When an electromagnetic wave travels from one medium to another, several phenomena can occur depending on the properties of the two media involved. The three main possibilities are reflection, refraction, and absorption.
Reflection: Reflection occurs when an electromagnetic wave encounters a boundary between two media and bounces back into the original medium. The angle of incidence (the angle at which the wave strikes the boundary) is equal to the angle of reflection (the angle at which the wave bounces off). This phenomenon is responsible for the reflection of light off a mirror or the echo of sound waves off a wall.
Refraction: Refraction occurs when an electromagnetic wave passes from one medium into another medium with a different optical density (refractive index). As the wave crosses the boundary, its speed and direction change. The change in speed causes the wave to bend, or refract, either toward or away from the normal (the line perpendicular to the boundary). The amount of bending depends on the difference in refractive indices between the two media. This phenomenon is responsible for the bending of light when it passes through lenses, the formation of rainbows, and the apparent displacement of objects seen underwater.
Absorption: Absorption occurs when an electromagnetic wave transfers its energy to the medium it encounters. The energy is absorbed by the atoms or molecules of the medium, leading to a decrease in the intensity of the wave. Different materials have different absorption properties, meaning they absorb specific wavelengths more effectively than others. This absorption can manifest as heating, such as when sunlight is absorbed by the Earth's surface, causing it to warm up.
It's important to note that these phenomena are not mutually exclusive, and multiple effects can occur simultaneously when an electromagnetic wave encounters a medium boundary. The behavior of the wave depends on the characteristics of the media involved, including their refractive indices, absorbance properties, and the angle of incidence.