Why cardboard mean the failure of law of reflection?

Cardboard does not necessarily mean the failure of the law of reflection. The law of reflection states that when light rays hit a smooth surface, they reflect off at the same angle at which they hit the surface. This law holds true for most smooth surfaces, including mirrors and glass.

However, cardboard is not a smooth surface. It is rough and uneven, which means that when light rays hit it, they scatter in different directions rather than reflecting off at a specific angle. This scattering of light is known as diffuse reflection.

So, while cardboard does not adhere to the law of reflection in the same way as smooth surfaces, it does not mean that the law of reflection fails. The law of reflection still holds true for smooth surfaces, but it does not apply to rough surfaces like cardboard.