When a ray of light passes from one medium to another, its frequency remains the same.

Option 1 states that the wavelength remains the same, but this is incorrect. When light travels from one medium to another, its wavelength can change due to the change in the refractive index of the medium. This change in wavelength is responsible for phenomena like refraction and dispersion.

Option 3 suggests that the frequency increases when light passes from one medium to another. However, this is also incorrect. The frequency of a ray of light is determined by the source that emits it and remains constant regardless of the medium it travels through.

Option 4 states that the wavelength increases when light passes from one medium to another. This is partially correct. The wavelength can either increase or decrease depending on the refractive indices of the two media involved. This change in wavelength is governed by the laws of refraction.

Therefore, the correct answer is option 2, which states that the frequency of a ray of light remains the same when it goes from one medium to another.