DOPPLER EFFECT IN LIGHT - Red & Blue Shift - Police use the Doppler Effect in the radar boxes they use to track speed. Radio waves are transmitted out, collide with a vehicle, and bounce back. Similar applications can be used to measure wind velocities in the atmosphere, which is the "Doppler radar" of which meteorologists are so fond. This Doppler shift is also used to track satellites. By observing how the frequency changes, you can determine the velocity relative to your location, which allows ground-based tracking to analyze the movement of objects in space. In astronomy, these shifts prove helpful. When observing a system with two stars, you can tell which is moving toward you and which away by analyzing how the frequencies change.

...............................................................................................................................................

Doppler Effect in Light

Red & Blue Shift

by Andrew Zimmerman Jones 

Light waves from a moving source experience the Doppler effect to result in either a red shift or blue shift in the light's frequency.

This is in a fashion similar (though not identical) to other sorts of waves, such as sound waves.

The major difference is that light waves do not require a medium for travel, so the classical application of the Doppler effect doesn't apply precisely to this situation.

Relativistic Doppler Effect for Light

Consider two objects: the light source and the "listener" (or observer).

Since light waves traveling in empty space have no medium, we analyze the Doppler effect for light in terms of the motion of the source relative to the listener.

We set up our coordinate system so that the positive direction is from the listener toward the source.

So, if the source is moving away from the listener, its velocity v is positive, but if it is moving toward the listener, then the v is negative.

The listener, in this case, is always considered to be at rest (so v is really the total relative velocity between them).

The speed of light c is always considered positive.

The listener receives a frequency f_L which would be different from the frequency transmitted by the source f_S.

This is calculated with relativistic mechanics, by applying necessary the length contraction, and obtains the relationship:

f_L = sqrt [( c - v)/( c + v)] * f_S

Red Shift & Blue Shift

A light source moving away from the listener (v is positive) would provide an f_L that is less than f_S. In the visible light spectrum, this causes a shift toward the red end of the light spectrum, so it is called a redshift.

When the light source is moving toward the listener (v is negative), then f_L is greater than f_S. In the visible light spectrum, this causes a shift toward the high-frequency end of the light spectrum.

For some reason, violet got the short end of the stick and such frequency shift is actually called a blue shift.

Obviously, in the area of the electromagnetic spectrum outside of the visible light spectrum, these shifts might not actually be toward red and blue.

If you're in the infrared, for example, you're ironically shifting away from red when you experience a "redshift."

Applications

Police use this property in the radar boxes they use to track speed. Radio waves are transmitted out, collide with a vehicle, and bounce back.

The speed of the vehicle (which acts as the source of the reflected wave) determines the change in frequency, which can be detected with the box.

(Similar applications can be used to measure wind velocities in the atmosphere, which is the "Doppler radar" of which meteorologists are so fond.)

This Doppler shift is also used to track satellites. By observing how the frequency changes, you can determine the velocity relative to your location, which allows ground-based tracking to analyze the movement of objects in space.

In astronomy, these shifts prove helpful. When observing a system with two stars, you can tell which is moving toward you and which away by analyzing how the frequencies change.

Even more significantly, evidence from the analysis of light from distant galaxies shows that the light experiences a redshift. These galaxies are moving away from the Earth.

In fact, the results of this are a bit beyond the mere Doppler effect. This is actually a result of spacetime itself expanding, as predicted by general relativity.

Extrapolations of this evidence, along with other findings, support the "big bang" picture of the origin of the universe.

Andrew Zimmerman Jones

Introduction

Academic researcher, educator, and writer with 23 years of experience in physical sciences

Works at Indiana Department of Education as senior assessment specialist in mathematics

Co-author of String Theory For Dummies

Member of the National Association of Science Writers

Experience

Andrew Zimmerman Jones is a former writer for ThoughtCo who contributed nearly 200 articles for more than 10 years. His topics ranged from the definition of energy to vector mathematics. Andrew is a dedicated educator; and he uses his background in the physical sciences, educational assessment, writing, and communications to advance that mission. 

Andrew is co-author of String Theory For Dummies, which discusses the basic concepts of this controversial approach. String theory tries to explain certain phenomena that are not currently explainable under the standard quantum physics model. 

Since 2018, Andrew has worked at the Indiana Department of Education as a senior assessment specialist in mathematics; prior to which he served as a senior assessment editor at CTB/McGraw Hill for 10 years. In addition, Andrew was a researcher at Indiana University's Cyclotron Facility. He is a member of the National Association of Science Writers. 

Education

Andrew Zimmerman Jones has a Master of Science (M.S.) in Mathematics Education from Indiana University–Purdue, Indianapolis, Ind.; and a Bachelor of Arts (B.A.) in Physics from Wabash College, Crawfordsville, Ind. 

Awards and Publications

String Theory For Dummies (Wiley–For Dummies Series, 2009)

Graduated magna cum laude (Wabash College, 1999)

Harold Q. Fuller Prize in Physics (Wabash College, 1998)

ThoughtCo and Dotdash

ThoughtCo is a premier reference site focusing on expert-created education content. We are one of the top-10 information sites in the world as rated by comScore, a leading Internet measurement company. Every month, more than 13 million readers seek answers to their questions on ThoughtCo.

For more than 20 years, Dotdash brands have been helping people find answers, solve problems, and get inspired. We are one of the top-20 largest content publishers on the Internet according to comScore, and reach more than 30% of the U.S. population monthly. Our brands collectively have won more than 20 industry awards in the last year alone, and recently Dotdash was named Publisher of the Year by Digiday, a leading industry publication.

https://www.thoughtco.com/doppler-effect-in-light-red-shift-and-blue-shift-2699033