Final answer:
The speed of a light wave will decrease when passing from helium (lower refractive index) to ethanol (higher refractive index), because the light wave travels slower in mediums with higher refractive indices. The frequency remains unchanged, while the wavelength decreases.
Step-by-step explanation:
When considering how the speed of a light wave changes as it passes from one medium to another, such as from helium to ethanol, we need to understand the concept of the refractive index. The refractive index of a medium determines how much it slows down light waves traveling through it. Helium has a lower refractive index compared to ethanol, meaning that light waves travel faster in helium and slower in ethanol. Therefore, as a light wave passes from helium to ethanol, its speed will decrease.
Sine the frequency of a photon remains the same when passing between media (option b), and considering that the speed of light in a vacuum is a constant 3.00×108 m/s, the change in speed observed is purely due to the differing refractive indices of the media. The wavelength of light will also change accordingly; as it enters a denser medium with a higher refractive index (ethanol), the wavelength of the light wave decreases (option d).
The concept that the speed of propagation is constant in a given medium (option d), and only the wavelength changes as the frequency remains constant, is fundamental. Thus, the answer to the student's question is that the speed of the light wave will decrease as it passes from helium to ethanol.