Block some light from a bright source with your hand to make a shadow: a region where there is less light.
The shape of your hand sets the shape of the shadow. Explain that easily if light appears to go in straight lines.
On a sunny-cloudy day you get beams of light through the clouds. On a foggy day car headlights appear as beams of light. Lighthouses produce beams of light. So do lasers in light-shows.
In all these cases it looks as if the light is going in a straight line. The edges of the beam are straight lines.
Some of the light reflects off the water droplets in the fog or other particles in the air, such as dust, in all directions. That's called
Scattering makes these particles non-luminous sources. You can see the beam if this scattered light enters your eye.
But the light that is scattered from the particles does not continue onward, so the beam gets dimmer.
Some particles found in the atmosphere can absorb beams of light. The beams of light stop, or become dimmer and the particles move more.