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Redshift Explained: Light and the Expanding Universe

Joseph Comunale, Amanda Robb, Christianlly Cena
  • Author
    Joseph Comunale

    Joseph Comunale obtained a Bachelor's in Philosophy from UCF before becoming a high school science teacher for five years. He has taught Earth-Space Science and Integrated Science at a Title 1 School in Florida and has Professional Teacher's Certification for Earth-Space Science.

  • Instructor
    Amanda Robb

    Amanda has taught high school science for over 10 years. She has a Master's Degree in Cellular and Molecular Physiology from Tufts Medical School and a Master's of Teaching from Simmons College. She is also certified in secondary special education, biology, and physics in Massachusetts.

  • Expert Contributor
    Christianlly Cena

    Christianlly has taught college Physics, Natural science, Earth science, and facilitated laboratory courses. He has a master's degree in Physics and is currently pursuing his doctorate degree.

Learn the meaning of redshift light, the Doppler Effect, and how waves work in physics. Explore the fundamentals of redshift light and the importance of redshift light in science. Updated: 12/07/2021

Table of Contents


What is Redshift Theory?

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.

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  • 0:04 What Is a Wave?
  • 0:50 The Doppler Effect
  • 1:51 Red Shift
  • 2:40 Implications of Red Shift
  • 3:35 Lesson Summary
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Redshift Explained

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."

What is a Wave?

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.

Wavelength is labeled in this image. Redshift explained as increasing the wavelength of light waves.

Redshift definition involves the wavelengths of light increasing. This diagram defines wavelength.

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.

Types of Redshift Light

There are three main ways for light to be redshifted:

  • Redshift through the expansion of the universe is the most effective at elongating wavelengths of light because the expansion of the universe at large scales can be happening at rates faster than the speed of light. This phenomenon was discovered by Edwin Hubble when he observed light from distant galaxies being red-shifted. Hubble discovered that the further away a galaxy is the faster it is moving away because the most distant galaxies emit light that is the most red-shifted. This meant that the universe is expanding like a balloon, with the distances between galaxies getting larger not because the galaxies are moving away from each other, but because the space between them is literally expanding. This is the only kind of redshift that actually results in the images of the galaxies appearing reddish, at least in the case of the most distant of galaxies. The light of very distant galaxies passes through the vastness of space which because of its expansion causes the wavelengths of the light to get stretched out toward the frequencies and wavelengths of red. This is also called cosmological redshift. However, other types of red-shifted light are undetectable by the human eye. For example, light-emitting from a car is not going to appear redder if it is driving away from an observer.

Some of the galaxies in this picture from the Hubble space telescope are more obviously red-shifted than others.

This picture is of very distant galaxies. Some of these galaxies are more obviously red-shifted than others.

  • In addition to expanding space pushing galaxies further apart, light emitted by galaxies is also red-shifted due to the movement of the galaxies themselves or the stars within them. Galaxies have rotation, therefore one side of a galaxy can be rotating toward an observer and therefore be blue-shifted, while the other side of the galaxy's center would be rotating away from the observer and be red-shifted. The light emitted by the individual stars within the galaxy can be red-shifted or blue-shifted depending on their movement in relation to an observer. The stars that are moving parallel to the observer would neither be red-shifted nor blue-shifted. This is also called relativistic redshift because it occurs due to the movement of sources of light relative to an observer. This type of redshift like most is not detectable by the human eye.
  • Visible light can also have its wavelengths physically stretched by gravity. Gravity can be thought of as the deformation of spacetime, through stretching like a fabric. Gravitational redshift can occur when the light emitted by a source moves toward an object that has weaker gravity and therefore has less strongly curved spacetime.

Redshift, especially relativistic redshift is a type of Doppler effect.

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  • Activities
  • FAQs

Red Shift: Multiple Choice Exercise

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.

Multiple Choice

1) A shift toward a lower frequency, or the red end of the color spectrum is known as a redshift.

A. True

B. False

2) __________ is a type of wave that can travel through any medium, including the vacuum in space.

A. Sound

B. Light

C. Air

D. Water

3) Which of the following is an example of a medium?

A. air

B. water

C. wood

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

C. Lightwaves

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

B. decreases

C. increases

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

Answer Key

  1. A
  2. B
  3. D
  4. C
  5. B
  6. A
  7. C
  8. D

What causes redshift in light?

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.

How does redshift show that the universe is expanding?

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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