I read that a (sufficiently) white surface reflects more (visible) light than a mirror. Is that true? And if yes, why?
Have you ever wondered why things show colour? Well when light is incident on a surface, light of certain wavelengths are absorbed and we see the complement colours.For example a green surface absorbs light of all colours other than green. But a white surface is a surface that reflects out light of all wavelengths unlike a black surface which absorbs all light.
Now imagine shining a green laser at a mirror . What we get is a reflected green beam.if we repeat this experiment for all other 6 colours we will get similar results.So we can conclude that a mirror reflects light of all colours just like a white surface .
So what makes a mirror different from a white surface?
A mirror possesses specularity i.e parallel rays are reflected parallel to each other thus maintaining an image which does not happen for all white surfaces. A surface which is not a mirror does not reflect the parallel rays in a specular fashion so the reflected rays mix into one homogeneous white light without producing image.
So basically if I take a white surface and give it the property of specularity I get a mirror.
Typically how much light is reflected depends on the surface.For example if you take a sufficiently white surface it will reflect 100 % light but if you compare it with a household mirror that reflects only 80% light then the white surface would reflect more than the mirror.