Why AMBULANCE Is Written Backwards on the Van
Learn how a pinhole camera flips an image, what reflection does to light, the four rules for a plane mirror image, and how periscopes and kaleidoscopes work.
Why is AMBULANCE written backwards on the front of the van?
So that a driver looking in their rear-view mirror sees it the right way round. A plane mirror swaps left and right — an effect called lateral inversion — so a word printed in reverse appears correct in the reflection.
This page covers everything in the CBSE Class 7 Science chapter's second half: how a pinhole camera forms an image, what reflection is, the characteristics of a plane mirror image, and the devices built from mirrors.
This page covers everything in the CBSE Class 7 Science chapter's second half: how a pinhole camera forms an image, what reflection is, the characteristics of a plane mirror image, and the devices built from mirrors.
How does a pinhole camera work?
A pinhole camera is a closed box with a tiny hole on one side and a translucent screen, such as tracing paper, on the opposite side.
Because light travels in straight lines, rays from the top of the object pass through the hole and land at the bottom of the screen, and rays from the bottom land at the top. The image is therefore inverted — upside down.
Its characteristics are:
- inverted (upside down)
- coloured, reproducing the object's real colours
- smaller or larger depending on distance — moving the camera closer to the object gives a bigger image, moving it away gives a smaller one
You can make one from a shoebox and view a bright object such as a tree against the sky.
The hole must be small. Make it too large and many overlapping rays from each point reach the screen, so the picture turns into a bright blur rather than an image.
Because light travels in straight lines, rays from the top of the object pass through the hole and land at the bottom of the screen, and rays from the bottom land at the top. The image is therefore inverted — upside down.
Its characteristics are:
- inverted (upside down)
- coloured, reproducing the object's real colours
- smaller or larger depending on distance — moving the camera closer to the object gives a bigger image, moving it away gives a smaller one
You can make one from a shoebox and view a bright object such as a tree against the sky.
The hole must be small. Make it too large and many overlapping rays from each point reach the screen, so the picture turns into a bright blur rather than an image.
What is reflection?
Reflection is the change in the direction of light when it falls on a surface and bounces back.
Smooth, shiny surfaces reflect light in a regular way and form clear images: a plane mirror, polished metal, still water. Rough surfaces scatter light in all directions, which is why you see a wall but no image in it.
This is also why the still water of a pond shows a clear reflection of the trees above it, while the same pond shows nothing recognisable once the surface is rippled by wind — the ripples scatter the light.
For example, a steel plate straight from the polishing wheel reflects your face, while the same steel scratched and dulled does not.
So a mirror is not special in its material but in its smoothness. That is exactly what a mirror is: glass with a very smooth reflecting coating behind it.
Smooth, shiny surfaces reflect light in a regular way and form clear images: a plane mirror, polished metal, still water. Rough surfaces scatter light in all directions, which is why you see a wall but no image in it.
This is also why the still water of a pond shows a clear reflection of the trees above it, while the same pond shows nothing recognisable once the surface is rippled by wind — the ripples scatter the light.
For example, a steel plate straight from the polishing wheel reflects your face, while the same steel scratched and dulled does not.
So a mirror is not special in its material but in its smoothness. That is exactly what a mirror is: glass with a very smooth reflecting coating behind it.
What are the characteristics of an image in a plane mirror?
A plane mirror image always has four properties, and questions ask for all four.
It is erect — upright, the same way up as you are.
It is the same size as the object.
It is formed as far behind the mirror as the object is in front of it. Stand 1 m from a mirror and your image appears 1 m behind it, so the image seems 2 m away from you.
It is laterally inverted — left and right are swapped. Raise your right hand and the image raises what appears to be its left.
That last property explains AMBULANCE, and also why the word MIRROR held up to a mirror reads oddly while a word like MUM looks unchanged — letters symmetrical about a vertical line survive the swap.
The image is also virtual: it cannot be caught on a screen, because the light only appears to come from behind the mirror. Nothing is actually there.
It is erect — upright, the same way up as you are.
It is the same size as the object.
It is formed as far behind the mirror as the object is in front of it. Stand 1 m from a mirror and your image appears 1 m behind it, so the image seems 2 m away from you.
It is laterally inverted — left and right are swapped. Raise your right hand and the image raises what appears to be its left.
That last property explains AMBULANCE, and also why the word MIRROR held up to a mirror reads oddly while a word like MUM looks unchanged — letters symmetrical about a vertical line survive the swap.
