Doppler rainbows

If c were slower, a yellow duck boat (inset) moving obliquely toward you might appear green in the front and violet in the back due to the Doppler shift as shown here. The front of the boat is closer to you, so it has a smaller component of velocity directed toward you, and thus is shifted less than the back of the boat. Near the center of the boat, the duck boat’s engine generates heat and infrared light, which is Doppler shifted to the red color seen on the roof of the boat. Length contraction is not so evident in this view because of the effect of the finite “runtime” of light, the term used to indicate that the light from the back of the boat starts from further away, and thus takes longer to get to your eye than the light from the front of the boat. See the...