Table of Contents
- What is Redshift Theory?
- Redshift Explained
- The Doppler Effect
- Redshift Uses in Science
- Lesson Summary
Redshift is a phenomenon that occurs with waves of electromagnetic radiation when they demonstrate the Doppler effect by their source moving away from an observer. When the source of the light is moving away from the observer, the frequency of the light observed is shifted toward the color red. When the source of light is moving toward the observer, the light waves get shifted toward the frequencies of blue and are, therefore, blue-shifted. Redshift is best explained when light is understood as a propagating wave.
Visible light is an electromagnetic wave composed of many frequencies. The varying frequencies of visible light are experienced as the varying colors of the rainbow. The following colors are in the order from lowest frequency to highest frequency of electromagnetic waves: red, orange, yellow, green, blue, indigo, violet. Again, redshift occurs when the source of light is moving away from an observer; this causes the frequency of the light to lower and be shifted toward the color red. But exactly what are electromagnetic waves and visible light? How does a source of light moving change the color of the light? First, the redshift meaning must be understood through redshift science or "the science of waves."
Within physics, a wave is a propagating dynamic deformation of a medium. Sound waves are waves of energy that propagate through the deformation of particles and molecules of air. Electromagnetic waves such as visible light are waves of energy that propagate through electric and magnetic fields. Light waves can be imagined as simple transverse waves.
The wavelength of a wave is the distance between common points on two waves, such as the crests or tops of two waves. The ''frequency'' of a wave is the number of waves that pass a given point per second. Waves that have shorter wavelengths have higher frequencies because the waves are closer together and therefore more waves can pass a given point per second. Whereas longer wavelengths create lower frequencies because it takes longer for a complete wave cycle to pass a given point. When a source of a continuous stream of sound waves or light waves is moving away from an observer, the wavelengths experienced by the observer are elongated. Because red has the longest wavelengths of the colors of light, light experienced by the observer are shifted toward the color red (or shifted toward longer wavelengths). But does this actually change the color of light? This depends on the different types of redshifted light.
There are three main ways for light to be redshifted:
Redshift, especially relativistic redshift is a type of Doppler effect.
Like sound waves, waves of visible light travel at a finite speed. When a source of a continuous propagation of sound or light is moving in reference to an observer, the Doppler effect is demonstrated by the waves of light or sound. In the case of sound waves, a common example of the Doppler effect is a car horn or a police siren passing by an observer. When a car is driving by an observer while honking its horn, the sound emitted by the horn changes pitch from the perspective of the observer. When the car is driving toward the observer, the frequency of the sound waves is experienced at a higher rate. Because the frequency of sound waves is experienced as pitch, the car horn sounds higher pitched while the car is driving toward the observer. Once the car passes the observer, the sound wave frequency is getting elongated due to the car's movement away from the observer. This lowers the frequency, and the observer experiences the car horn as having a lower pitch.
The frequency for sound waves is experienced as pitch, but the frequency for light waves is experienced instead of color. Light can become red-shifted or blue-shifted when the source of the light relative to an observer is moving toward or away from the observer. When the source of light is moving toward the observer, the frequency of the light waves increases because the wavelengths of the light waves get shorter or more scrunched up. Blue and violet have the shortest wavelengths and highest frequencies. Therefore, if a source of light is moving toward an observer, the observer will experience the light at a higher frequency and it will therefore be blue-shifted. If the source of light is moving away from the observer, its wavelengths get elongated and the frequency of the light experienced by the observer will be lower and therefore will be red-shifted.
The major implication for red-shifted light is what Edwin Hubble discovered by observing very distant galaxies. As previously mentioned, Hubble discovered that the further away a galaxy is, the more red-shifted its light. Therefore, the further away a galaxy is, the faster it is moving away from Earth. However, this means that galaxies at a certain distance would therefore be moving away from Earth faster than the speed of light. This meant that the galaxies themselves could not be moving, but the space between them must be getting larger. If almost all galaxies are red-shifted and expanding away from each other, then they must have been closer together in the distant past. This is one of the major pieces of evidence for the Big Bang Theory as the best possible explanation for the origin of the universe. The Big Bang Theory explains how and why almost all galaxies are moving away from each other besides the ones that are gravitationally bound in local groups. This discovery made by Edwin Hubble shifted the paradigm in cosmology, astronomy, and astrophysics.
Overall, redshift is a type of Doppler effect that occurs with visible light. The Doppler effect is the increase or decrease of wavelength when the source of propagating waves is moving away from or toward an observer. Wavelength is the distance between common points on two waves such as the distance between two wave crests or peaks of waves.
Redshift occurs when a light source moves away from an observer and therefore increases and elongates its wavelengths of light, resulting in the light having redder frequencies. Light can also be blue-shifted when the source of the light is moving toward an observer. Scientists use blueshift and redshift to study the universe by studying how redshift occurs with light from distant galaxies and stars which is therefore supporting evidence for the universe's expansion.
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This activity will help you assess your knowledge of the definition, theory, and effect of a redshift.
For this activity, carefully read and select the best answer that completes each of the given statement. To do this, print or copy this page on a blank paper and circle the letter of your answer.
1) A shift toward a lower frequency, or the red end of the color spectrum is known as a redshift.
2) __________ is a type of wave that can travel through any medium, including the vacuum in space.
3) Which of the following is an example of a medium?
D. all of the above
4) The waves are __________ as the __________ moves closer and closer to the observer.
A. heated, medium
B. transparent, signal
C. compressed, source
D. stretched, source
5) Doppler effect is observed whenever the __________ of waves is moving relative to the __________.
A. medium, source
B. source, observer
C. observer, medium
D. source, wave
6) __________ are produced from the compression and collision of air molecules.
A. Sound waves
B. Ocean waves
D. none of the above
7) As a wave source moves away from an observer, the wave spreads out, and its wavelength __________.
A. remains the same
D. cannot be determined
8) From the redshift data, scientists observed that stars farther away from Earth are moving __________, which proves that the universe is __________.
A. slower, evolving
B. slower, shrinking
C. faster, exploding
D. faster, expanding
Light gets red-shifted when the source of the light is moving away from an observer. When this occurs, the wavelengths or the distances between waves of light get elongated because light travels at a finite speed. Because red light has the longest wavelengths, light from a source that is moving away from an observer, therefore, has redder light. This however is mostly undetectable by the human eye.
Redshift shows scientists that the universe is expanding because of how redshift occurs. Redshift occurs in light when the source of light is moving away from an observer. Scientists have observed that almost all galaxies observed from Earth are red-shifted. Therefore almost all galaxies are moving away from each other and the universe is therefore expanding.
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