Why AMBULANCE Is Painted Backwards on the Van

So that a driver ahead, glancing in the rear-view mirror, reads it the right way round. The mirror swaps left and right, and painting the word already swapped cancels that out.

That swap is called lateral inversion, and it follows from the laws of reflection. This page covers everything in the ICSE Class 7 Physics chapter's first half: regular and diffused reflection, the two laws of reflection, the image formed by a plane mirror, and the uses of plane mirrors.

Reflection is the bouncing back of light from a surface. Whether you get an image depends entirely on how smooth that surface is.

In regular reflection, a smooth polished surface such as a plane mirror or still water sends a parallel beam away still parallel. Because the rays keep their arrangement, a clear image forms.

In diffused (irregular) reflection, a rough surface such as a wall, paper or cloth scatters the parallel beam in all directions. No image forms, but the surface becomes visible from every angle.

A still pond reflects the sky sharply; the moment a breeze ripples it the reflection breaks up, because the same water has stopped being a smooth surface.

Here is the important consequence, and it is often missed: diffused reflection is not a failure of light. It is the reason you can see a page of a book from anywhere in the room, and the reason a whitewashed wall brightens a room by spreading daylight.

Note also that no law is broken in diffused reflection. Each tiny part of the rough surface obeys the laws of reflection exactly; the parts simply face slightly different ways.

The two laws are:

1. The angle of incidence equals the angle of reflection, $\angle i=\angle r$.
2. The incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane.

The terms have to be labelled correctly in a ray diagram:

- Incident ray — the ray striking the mirror.
- Point of incidence — where it strikes.
- Normal — the line drawn at $90°$ to the mirror at that point, always shown dotted.
- Reflected ray — the ray leaving the mirror.
- Angle of incidence ($i$) — between the incident ray and the normal.
- Angle of reflection ($r$) — between the reflected ray and the normal.

Worked example. A ray strikes a plane mirror at $30°$ to the normal, so $\angle i=30°$ and therefore $\angle r=30°$. The angle between the incident and reflected rays is $30+30=60°$.

Now the boundary case worth knowing. If a ray falls normally on the mirror, then $\angle i=0°$, so $\angle r=0°$ and the ray returns straight back along its own path.

And this is the single commonest error: both angles are measured from the normal, never from the mirror surface. A ray at $30°$ to the mirror has an angle of incidence of $90-30=60°$.

A plane mirror image has five features, and questions expect all five:

- Virtual — it cannot be caught on a screen, because the rays only appear to come from behind the mirror.
- Erect — the right way up.
- Same size as the object.
- Laterally inverted — left and right are interchanged.
- As far behind the mirror as the object is in front.

That last point is a measurement you can be asked to calculate. Stand $1.5m$ in front of a mirror and your image is $1.5m$ behind it, so the distance between you and your image is

$$1.5+1.5=3m$$

Walk $0.5m$ closer and the gap becomes $1+1=2m$ — it closes at twice your own speed.

A bathroom mirror shows the same-size rule plainly: your reflection never looks larger or smaller as you approach, only nearer.

The misconception to clear up is about "virtual". A virtual image is perfectly visible — you see it with your eyes every morning. What it cannot do is fall on a screen held behind the mirror, because no light actually reaches that point.

Lateral inversion is the interchange of the left and right sides of an object in its mirror image. Raise your right hand and the image raises what looks like its left.

This is why AMBULANCE is painted reversed on the front of a van, and why a doctor's prescription held up to a mirror suddenly reads normally.

Some letters survive it. Letters symmetrical about a vertical line — A, H, I, M, O, T, U, V, W, X, Y — look unchanged in a mirror, so a word such as MOM appears identical while a word such as BOOK does not.

Two devices in the syllabus use plane mirrors:

- A periscope uses two plane mirrors fixed parallel at $45°$ to the tube, one at each end. Light from an object enters the top mirror, turns through $90°$, travels down the tube, meets the second mirror and turns through $90°$ again into the eye — letting you see over a wall or a crowd.
- A kaleidoscope uses three plane mirrors in a triangular tube, with coloured pieces at one end. Repeated reflections between the mirrors produce many symmetrical images, and shaking the tube rearranges the pattern.

The periscope has a neat consequence: because the light is reflected twice, the second inversion undoes the first, so the final image is not laterally inverted.