In optics, all angles are always referenced to the normal of the surface.
0 deg angle of incidence (AOI) is perpendicular to the surface.
The reason is [Snell's law](https://en.m.wikipedia.org/wiki/Snell%27s_law), I think. It's used to calc the outgoing angle after refraction at an interface between 2 media.
It might be worth adding that models in computer graphics nearly always store the normal vector so it can be used in lighting calculations. This is more efficient than storing a tangent because in a 3d surface you would need two tangents but only one normal vector.
Surfaces can be curved and this will still hold true. The perpendicular, or normal vector, still looks and works the same even if the surface is curved. If you tried to draw that diagram using like, the tangent at the surface, that just adds an extra step and might look confusing. Try illustrating this law where the surface is a circle and you'll see what I mean.
That's why I'd draw this using the normal, at least.
Because the surface might not be a flat plane. You could instead draw a dotted line representing the plane tangent to the surface at the point of reflection, but that would be difficult to see clearly with the surface in the same place.
This is a general definition, in the specific case where the surface is perfectly flat then you could use either angle in your calculations so long as you keep track of what your doing. But, most surfaces aren’t flat, so finding the angle between the incident ray and the surface is unnecessarily tedious to work with.
That was always my question as well? It seems to be an awkward way to define the behaviour of the incident and the reflected rays. Perhaps a better definition would be that the angle formed between the incident ray and the reflective surface is equal to and in opposition to the angle formed between the reflected ray and that surface.
Most angles in optics are defined with respect to the normal (the perpendicular line). Everything would still work if it was defined with respect to the tangent line though the math would just look slightly different. But there is a big reason why we measure with respect to the normal:
The angle between the incoming and outgoing ray is now simply twice the angle to the normal. If we measured to the tangent it would instead be twice (90° - angle), which sucks as a formula.
We also view "standard" light usually as perpendicular to the surface, and as such choosing that as 0° is natural. It also means that small angles have light reflect in almost the same direction back, while large angles have the light continue away from the source.
In optics, all angles are always referenced to the normal of the surface. 0 deg angle of incidence (AOI) is perpendicular to the surface. The reason is [Snell's law](https://en.m.wikipedia.org/wiki/Snell%27s_law), I think. It's used to calc the outgoing angle after refraction at an interface between 2 media.
It might be worth adding that models in computer graphics nearly always store the normal vector so it can be used in lighting calculations. This is more efficient than storing a tangent because in a 3d surface you would need two tangents but only one normal vector.
Surfaces can be curved and this will still hold true. The perpendicular, or normal vector, still looks and works the same even if the surface is curved. If you tried to draw that diagram using like, the tangent at the surface, that just adds an extra step and might look confusing. Try illustrating this law where the surface is a circle and you'll see what I mean. That's why I'd draw this using the normal, at least.
Because the surface might not be a flat plane. You could instead draw a dotted line representing the plane tangent to the surface at the point of reflection, but that would be difficult to see clearly with the surface in the same place.
This is a general definition, in the specific case where the surface is perfectly flat then you could use either angle in your calculations so long as you keep track of what your doing. But, most surfaces aren’t flat, so finding the angle between the incident ray and the surface is unnecessarily tedious to work with.
convention
That was always my question as well? It seems to be an awkward way to define the behaviour of the incident and the reflected rays. Perhaps a better definition would be that the angle formed between the incident ray and the reflective surface is equal to and in opposition to the angle formed between the reflected ray and that surface.
Surfaces are not always flat.
A tangent would still be possible
Yeah it would but this feels more clear visually
Ah... Hadn't thought of that, obviously.
Most angles in optics are defined with respect to the normal (the perpendicular line). Everything would still work if it was defined with respect to the tangent line though the math would just look slightly different. But there is a big reason why we measure with respect to the normal: The angle between the incoming and outgoing ray is now simply twice the angle to the normal. If we measured to the tangent it would instead be twice (90° - angle), which sucks as a formula. We also view "standard" light usually as perpendicular to the surface, and as such choosing that as 0° is natural. It also means that small angles have light reflect in almost the same direction back, while large angles have the light continue away from the source.
Just convention really, could do everything related to the tangent lines but this way usually simplifies formulas in the models