Every Surface Reflects Light, but Only Smooth Ones Do It Neatly

Both surfaces reflect light. The wall reflects just as obediently as the mirror — that is the only reason you can see the wall at all.

The difference is where the reflected rays go. A mirror is smooth, so parallel rays arriving together leave together, still parallel, and your eye can reassemble them into a face. A wall is rough at the scale of light, so each tiny patch faces a slightly different way and scatters its rays in every direction. Nothing is left to reassemble.

So no image does not mean no reflection. This page covers the first part of the ICSE Class 8 Physics chapter on light: light as energy, the laws that fix a reflected ray, the two kinds of reflection, and the image in a plane mirror.

Light is a form of energy that travels as a wave and needs no material medium — which is why sunlight crosses empty space to reach us, while sound cannot.

Its speed in air, and in vacuum, is

$$c=3\times 10^8\text{m/s}$$

This is the fastest speed anything can have, and it is worth getting a feel for. Worked example — how long does light take to travel $300\text{km}$?

$$d=300\text{km}=3\times 10^5\text{m}$$

$$t=\frac{d}{c}=\frac{3\times 10^5}{3\times 10^8}=10^{-3}\text{s}=1\text{millisecond}$$

So light covers the distance between two distant cities in about a thousandth of a second. This is why a lamp being switched on seems instantaneous.

Light slows in a denser medium. It moves fastest in vacuum and air, more slowly in water, and more slowly still in glass. In glass its speed drops to about

$$v=\frac{3\times 10^8}{1.5}=2\times 10^8\text{m/s}$$

because glass is around $1.5$ times optically denser than air.

That slowing has a visible consequence. A ray crossing from air into glass changes direction, which is why a straw in a glass of water looks bent at the surface. Light travelling straight inside each medium but changing speed at the boundary is all that is needed to produce the effect.

Light travels in straight lines within a single medium. That is why shadows have sharp edges and why you cannot see round a corner.

In regular reflection a parallel beam stays parallel; in diffuse reflection it is scattered in all directions. Both obey the laws of reflection at every point.

Regular (specular) reflection happens at a smooth, polished surface — a plane mirror, still water, a polished steel plate. Every point of the surface faces the same way, so every normal points the same way, so all the reflected rays turn through the same angle. A clear image results, and the surface looks shiny.

Diffuse (irregular) reflection happens at a rough surface — a wall, paper, cloth, wood, a road. The surface is made of countless tiny patches tilted at different angles, so the normals point in many directions. Each ray still obeys $\angle i=\angle r$ at its own patch, but the patches disagree, so the beam is scattered. No image forms, and the surface looks matt.

Why non-luminous objects are visible at all. A luminous body makes its own light — the Sun, a flame, a bulb. A non-luminous body does not — a book, a wall, a chair, the Moon. We see non-luminous objects only because they scatter light falling on them into our eyes.

That scattering is diffuse reflection. So diffuse reflection is not a defect; it is the reason the world is visible from every angle instead of only from one special direction.

And the everyday proof. A blackboard is deliberately made matt so every student in the room receives some scattered light from it. Polish it to a mirror finish and only one student, sitting in exactly the right place, would see the writing — everyone else would see a reflection of the window.

A plane mirror gives an image that is virtual, erect, the same size as the object, as far behind the mirror as the object is in front, and laterally inverted.

Taking those one at a time:

- Virtual — it cannot be caught on a screen, because the reflected rays only appear to come from behind the mirror; they never actually go there.
- Erect — the right way up, never upside down.
- Same size — the magnification is exactly $1$, whatever the distance.
- Equally distant — object distance equals image distance.
- Laterally inverted — left and right are interchanged.

Worked example. Stand $2\text{m}$ in front of a plane mirror. The image forms $2\text{m}$ behind it, so the distance between you and your image is

$$2+2=4\text{m}$$

Now walk $0.5\text{m}$ towards the mirror. Your distance is $1.5\text{m}$, the image is $1.5\text{m}$ behind, and the separation becomes

$$1.5+1.5=3\text{m}$$

So the separation fell by $1\text{m}$ while you moved only $0.5\text{m}$ — the image approached you at the same time, and the gap closes at twice your speed.

Lateral inversion in practice. Raise your right hand and the image raises what looks like its left. The word AMBULANCE is painted reversed on the front of the vehicle so that a driver looking in the rear-view mirror reads it the right way round.

What lateral inversion is not. It is not a top-to-bottom flip. The image is never upside down — only left and right are exchanged, which is a swap along the direction facing the mirror. This is why letters like A, H, I, M, O, T, U, V, W, X and Y look unchanged in a mirror: each is symmetrical about a vertical line, so swapping left and right leaves it as it was.