WHY SHOULD LIGHT BEND?

     put a spoon in a glass which is half filled with water.  The spoon looks bent, because light traveling from air to water is bent(refracted).  why should light refract?
     When light waves travel in a denser medium like water and glass, its slows down.  like we slow down, when we move into a knee deep water. 
     If all the light waves travels straight(perpendicular to the line of separation of two media) and enter the denser medium, the wave speed will decrease, but they will go undeviated.
     But if the waves enter the line of separation at an angle(obliquely), part of the first wave front will enter the denser medium and"that part" will be slowed down; remaining part will move with original speed in the first medium.  This process causes the wavefront to bend.  similarly all the waves that follow will bend (when they enter) and follow a different path in the denser medium.
     Without refraction of light,   lenses, prism, microscope, telescope and other optical devices are not possible.

     Note:  The second medium, the waves' speed will change and hence wavelength will change.  But the frequency can not change.