|Headlights: evil harbingers of environmental doom.|
Most car lights look yellow. Yellow light has a wavelength and frequency of λ = 570 nm and f = 5×1014 Hz, respectively. Each photon of light that leaves the car carries with it some energy given by1
E = h f .
If you have N photons emitted by the car, the car will have lost an amount of energy
ΔE = N h f .
Car lights consume energy at a rate of about P = 50 W. This P is known as power, and it's the rate at which energy is transferred,
P = ΔE / Δt.Assuming all the energy goes into creating yellow photons, we can find how many of particles of light leave the car each second by solving for N:
N = P Δt / h f,
= (50 W) · (1 sec) / (6.63×10-34 J · s) · (5×1014 Hz),
= 1.5×1020 photons.
Each photon that leaves carries with it some momentum,
p = h / λ.
As it leaves, the photon imparts some force on the car,
F = Δp / Δt.
The total force on the car is then
F = N h / λ Δt ,
= (1.5×1020 photons) · (6.63×10-34 J · s per photon) / (540 nm) · (1 sec),
= 1.8×10-7 N.
That's a tiny force. Just how tiny? If you drive 3000 miles across the United States, it'll cost only 1 Joule of energy.2 One gallon of gasoline contains 1.3×108 J of energy. You would need to drive around the U.S. with your lights on 130 million times before you'd wasted a gallon of gas.
 Photons are particles of light.
 You can obtain this by multiplying the force times the distance.