Light travelling in straight lines(Exposition)

Linear paths

Not only does light travel, it travels in straight lines through a given medium.

Various first-hand sources of evidence point to light's linear path.

Shadows

Further evidence is provided by shadows. If some light from an object (usually a luminous object) is blocked by something, then that something causes a shadow: a region where there's less light.

The shape of the something doing the blocking determines the shape of the shadow.

Scattering

Some particles found in the atmosphere have the ability to scatter beams of light.

The incident beams of light are scattered in all directions. In some cases the scattering results from relatively large particles in the air (such as dust particles). In other cases the scattering is thought to be due to interactions between the incident light and molecules in the air.

Absorption

Some particles found in the atmosphere have the ability to absorb beams of light.

The incident beams of light stop, or become dimmer and the particles move more.