Shadows can provide evidence that light travels in straight line. Think of the shadows that are formed on a cloudless sunny day, they are very well defined shadows. Supposing at this point that light did not travel in straight lines what would the shadows look like? They would probably be very fuzzy just like you would get on a sunny day with lots of clouds in the sky. When the sky is cloudy the sunlight is reflected of the clouds and they become secondary sources of light so on such a day the shadow is formed from lots of different light sources coming from a variety of directions.