Fog lights must have sufficient penetrating power when used in low visibility conditions. Before talking about the principle of fog lights, let’s start with the "rising posture": the air is an aerosol filled with various tiny water droplets, suspended particles and other "impurities". The light from the headlights must pass through these impurities to reach the ground. Since it is an aerosol, the light from the headlamp will inevitably produce the "Tyndall effect" when passing through the air, which is a phenomenon in which parallel rays of light diffract a bright beam of light when passing through the air. When the weather is good, these "impurities" are harmless and we won't be aware of it. However, it is different when visibility is low. The light of ordinary headlights is gathered together, and it is difficult to penetrate thick fog and water vapor. The high-brightness light hits the fog, and the Tyndall effect is very obvious. The result is a vast expanse of whiteness in front of me, and nothing can be seen on the road. This is why high beams cannot be used in foggy days. The strong and concentrated light of high beams can turn the fog in front of you into a bright "wall".
The principle of the "Tyndall effect" is that the visible light wavelengths of certain bands coincide with the aerosol particles, and diffraction occurs, so that parallel light cannot pass through in a straight line. Therefore, the best way to reduce the "Tyndall Effect" is to keep the light source as far away as possible from these bands. The fog lamp has a relatively long wavelength, which can minimize diffraction, allowing more effective light to pass through dense fog and tiny water droplets. At the same time, the light of the fog lamp is divergent rather than parallel, and will not converge like a headlight. If the light does not converge, the Tyndall effect will not be amplified.
To put it simply, the fog lamp is specially designed and has two major advantages: first, it has strong penetrating ability and high recognition; second, it hardly produces Tyndall effect, and the light will not hit the fog and block the line of sight. It can also illuminate the ground nearby. This is the key to distinguishing fog lights from other car lights.