Electromagnetic waves, such as light, can travel in straight lines in a vacuum or through a homogeneous medium. This straight-line propagation is a result of the waves following the principle of rectilinear propagation, which states that they travel in a straight path unless they encounter an obstacle or are influenced by external factors.
However, when electromagnetic waves encounter a medium with varying refractive index, such as the Earth's atmosphere, they can be bent or refracted. Refraction occurs because the speed of light changes as it passes from one medium to another with a different refractive index. The change in speed causes the waves to change direction, resulting in bending.
The Earth's atmosphere has a gradually changing refractive index due to variations in temperature, pressure, and density at different altitudes. As a result, when light travels through the atmosphere, it encounters regions with different refractive indices, leading to the bending or refraction of the light rays.
The bending of light due to refraction is responsible for various optical phenomena we observe, such as the apparent displacement of objects in water, the formation of rainbows, and the mirage effect. In these cases, the varying refractive index of the medium causes the light waves to change direction and deviate from their original path.
It's important to note that the bending of electromagnetic waves by the Earth's atmosphere is a gradual process that occurs over large distances and is influenced by the specific atmospheric conditions and the geometry of the situation. In extreme cases, such as in the presence of strong gravitational fields, electromagnetic waves can be significantly bent or distorted, as predicted by Einstein's theory of general relativity.
In summary, electromagnetic waves travel in straight lines in a vacuum or uniform medium according to the principle of rectilinear propagation. However, when they encounter a medium with varying refractive index, such as the Earth's atmosphere, they can be bent or refracted, leading to their apparent deviation from a straight path.