During fog, why are we unable to see objects clearly?

Concept: Scattering of light happens when a light ray traveling through a transparent medium meets tiny suspended particles - such as water droplets, dust, or smoke - whose size is comparable to or larger than the wavelength of light. On striking such a particle, the ray is redirected away from its original path, spreading out over a range of directions instead of continuing along one straight line.

Application: Fog is a thick suspension of countless tiny water droplets floating in air near the ground. As light travels from an object toward an observer's eye, it repeatedly strikes these droplets and gets scattered away from its original path in many directions. This scattered light mixes with the direct light from the object, washing out the sharp outline and reducing the contrast between the object and its background, so the object looks blurred, hazy, or invisible even at short distances.

Cross-check - why the other listed phenomena do not fit:

• Reflection describes a ray bouncing off a surface at a definite angle (angle of incidence equals angle of reflection), as with a mirror or still water; it does not describe light being spread over many directions by countless suspended droplets.

• Internal reflection describes a ray inside a denser medium striking the boundary with a rarer medium; when the angle of incidence exceeds the critical angle this becomes total internal reflection and the ray bounces entirely back inside (the basis of optical fibres and a diamond's sparkle) - either way it needs a single well-defined boundary, not a cloud of droplets spread through open air.

• Refraction describes a ray bending as it crosses one interface between two media of different optical density, such as a straw appearing bent in water; a single bending event at one interface does not account for the overall haze produced across an entire foggy path.

• Diffraction describes waves spreading around the edge of an obstacle or through a narrow slit, noticeable when the obstacle or slit size is close to the light's wavelength; it is not the mechanism by which a field of much larger water droplets reduces visibility.

Result: Because countless water droplets in fog scatter light over a spread of directions rather than reflecting, refracting, or diffracting it at a single well-defined surface, the direct image of an object is blurred and its contrast with the background is lost. This is also why a vehicle's headlight beam looks like a hazy, spread-out glow in fog instead of a clean, focused beam - scattered light returns toward the observer from many of those directions. Scattering of light is therefore the phenomenon responsible for reduced visibility in fog.