Final answer:
A wave that hits perspex at a 90-degree angle does not change its angle thanks to normal incidence, where there is no bending of the wavefront, just a change in wave speed.
Step-by-step explanation:
When a wave hits perspex at a 90-degree angle, it does not change its angle of incidence, and hence its angle of refraction remains 0 degrees. This is because, according to Snell's law, a light wave entering a new medium at a normal incidence (90 degrees to the surface) will not bend. As the wave passes from one medium to another, such as air into perspex (a type of plastic with similar properties to glass), the only effect observed is a change in speed due to the differing refractive indices of the materials, but the direction of the wavefront remains unaltered.