The image is also virtual: it cannot be caught on a screen, because the light only appears to come from behind the mirror. Nothing is actually there.
How do a periscope and a kaleidoscope use mirrors?
Both are built from plane mirrors arranged at carefully chosen angles.
A periscope uses two plane mirrors, fixed parallel to each other at 45° inside a long tube. Light from the object strikes the upper mirror, is reflected down the tube, strikes the lower mirror, and is reflected into your eye. This lets you see over a wall or a crowd, or from inside a submarine — you see things that are not in your straight line of sight.
A kaleidoscope uses three plane mirrors joined to form a triangular tube, with small coloured pieces of glass or bangle at one end. Each mirror reflects the others' reflections many times over, producing beautiful symmetrical patterns that change as you rotate it.
So the periscope is about changing direction — bending your line of sight around an obstacle — while the kaleidoscope is about multiple reflections creating repeated patterns.
In both cases light still travels only in straight lines; the mirrors simply redirect it at each bounce.
A periscope uses two plane mirrors, fixed parallel to each other at 45° inside a long tube. Light from the object strikes the upper mirror, is reflected down the tube, strikes the lower mirror, and is reflected into your eye. This lets you see over a wall or a crowd, or from inside a submarine — you see things that are not in your straight line of sight.
A kaleidoscope uses three plane mirrors joined to form a triangular tube, with small coloured pieces of glass or bangle at one end. Each mirror reflects the others' reflections many times over, producing beautiful symmetrical patterns that change as you rotate it.
So the periscope is about changing direction — bending your line of sight around an obstacle — while the kaleidoscope is about multiple reflections creating repeated patterns.
In both cases light still travels only in straight lines; the mirrors simply redirect it at each bounce.
Exam tip
Exam tip: giving all four properties of a mirror image
Asked to state the characteristics of an image formed by a plane mirror, most students name two and lose half the marks.
Memorise the set of four: erect, same size, as far behind as the object is in front, laterally inverted — and add virtual where the question asks about the nature of the image.
For lateral inversion questions, do not just define it. Give an example that shows it: AMBULANCE on a van, or raising your right hand and the image appearing to raise its left.
And watch the distance wording. The image is as far behind the mirror as the object is in front of it — so an object 1 m away has an image 1 m behind the mirror, which is 2 m from the object itself. Confusing those two numbers is a frequent slip.
Memorise the set of four: erect, same size, as far behind as the object is in front, laterally inverted — and add virtual where the question asks about the nature of the image.
For lateral inversion questions, do not just define it. Give an example that shows it: AMBULANCE on a van, or raising your right hand and the image appearing to raise its left.
And watch the distance wording. The image is as far behind the mirror as the object is in front of it — so an object 1 m away has an image 1 m behind the mirror, which is 2 m from the object itself. Confusing those two numbers is a frequent slip.
Did you know
Why can't a mirror image be caught on a screen?
The light never actually travels to the place the image appears to be. It bounces off the mirror's surface and comes towards your eye, and your brain traces those rays straight back to a point behind the glass.
Since no light truly reaches that point, putting a screen there catches nothing. That is what makes the image virtual, as opposed to the real image a pinhole camera casts onto its screen.
Since no light truly reaches that point, putting a screen there catches nothing. That is what makes the image virtual, as opposed to the real image a pinhole camera casts onto its screen.
Key takeaways
Reflection and mirrors: quick revision
- A pinhole camera forms an inverted, coloured image whose size changes with distance, because light travels in straight lines through the small hole.
- Reflection is light changing direction on a surface; smooth surfaces give clear images, rough ones scatter light.
- A plane mirror image is erect, the same size, as far behind the mirror as the object is in front, laterally inverted, and virtual.
- Lateral inversion swaps left and right, which is why AMBULANCE is printed reversed to read correctly in a rear-view mirror.
- A periscope uses two mirrors at 45° to see past obstacles; a kaleidoscope uses three mirrors and repeated reflections to make symmetrical patterns.
You will remember all of this far better after answering five questions on it than after reading it twice.
- Reflection is light changing direction on a surface; smooth surfaces give clear images, rough ones scatter light.
- A plane mirror image is erect, the same size, as far behind the mirror as the object is in front, laterally inverted, and virtual.
- Lateral inversion swaps left and right, which is why AMBULANCE is printed reversed to read correctly in a rear-view mirror.
- A periscope uses two mirrors at 45° to see past obstacles; a kaleidoscope uses three mirrors and repeated reflections to make symmetrical patterns.
You will remember all of this far better after answering five questions on it than after reading it twice.