If we keep rectangular block straight in front of light ray what happens
Answers
It actually depends on angle of incidence but before that we must know why it deviates? why light bends when it travels from one medium to the other? to know the answer for this question.
why refraction? why direction of wave changes?
You can explain this using the Principle of least action or equivalently Fermat's principle. If you want light to get from place A to place B it will take the fastest route. As the speeds differ in the two different medium the fastest route turns out to be one with an bend at the boundary.
Fermat's principle of least time tells us that light always takes the path which is extreme in nature (i.e. minimum, maximum or constant). When the velocity of light changes it has to change its direction accordingly to arrive in the same phase.
Suppose a plane wave front of light is incident on water from air making some angle as shown in the figure. Now since the speed of light in water is less than that in air, the portion of the wave front which touches water before other portions propagates slower than those portion. As a result the wave front changes its direction towards the normal. If light is incident on a rarer medium from a dense medium the effect will be opposite and light will change its direction away from the normal.
So i think its clear now why light doesnt deviate for normal incidence because each part of wavefront approaches medium at the same time and also the extreme nature is to travel without deviation.