When light travels from one medium to another, which property of the wave REMAINS UNCHANGED?

1. syllabus explains the boundary conditions for refraction. 2. FREQUENCY is a property of the SOURCE. When light crosses into a new medium, the atoms at the boundary oscillate at the same rate as the incident wave drives them — so frequency stays the same. 3. The SPEED changes (slower in denser media). Since $v = f\lambda$ and $f$ is fixed, the WAVELENGTH also changes inversely with the speed. 4. In a medium of refractive index $n$: $v_\text{med} = c/n$, $\lambda_\text{med} = \lambda_\text{vac}/n$, $f$ unchanged. 5. Option A is wrong (wavelength changes). Option B is wrong (speed changes). Option D is a property of the medium, not the wave. _Source: syllabus Physics Part 2, Ch 9, §9.3 (Refraction — frequency, wavelength, speed), p. 2._