Snell's circle                                                                                   Explanation

The boundary part

Gap is large compared to wavelength                                                       Gap is small compared to wavelength

Reason for interference pattern.

According to Huygen's principle, the gap can be seen to consist of many small individual wave sources (blinking points). The right side shows how the intensity of the waves hitting the right wall varies.

• The grey "lines" show points where these waves have destructive interference (cancel each other out).
• Between the grey "lines" the waves have constructive interference and the wave (intensity) gets stronger
• Many waves have constructive interference in the middle. Here the intensity is highest.
• There are many "dark spots" (black lines in figure 2). Here you have destructive interference and the waves cancel each other.

                       Figure 1                                                                                                Figure 2

Same thing here. The blue area shows constructive interference, the green destructive. The constructive interference is strongest in the middle, hence the peak in intensity. 

Angle of incidence is not 90°                                                     Angle of incidence is 90°

Explanation                                                                         In glass fiber

In water

Hard boundary                                                                                                     Soft boundary

Plane polarized wave shown from two different angles.

The variation of the electric field is shown in red and the variation of the magnetic field is shown in blue 

                           Stationary source                                                            Moving source

         Speed of source = speed of wave                                         Speed of source > speed of wave

       Standing waves (different harmonics)                                              Standing waves from travelling waves.

Standing longitudinal waves

Pulses in opposite direction                                                                  Constructive and destructive interference

Superposition on a slinky