Reflection

You look into a still lake and see the mountains reflected perfectly. You look at the rough pavement next to it and see no reflection at all — even though the same sunlight is hitting both surfaces.

The difference isn’t whether light reflects (it always does, off basically everything you can see). The difference is whether the surface is smooth enough to keep the reflected rays organized into a coherent image.

That distinction — between specular reflection (mirrors, calm water) and diffuse reflection (paper, wood, your skin) — is what determines whether you see a clean image or just a dimly lit object.

The Law of Reflection

When light hits a surface, the angle at which it arrives equals the angle at which it bounces off. Both angles are measured from the normal — an imaginary line perpendicular to the surface at the point of contact.

Specular vs. Diffuse Reflection

• Specular reflection happens on smooth surfaces (mirrors, calm water, polished metal). All incoming parallel rays reflect in the same direction, preserving the image.
• Diffuse reflection happens on rough surfaces (paper, unpolished wood, concrete). The surface has microscopic bumps, so each ray hits a slightly different normal. Reflected rays scatter in all directions. You can still see the object (diffuse reflection is how you see most things — paper, walls, skin), but you don’t see a mirror image.

Both types obey the law of reflection at each individual point. The difference is just whether the surface is smooth enough at the scale of the wavelength of light for all the local normals to point in the same direction. A piece of paper looks “smooth” to your eye but has surface roughness comparable to or larger than 500 nm (visible-light wavelength), so each tiny patch reflects in a different direction → diffuse.

Plane Mirrors

A plane mirror is a flat reflective surface. It produces images with very predictable properties:

• The image is virtual (light rays don’t actually converge behind the mirror).
• The image is upright (same orientation as the object).
• The image is the same size as the object (magnification = 1).
• The image distance equals the object distance: $d_i = d_o$ (behind the mirror).
• The image is laterally inverted (left and right are swapped — which is why a written word in the mirror reads “backward”).

Why Virtual?

A real image forms where light rays actually converge — you could place a screen there and see the image projected onto it. A virtual image forms where light rays only appear to come from when your brain traces them backward.

In a plane mirror, reflected rays diverge (spread apart) after bouncing off the surface. Your eye traces them back along straight lines to a point behind the mirror. No light actually passes through that point. The image is virtual.

This distinction between real and virtual becomes critical in the mirror and lens sections coming up. Get it locked in now and the rest of the chapter will be much easier.