Light can be assumed to be approximately travel in straight line because :

Wavelength of light is much smaller than the dimensions of the objects.

The wavelength of visible light was measured and found to be extremely small; for example, the wavelength of yellow light is about 0.6 µm. Because of the smallness of the wavelength of visible light (in comparison to the dimensions of typical mirrors and lenses), light can be assumed to approximately travel in straight lines.