Specular vs diffuse reflection

Elowen can see her face in the bathroom mirror. She holds up a sheet of printer paper: same light, same room, no face at all. Both surfaces reflect light, so what is the paper doing differently?

Every surface you can see reflects some light; that is why you can see it. This lesson sorts reflections into two kinds by one feature of the surface: how smooth it is at the small scale.

Here is a strip of aluminum foil, smooth from the roll, with four parallel laser beams aimed at it.

The smooth foil is flat at every strike point.

Smooth foil: every strike point faces the same way, so the reflected beams leave together.

So the four reflected beams leave together, in one direction, and the foil shows a clear image.

Now Elowen crumples the same foil and presses it onto the table.

The crumpled foil is rough at the small scale: each tiny facet faces its own way.

Each beam still obeys the law of reflection at its own strike point, but each strike point's normal points its own way.

Crumpled foil: each facet obeys the law at its own normal, so the beams scatter.

So the reflected beams scatter in many directions, and the image is gone.

Reflection from a smooth surface, with parallel rays leaving in one direction, is called specular reflection.

Reflection from a rough surface, with the rays scattered in many directions, is called diffuse reflection.

Paper up close: rough fibers scatter the light, so no image forms.

Elowen's mirror is smooth at the small scale, so its reflection is specular and shows her face; the paper is rough at the small scale, so its reflection is diffuse and shows no face.

The sort in one table: smooth means specular, rough means diffuse.

Why can you not see your face in a sheet of printer paper?

Paper looks flat, but at the small scale its surface is a tangle of rough fibers.

Each ray reflects by the law of reflection at its own fiber, and each fiber's normal points its own way.

So the reflected rays scatter in many directions, and scattered rays build no image.

The paper's reflection is diffuse, so it shows no face.

A calm pond shows the trees on the far bank. A breeze ruffles the water and the picture breaks up. Name the kind of reflection before the breeze, and the kind after.

Calm water is smooth, so parallel rays leave together in one direction: specular.

Ruffled water is rough at the small scale, so the rays scatter: diffuse.

Before the breeze: specular. After: diffuse.

Fintan sees the ceiling lights mirrored in a polished gym floor. Is the floor's reflection specular or diffuse?

A movie screen reflects the projector's light so that every seat in the theater sees the picture. Is the screen's reflection specular or diffuse?

A chrome bumper shows Greer a stretched picture of the street. Is the bumper's reflection specular or diffuse?

Your turn

What makes a surface reflect specularly?

Haldis shines one flashlight beam at a brick wall in a dark yard, and people standing all around the yard can see the lit patch. Which statement explains this?

Which surface gives specular reflection?