Light does bend or change direction when it transitions from one medium to another, such as from air to water or from air to glass. This phenomenon is called refraction and occurs due to the change in the speed of light as it travels through different substances with varying optical densities.
However, in a vacuum, where there is no medium to interact with, light does not undergo refraction. According to the wave theory of light, light is an electromagnetic wave that consists of electric and magnetic fields oscillating perpendicular to each other and to the direction of propagation. When a light wave encounters a boundary between two media, such as air and water, the change in optical density causes the speed of light to change, and as a result, the wavefront bends or refracts.
In a vacuum, the speed of light is considered to be its maximum value, which is approximately 299,792 kilometers per second in the classical theory. Since there is no change in the speed of light when it passes from a vacuum to another medium or vice versa, it does not refract or bend in a vacuum.
It's worth noting that in extreme gravitational fields, such as those near massive objects like black holes, light can be affected by gravity and exhibit gravitational lensing, which causes its path to curve. However, this is a result of the curvature of spacetime caused by the massive object and is different from refraction in a medium.
In summary, light does not bend in a vacuum because there is no change in its speed when transitioning from a vacuum to another medium or vice versa. Refraction occurs when light travels through different substances with varying optical densities.