help with refraction

Question

refraction occurs when

 

a.  a wave strikes and is re-emitted from an object

b.  there is a difference in the speed waves  

c.  a wave bends around a barrier

d.  the crest of one wave overlaps the crest of another

 

 

I think the answer is either b or c

 

thanks

Answer 1

The refraction occurs when there is a change of a phase velocity. The frequency remains constant. This may occur when the medium where the wave travels changes. The answer is b.