Final answer:
The property of light that remains unchanged as light passes through a boundary from air into water is its frequency. While wavelength and speed can change due to the medium's refractive index, frequency is intrinsic to the source and remains constant. The correct option is C) Frequency.
Step-by-step explanation:
When a beam of light passes from air into water, there are a number of properties related to the light wave that can change, such as its speed and wavelength. However, the property of light that stays the same as it crosses the boundary is the frequency of the light. This is because frequency is determined by the source of light and remains unaffected by the medium through which the light travels.
Light is an electromagnetic wave and is characterized by its wavelength (λ), frequency (ν), and amplitude. The relationship between these properties is given by the equation c = λν, where c is the speed of light in a vacuum. This relationship implies that if the speed of light changes as it enters a different medium, the wavelength will also change to maintain the constant value of c when the frequency remains constant.
As light transitions from a less dense medium like air to a more dense medium like water, its speed decreases due to the higher refractive index of water. However, as per the equation v= c/n, where n is the refractive index of the medium, only the wavelength of the light wave is altered. The frequency, which is the number of wave cycles per second, does not change because it is intrinsic to the wave's source.
The correct option here is therefore C) Frequency. The frequency of the light wave remains unchanged as the light crosses the boundary from air to water.