Critical Angle and Refraction at 90° Steps: - Light refracting along the surface means the angle of refraction is 90° to the normal. - This occurs only when light travels from higher to lower refractive index, with incident angle at the critical value. - The incident medium must be X (higher n), boundary to W (lower n), so n_X > n_W. - Angle of incidence equals angle of reflection (both z), making z the critical angle; y is the 90° refraction angle, so y > z. Why C is correct: - By Snell's law at critical angle, sin z = n_W / n_X < 1, with refraction at 90° (y > z), confirming incidence from denser X to rarer W. Why the others are wrong: - A: If n_W > n_X, incidence would be from rarer W to denser X, impossible for 90° refraction (requires sin i > 1). - B: n_W > n_X implies wrong direction for critical angle; z as critical doesn't fit rarer-to-denser path. - D: n_X > n_W is right, but y = 90° isn't the …