In case of laws of reflection , first laws says that incident ray, reflected ray and incident ray lies in the same plane , it means that all three rays lies in the same plane ( it may be xy plane, yz plane or xz plane).

For ex: The points in a triangle lie in a same plane means the points are present in the same line of the triangle.

1

Snigdha answered this

example: in physics the 1st law of light is the reflected ray,incident ray and normal lie on the same plane that means all these three lie on the same line of the surface