In general, light tends to travel in straight lines through a medium or in a vacuum. This phenomenon is known as rectilinear propagation. When light encounters a boundary between two different mediums, such as air and water, it can undergo refraction, causing the path of light to bend. However, even in such cases, the light still continues to travel along a straight line within each medium.
Regarding the speed of light, in a vacuum, all types of light travel at the same speed, which is approximately 299,792,458 meters per second (often approximated as 3 x 10^8 meters per second). This speed is often denoted by the symbol 'c' in physics and is considered a fundamental constant of nature.
However, when light passes through different materials, such as glass or water, its speed can vary. This change in speed leads to a phenomenon called "refraction." Each material has a property known as the refractive index, which indicates how much the speed of light is reduced when passing through that material. This variation in speed causes light to change direction at the boundary between two materials, as observed, for example, when a straw appears bent when partially submerged in water.
Lasers, on the other hand, are devices that produce an intense and focused beam of light. While the light emitted by lasers can be highly directional and concentrated, it still follows the basic principles of light propagation. Laser light can still undergo refraction or scattering, just like any other light, and can travel in straight lines unless affected by external factors or optical components specifically designed to manipulate its path.
In summary, while light generally travels in straight lines and at a constant speed in a vacuum, its behavior can be modified by interaction with different materials or specific optical devices, including lasers.