Physics

The key insight is that the normal to the mirror surface is perpendicular to the mirror itself, not to the floor. Since the reflected ray travels vertically upward and the law of reflection says the angle of incidence equals the angle of reflection (both measured from the normal), you need to find what angle the incident ray must make with the normal so that its reflection points straight up. Sketch the geometry: if the mirror tilts 15° from horizontal, its normal tilts 15° from vertical—use this to relate the incident ray's direction to the vertical reflected ray.

The key insight is that the normal to the mirror surface is perpendicular to the mirror itself, not to the floor. Since the reflected ray travels vertically upward and the law of reflection says the angle of incidence equals the angle of reflection (both measured from the normal), you need to find what angle the incident ray must make with the normal so that its reflection points straight up. Sketch the geometry: if the mirror tilts 15° from horizontal, its normal tilts 15° from vertical—use this to relate the incident ray's direction to the vertical reflected ray.