Döppler Effect (Light)

The Physics Hypertextbook™
© 1998-2008 by Glenn Elert -- A Work in Progress
All Rights Reserved -- Fair Use Encouraged

prev | up | next

---

Discussion

Red-shift away. Blue-shift towards.

Symbol: Z

The red shift doesn't literally look red.

Handheld radar gun: 35.1 GHz signal, 0-300 mph speed range, 0.1 mph sensitivity, 30 Hz sampling rate.

Summary

• bullet

Problems

practice

1. There are hundreds if not thousands of television stations across North America that claim to use "doppler radar" when reporting weather related news. In keeping with the general level of hype that is Twentifirst Century TV weather, most if not all of these stations never show actual doppler radar images in their broadcasts. Instead, what they show are reflectivity images. A radar pulse is sent out, the strength of the echo is measured, and the results are displayed as a certain color that indicates the intensity of precipitation. A typical color code would be green for light rain, yellow for moderate, red for intense, and so on.
   
   [magnify]

   A real doppler radar image like the one on the right shows something different. In this mode, a radar pulse is sent out, the frequency of the echo is measured, and the results are displayed as a certain color that indicates the velocity of the precipitation. The colors are often assigned in a manner that imitates the color shifts seen when the doppler effect is applied to visible light — red, orange, and yellow for raindrops blowing away from the radar antenna and green, blue, and violet for raindrops blowing toward the radar antenna. (Color assignments vary from one radar system to another, however, so these are not absolute rules.)

   The image to the right was taken from the NOAA National Weather Service radar station in Dodge City, Kansas on 7 May 2007 that was operating in doppler mode.

   1. What was the direction of the wind at the radar site in Dodge City (located near the upper left hand corner of the image)?
   2. There is an unusual two-color region located in the center of this image just to the southwest of a small town called Greensburg.
      1. What is the wind direction in the blue region?
      2. What is the wind direction in the red region?
      3. What is the overall airflow pattern in this unusual two-color region?
      4. What is probably going on here?

   Solution

2. Use this variation of the doppler effect formula …

   Δλ	≈ ±	v
   λ		c

   to answer this set of related questions …
   1. How fast and in what direction would a car have to move to make a yellow traffic light (580 nm) appear green (530 nm)?
   2. How fast and in what direction would a car have to move to make a red traffic light (680 nm) appear green (530 nm)?
   3. "I swear I saw a green light. The doppler efffect must have made it look green. You shouldn't give me a ticket for running a red light. It's not my fault. You should give a ticket to Christian Döppler." Would this reasoning get you out of a traffic ticket?

   Solutions …

   Don't break the law.

3. Write something different.
   • Answer it.
4. Write something completely different.
   • Answer it.

numerical

1. problems

Resources

• no resources for this topic

---

prev | up | next

Another quality webpage by Glenn Elert

home | contact bent | chaos | eworld | facts | physics