"Shadow" is a common light phenomenon in our daily life. Please write down the physical knowledge corresponding to "shadow": (1) Hand shadow formed by playing optical games: _

(1) The "hand shadow" formed by playing optical games is the shadow. When light travels in a straight line, it encounters an opaque object, and the dark area behind the object is called a shadow.

(2) The shadow play of folk shadow play is the straight-line propagation of light;

(3) The "reflection" of the lakeshore scenery in the water: it is imaged by a flat mirror, and the calm water surface is equivalent to a flat mirror. The image formed by the plane mirror is symmetrical with the object about the water surface, so it is inverted, and its principle is the reflection of light;

(4) "Photography" during the spring outing: the inverted image, the restored image and the real image are all made of convex lenses, and the lens is the refraction principle of light.

So the answer is: (1) the straight-line propagation of light; (2) the straight-line propagation of light; (3) reflection of light; (4) Refraction of light.