Refraction is caused by waves changing speed.
Refraction is when waves change direction as they enter a different medium caused by a change in density which changes the speed of the waves.
When waves slow down, they bend towards the normal.
When light enters glass or plastic, it slows down.
If a wave hits a boundary at 90 degrees (I.e. along the normal line) it will not change direction but it will still slow down.
When light hits a different medium (e.g. Plastic or…