appear to our eyes? The observations and measurements made by different observers
do differ. In discussions of relativity, much focus is placed on observers and
measurements, since experiments show effects such as time dilation and length
contraction are real.On the other hand, we are taking liberties in showing the object’s contraction in these
illustrations. If the train car were photographed from a station as it passed by, it would
appear rotated. The rear of the car is farther away from the stationary observer, and
light from the rear will take longer to reach her than light from the front of the vehicle.
The light that arrives simultaneously for the observer is emitted at different instants in
time from the car, which moves during that interval of time. Using geometrical
arguments, it can be shown that this effect creates the perception of rotation.L = length measured by observer
watching object pass by
L 0 = proper length (measured by
observer stationary relative to object)
v = speed of reference frame
c = speed of light
How long is the box on the train
from Sara’s perspective? The
professor measures its length as
4.0 meters.
L = 4.0(0.6) = 2.4 m
35.10 - Doppler shift for light
The Doppler effect is frequently thought of in terms of sound. The siren on a police car heading toward you will sound different than when it isDoppler shift: light
Motion changes light frequency (color)
Shift depends on velocity between
source, observer(^648) Copyright 2007 Kinetic Books Co. Chapter 35