the speed of light in the seventeenth century. An early experiment to measure the speed of light was conducted by Ole Rømer, a Danish physicist, in 1676. Using a telescope, Ole observed the motions of Jupiter and one of its moons, Io. Noting discrepancies in the apparent period of Io's orbit, Rømer calculated that light takes about 22 minutes to traverse the diameter of Earth's orbit.[4] Unfortunately, its size was not known at that time. If Ole had known the diameter of the Earth's orbit, he would have calculated a speed of 227,000,000 m/s.
Another, more accurate, measurement of the speed of light was performed in Europe by Hippolyte Fizeau in 1849. Fizeau directed a beam of light at a mirror several kilometers away. A rotating cog wheel was placed in the path of the light beam as it traveled from the source, to the mirror and then returned to its origin. Fizeau found that at a certain rate of rotation, the beam would pass through one gap in the wheel on the way out and the next gap on the way back. Knowing the distance to the mirror, the number of teeth on the wheel, and the rate of rotation, Fizeau was able to calculate the speed of light as 313,000,000 m/s.
Léon Foucault used an experiment which used rotating mirrors to obtain a value of 298,000,000 m/s in 1862. Albert A. Michelson conducted experiments on the speed of light from 1877 until his death in 1931. He refined Foucault's methods in 1926 using improved rotating mirrors to measure the time it took light to make a round trip from Mt. Wilson to Mt. San Antonio in California. The precise measurements yielded a speed of 299,796,000 m/s.
Two independent teams of physicists were able to bring light to a complete standstill by passing it through a Bose-Einstein Condensate of the element rubidium, one led by Dr. Lene Vestergaard Hau of Harvard University and the Rowland Institute for Science in Cambridge, Mass., and the other by Dr. Ronald L. Walsworth and Dr. Mikhail D. Lukin of the Harvard-Smithsonian Center for Astrophysics, also in Cambridge.[citation needed]
Electromagnetic spectrum
Main article: Electromagnetic spectrum
Electromagnetic spectrum with light highlighted
Generally, EM radiation (the designation 'radiation' excludes static electric and magnetic and near fields) is classified by wavelength into radio, microwave, infrared, the visible region we perceive as light, ultraviolet, X-rays and gamma rays.
The behavior of EM radiation depends on its wavelength. Higher frequencies have shorter wavelengths, and lower frequencies have longer wavelengths. When EM radiation interacts with single atoms and molecules, its behavior depends on the amount of energy per quantum it carries.
Refraction
Main article: Refraction
Refraction is the bending of light rays when passing from one transparent material to another. It is described by Snell's Law:
where θ1 is the angle between the ray and the normal in the first medium, θ2 is the angle between the ray and the normal in the second medium, and n1 and n2 are the indices of refraction, n = 1 in a vacuum and n > 1 in a transparent substance.
When a beam of light crosses the boundary between a vacuum and another medium, or between two different media, the wavelength of the light changes, but the frequency remains constant. If the beam of light is not orthogonal (or rather normal) to the boundary, the change in wavelength results in a change in the direction of the beam. This change of direction is known as refraction.
The refractive quality of lenses is frequently used to manipulate light in order to change the apparent size of images. Magnifying glasses, spectacles, contact lenses, microscopes and refracting telescopes are all examples of this manipulation.
Light refraction is the main basis of measurement for gloss. Gloss is measured using a glossmeter
Electrons hold the key to understanding why substances behave as they do. When atoms react it is their outer pars, their electrons, that interact.
We refer to the arrangements of electrons in atoms as their electronic structure.
• Number of electrons
• Where they can be found
• The energies they possess
Be warned!: electrons to not behave like anything we are familiar with in the macroscopic world
The Wave Nature of Light
Much of our present understanding of the electronic structure of atoms has come from analysis of the light emitted or absorbed by substances
Electromagnetic radiation
• Carries energy through space (also known as radiant energy)
• Includes visible light, dental x-rays, radio waves, heat radiation from a fireplace
• Share certain fundamental characteristics
• All move through a vacuum at 3.00 x 108 m/s ("speed of light")
• Have "wave-like" characteristics
• The number of complete wavelengths, or cycles, that pass a given point in 1 second is the frequency of the wave
(frequency=cycles/second)
Electromagnetic radiation has both electric and magnetic properties. The wave-like property of electromagnetic radiation is due to the periodic oscillations of these components.
We can assign a frequency and a wavelength to electromagnetic radiation Because all electromagnetic radiation moves at the same speed (speed of light) wavelength and frequency are re
lated
• If the wavelength is long, there will be fewer cycles passing a given point per second, thus the frequency will be low
• If the wavelength is short, there will be more cycles passing a given point per second, and the frequency will be high
• Thus, there is an invers e relationship between wavelength and frequency
• (frequency [nu] * wavelength[lambda]) is a constant (c)
________________________________________
What is the speed of a wave?
