The correct answer is C. Frequency.
When light travels from a vacuum into water, its speed, wavelength, and direction of propagation all change. However, its frequency remains unchanged.
The frequency of light is the number of waves that pass a given point in a given amount of time. It is measured in hertz (Hz), which is equal to one wave per second. The frequency of light is determined by its source, and it does not change when light travels from one medium to another.
The speed of light is the distance that light travels in a given amount of time. It is measured in meters per second (m/s). The speed of light in a vacuum is 299,792,458 m/s. When light travels from a vacuum into a medium, its speed decreases. The amount of decrease depends on the refractive index of the medium. The refractive index of a medium is a measure of how much light slows down when it travels through the medium.
The wavelength of light is the distance between two consecutive crests or troughs of a wave. It is measured in meters (m). The wavelength of light is inversely proportional to its frequency. This means that light with a higher frequency has a shorter wavelength, and light with a lower frequency has a longer wavelength.
The direction of propagation of light is the path that light travels. When light travels from a vacuum into a medium, its direction of propagation changes. This is because the light waves are refracted, or bent, when they enter the medium. The amount of refraction depends on the angle of incidence, which is the angle between the incoming light ray and the normal, which is an imaginary line perpendicular to the surface of the medium.
In conclusion, the frequency of light remains unchanged when light travels from a vacuum into water. The speed, wavelength, and direction of propagation of light all change when light travels from one medium to another.