Doppler Effect in Light
Overview
This lecture explains how the spectrum of light can reveal the speed and direction of moving objects using the Doppler effect, focusing on the concepts of red shift and blue shift.
Light Color Perception Basics
- A light bulb emits light in all directions, and stationary observers see its true color (e.g., green).
- Color perception depends on the wavelength of light reaching the observer.
Effect of Motion on Light Wavelength
- If a light source moves towards an observer, the emitted light wavelengths are compressed (shorter).
- If the source moves away, the wavelengths are stretched (longer).
- Compressed wavelengths (shorter) result in observed colors shifting toward blue (blue shift).
- Stretched wavelengths (longer) result in observed colors shifting toward red (red shift).
- The faster the movement, the greater the shift from the original color.
Blue Shift and Red Shift
- Blue shift: Observed when a light source moves toward you, causing shorter-than-actual wavelengths.
- Red shift: Observed when a light source moves away, causing longer-than-actual wavelengths.
- Both shifts cause the observed color to differ from the actual emitted color.
Determining Speed and Direction
- By comparing the observed color with the true emitted color, the speed and direction (towards or away) of the object can be calculated.
- The degree of color shift indicates how fast the object is moving relative to the observer.
Applications of the Doppler Effect
- The Doppler effect describes the change in observed wavelength (and thus color) due to motion towards or away from the observer.
- Doppler radar uses this effect to track the movement of objects such as clouds or planes by analyzing the shift in reflected light.
Key Terms & Definitions
- Wavelength — The distance between two peaks in a wave, determining perceived color.
- Blue Shift — Shortening of wavelength seen when a source moves toward the observer.
- Red Shift — Lengthening of wavelength seen when a source moves away from the observer.
- Doppler Effect — The change in observed wavelength or frequency due to motion of the source relative to the observer.
- Doppler Radar — Technology that measures motion using the Doppler effect with reflected light or radio waves.
Action Items / Next Steps
- Review examples of objects exhibiting red shift and blue shift.
- Prepare to identify Doppler effect scenarios in upcoming exercises or discussions.