As light from air to water to glass, it will refract. the best explanation for this would be

 Light travels at different speeds in different medium because of the varying densities.

Explanation: The reason for the straw to break at the water's surface is because of the process of refraction. Light travels at different speeds in different medium because of the varying densities.

The light travels from air to water and thus the difference in speed results in an apparent shift of the position of part of straw under water and thus the part appears broken. This phenomenon is called as refraction.

this is poiselle's equation from fluid mechanic in physics. mech eng