"Shadow" is a common light phenomenon in our daily life, such as "hand shadow" formed by playing optical games; "Shadow Play" of folk shadow play; Lake shore scenery in the water.

Shadow play is the same as hand shadow, which is the straight-line propagation of light;

The reflection is imaged by a plane mirror, and the calm water surface is equivalent to a plane mirror. The image formed by the plane mirror is symmetrical with the object about the water surface, and its principle is the reflection of light.

photography is inverted, reduced and real by using a convex lens, which belongs to the refraction of light.

Therefore, the answer is: the straight-line propagation of light; Reflection of light; The refraction of light.