This is mainly a reflection problem, not Snell’s law/refraction. For a whiteboard’s shiny surface, use the law of reflection: the angle of incidence equals the angle of reflection.

That means the bright spot appears where light from a source bounces off the board into the camera. If you move the camera and the hotspot moves, that’s consistent with a specular reflection from the board.

A practical way to find the source is:

• Turn off unnecessary lights one by one.
• Wave your hand/arm around between the board and possible light sources, including near the camera position. When the glare disappears, you’ve blocked the offending path.
• Think of it like a billiard-bank shot: from the camera’s viewpoint, where would a ball “bounce” off the board? That points toward the light source.

To reduce glare:

• Reposition the light so it reflects away from the camera.
• Side-light the whiteboard instead of lighting it from near the camera axis.
• Use only the lights you need; extra room lights often create hotspots.

There is reflection across the whole board, but the camera only sees the strongest one at the matching angle.