The rectilinear propagation of light explains a number of common phenomena, such as the formation of shadows. Suppose light is moving outward from a point source (like the sun) and it encounters an obstacle (like a ball). The light will be stopped (absorbed and reflected) by the ball. Around the "edges" of the ball, the light will continue so that a shadow of the ball appears on the ground. If the light source is extended, rather than a point source, the shadow will consist of very dark regions (umbra) surrounded by partially illuminated regions (penumbra). These can also be explained by rectilinear propagation.
You can observe shadows from a point source (which consist of a dark portion only) by looking at shadows cast by the sun on a bright day. To see the two-part shadows which have both darker and lighter portions, look at the shadow your hand casts under a table lamp. As you move your hand toward the table, the darker part (umbra) grows larger.
During an eclipse of the moon, the two-part shadow (umbra and penumbra) cast by the earth can be seen.