Refraction

The index of refraction is $n = c/v$, so a higher index means slower light, and Snell's law $n_1\sin\theta_1 = n_2\sin\theta_2$ pairs each index with the angle in its own medium. Entering a slower medium the ray bends toward the normal and ends at a smaller angle; entering a faster one it bends away; arriving along the normal it does not bend at all while still changing speed. Frequency is fixed by the source and does not change at a boundary, so the wavelength shrinks by the same factor as the speed. Total internal reflection requires travel from high index to low, with $\sin\theta_c = n_2/n_1$.

Five errors dominate. Crossing the pairs in Snell's law so each index multiplies the angle in the other medium, which bends the ray the wrong way with a plausible-looking number. Choosing the direction of the bend from a feeling rather than from which medium is slower, and missing that a ray along the normal does not bend. Letting the frequency fall along with the speed, which would recolour everything seen underwater. Computing a critical angle for a trip from low index to high, where none exists and the equation returns a sine above one. And explaining dispersion by energy rather than by the index, which works in glass by accident and fails everywhere else.