Figure 12-28The Speed of Light Is Constant (a) As seen from the ground, photons of light from the lamp are traveling toward the car with a speed v = c (ignoring the slight decrease in speed due to the presence of the air). (b) As seen from the car, moving toward the lamp at v = 0.5c, the photons are also traveling at the speed v = c. The difference in color between the light in the two figures is due to the Doppler shift.