Imagine you are on the beach watching the ocean waves go by, and you want to know the speed of the waves. There is an island offshore with a palm tree that will serve as a convenient frame of reference. You count the number of waves that pass by the tree in one minute:
In this case, two peaks (two wavelengths) pass by the tree in one minute. Thus, the frequency is 2 wavelengths/minute. If we measure the distance between the peaks (i.e. the wavelength) we can determine the speed of the wave:
Speed of the wave = (distance between peaks) * (frequency)
= (wavelength) * (frequency)
The unit of length chosen to describe a particular wavelength is typically dependent on the type of electromagnetic radiation
Unit Symbol Length (m) Type of Radiation
Angstrom Å 10-10 X-ray
Nanometer nm 10-9 UV, visible
Micrometer m 10-6 Infrared
Millimeter mm 10-3 Infrared
Centimeter cm 10-2 Microwave
Meter m 1 TV, radio
The range of EM wavelengths is dramatic
• The wavelengths of gamma-rays (<0.1 Å) are similar to the diameter of atomic nuclei
• The wavelengths of some radio waves can be larger than a football field
Frequency
• Frequency is expressed in cycles per second, also known as hertz (Hz)
• Usually the dimension 'cycles' is omitted and frequencies thus have the dimension of s-1
Sodium vapor lamps are sometimes used for public lighting. They give off a yellowish light with a wavelength of 589 nm. What is the frequency of this radiation?
frequency*wavelength = speed of light
frequency = speed of light/wavelength
= (3.00x108 m/s)/(589x10-9m)
= 5.09 x 1014 s-1
= 5.09 x 1014 cycles per second or 5.09 x 1014 hertz
Speed of light
Main article: Speed of light
The speed of light in a vacuum is presently defined to be exactly 299,792,458 m/s (approximately 186,282 miles per second). This definition of the speed of light means that the metre is now defined in terms of the speed of light. Light always travels at a constant speed, even between particles of a substance through which it is shining. Photons excite the adjoining particles that in turn transfer the energy to the neighbor. This may appear to slow the beam down through its trajectory in realtime. The time lost between entry and exit accounts to the displacement of energy through the substance between each particle that is excited.
Different physicists have attempted to measure the speed of light throughout history. Galileo attempted to measure the speed of light in the seventeenth century. An early experiment to measure the speed of light was conducted by Ole Rømer, a Danish physicist, in 1676. Using a telescope, Ole observed the motions of Jupiter and one of its moons, Io. Noting discrepancies in the apparent period of Io's orbit, Rømer calculated that light takes about 22 minutes to traverse the diameter of Earth's orbit.[4] Unfortunately, its size was not known at that time. If Ole had known the diameter of the Earth's orbit, he would have calculated a speed of 227,000,000 m/s.
Another, more accurate, measurement of the speed of light was performed in Europe by Hippolyte Fizeau in 1849. Fizeau directed a beam of light at a mirror several kilometers away. A rotating cog wheel was placed in the path of the light beam as it traveled from the source, to the mirror and then returned to its origin. Fizeau found that at a certain rate of rotation, the beam would pass through one gap in the wheel on the way out and the next gap on the way back. Knowing the distance to the mirror, the number of teeth on the wheel, and the rate of rotation, Fizeau was able to calculate the speed of light as 313,000,000 m/s.
Léon Foucault used an experiment which used rotating mirrors to obtain a value of 298,000,000 m/s in 1862. Albert A. Michelson conducted experiments on the speed of light from 1877 until his death in 1931. He refined Foucault's methods in 1926 using improved rotating mirrors to measure the time it took light to make a round trip from Mt. Wilson to Mt. San Antonio in California. The precise measurements yielded a speed of 299,796,000 m/s.
Two independent teams of physicists were able to bring light to a complete standstill by passing it through a Bose-Einstein Condensate of the element rubidium, one led by Dr. Lene Vestergaard Hau of Harvard University and the Rowland Institute for Science in Cambridge, Mass., and the other by Dr. Ronald L. Walsworth and Dr. Mikhail D. Lukin of the Harvard-Smithsonian Center for Astrophysics, also in Cambridge.[citation needed]
Electromagnetic spectrum
Main article: Electromagnetic spectrum
Electromagnetic spectrum with light highlighted
Generally, EM radiation (the designation 'radiation' excludes static electric and magnetic and near fields) is classified by wavelength into radio, microwave, infrared, the visible region we perceive as light, ultraviolet, X-rays and gamma rays.
The behavior of EM radiation depends on its wavelength. Higher frequencies have shorter wavelengths, and lower frequencies have longer wavelengths. When EM radiation interacts with single atoms and molecules, its behavior depends on the amount of energy per quantum it carries.
Refraction
Main article: Refraction
Refraction is the bending of light rays when passing from one transparent material to another. It is described by Snell's Law:
where θ1 is the angle between the ray and the normal in the first medium, θ2 is the angle between the ray and the normal in the second medium, and n1 and n2 are the indices of refraction, n = 1 in a vacuum and n > 1 in a transparent substance.
When a beam of light crosses the boundary between a vacuum and another medium, or between two different media, the wavelength of the light changes, but the frequency remains constant. If the beam of light is not orthogonal (or rather normal) to the boundary, the change in wavelength results in a change in the direction of the beam. This change of direction is known as refraction.
The refractive quality of lenses is frequently used to manipulate light in order to change the apparent size of images. Magnifying glasses, spectacles, contact lenses, microscopes and refracting telescopes are all examples of this manipulation.
Light refraction is the main basis of measurement for gloss. Gloss is measured using a glossmeter
