PhysicsICSE Class 9 · Reflection of Light by Plane Mirrors

Lateral Inversion and Multiple Images in Two Parallel Mirrors: Positions and Reasoning (ICSE Class 9)

Updated 17 October 2026 · Fact-checked

Lateral inversion means the left side of an object appears as the right side of its image in a plane mirror. Between two parallel mirrors d apart, an object x from mirror M₁ gives images behind M₁ at x, 2d − x, 2d + x, 4d − x, … and an infinite series in principle; they get fainter because each reflection absorbs some light.

Understand Lateral Inversion and Multiple Images in Two Parallel Mirrors

Lateral inversion. In a plane mirror the image is as far behind the mirror as the object is in front, and it is reversed left to right while remaining upright. Your right hand looks like the image’s left hand; printed words look back to front.

Why? The mirror reverses the *front-to-back* direction (the part nearest the mirror stays nearest). To compare your image with yourself you imagine turning it round, and that turning swaps left and right.

Effects of lateral inversion.

*AMBULANCE* is painted in reverse on the front of the vehicle so that a driver ahead reads it correctly in the rear-view mirror.
Letters with a vertical line of symmetry (A, H, I, M, O, T, U, V, W, X, Y) look unchanged; a word made only of such letters whose reverse is the same word (such as TOOT or MOM) looks unchanged.
Clock in a mirror. For an analogue clock the mirror shows the time $t$ such that actual time $= 11 : 60 - t$ (for example a mirror time 4:20 means 7:40).

Two parallel mirrors. An object between two parallel mirrors is reflected repeatedly: the image in one mirror is an object for the other, and so on. In theory an infinite number of images form along a straight line perpendicular to the mirrors. They become fainter because every reflection absorbs a little light.

Positions of images. Let the mirrors $M_{1}$ and $M_{2}$ be $d$ apart and the object $x$ from $M_{1}$ (so $d - x$ from $M_{2}$ ).

Behind $M_{1}$ (distances from $M_{1}$ ): $x, 2 d - x, 2 d + x, 4 d - x, 4 d + x, \dots$
Behind $M_{2}$ (distances from $M_{2}$ ): $(d - x), 2 d - (d - x), 2 d + (d - x), \dots$ (swap the roles of the mirrors).

Orientation of successive images. Each reflection flips left and right. The first image (one reflection) is laterally inverted; the second image (two reflections) is not laterally inverted; the third is inverted again, and so on.

Other uses. In shops and lifts, parallel mirrors create a sense of space; the many reflections in a barber shop show the back of your head.

Lateral Inversion and Multiple Images in Two Parallel Mirrors: formulas and key facts

Clock in a mirror: $actual time = 11 : 60 - mirror time$; Analogue clock facing a mirror.
Images behind M₁: $x, 2 d - x, 2 d + x, 4 d - x, \dots$; x from M₁; mirrors d apart.
Image distance: $v = u$; Each single reflection.

How to solve Lateral Inversion and Multiple Images in Two Parallel Mirrors questions

Work reflection by reflection.

1For parallel mirrors list distances from the mirror that forms each image.
2Use $x$ , $2 d - x$ , $2 d + x$ , $4 d - x$ for the mirror nearest the object’s first reflection.
3For the other side swap $x$ with $d - x$ .
4Count reflections to decide whether an image is laterally inverted (odd number: inverted; even: not).
5For clocks subtract the mirror time from 11:60.

Lateral Inversion and Multiple Images in Two Parallel Mirrors in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Reading a clock in a mirror

Suppose you read the clock on the wall behind you in a mirror and it appears to show 8:25.

What is the actual time?

Try it first, then show the solutionSolution

Actual time $= 11 : 60 - 8 : 25 = 3 : 35$ .

Answer: 3:35.

Memory hook: Subtract from 11:60.

Tricks to remember Lateral Inversion and Multiple Images in Two Parallel Mirrors

Short memory aids. Say each one aloud once, then use it on a question the same day.

One flip per mirror
Odd number of reflections: laterally inverted. Even: same as the object.
Use it when: Orientation in multiple images.
Eleven-sixty
Mirror clock: 11:60 minus mirror time.
Use it when: Clock questions.
x, 2d − x, 2d + x
Images alternate: minus, plus, minus, plus, adding 2d every two images.
Use it when: Parallel mirrors.

Lateral Inversion and Multiple Images in Two Parallel Mirrors: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Which word looks unchanged? · 1 mark · MCQBoard level

Which of the following words looks the same in a plane mirror (vertical mirror)?
(a) BOOK
(b) TOOT
(c) MATH
(d) CODE

Try it first, then show the solutionSolution

T and O are symmetric about a vertical axis, and TOOT reads the same reversed. In the other words the reversed order or the shapes change.

Answer: (b) TOOT

Example 2: Nature of the image · 2 marks · Short answerBoard level

State the nature of the image formed by a plane mirror and explain why a student cannot get it on a screen.

Try it first, then show the solutionSolution

Virtual, erect, same size as the object, laterally inverted, as far behind the mirror as the object is in front. (1)
The reflected rays do not actually meet; they only appear to come from the image, so no screen can receive it. (1)

Answer: Virtual, erect, same size, laterally inverted

Example 3: Parallel mirrors 40 cm apart · 3 marks · Short answerHarder board level

Two parallel plane mirrors $M_{1}$ and $M_{2}$ are 40 cm apart. A candle is 15 cm from $M_{1}$ . Find the distances (from the mirror in which they appear) of the first three images behind $M_{1}$ and the first two behind $M_{2}$ .

Try it first, then show the solutionSolution

Behind $M_{1}$ : $x = 15$ , $2 d - x = 80 - 15 = 65$ , $2 d + x = 95$ cm. (1½)
Candle is $40 - 15 = 25$ cm from $M_{2}$ . Behind $M_{2}$ : $25$ cm and $2 d - 25 = 55$ cm. (1½)

Answer: Behind M₁: 15, 65, 95 cm. Behind M₂: 25, 55 cm

Example 4: Shifting the object · 3 marks · Short answerHarder board level

In the previous arrangement (d = 40 cm), the candle is moved 5 cm towards $M_{1}$ . Find the new distances of the first three images behind $M_{1}$ and describe how each image moved.

Try it first, then show the solutionSolution

New $x = 10$ cm. (½)
Images behind $M_{1}$ : $10$ , $2 d - 10 = 70$ , $2 d + 10 = 90$ cm. (1)
First image: 5 cm nearer to $M_{1}$ ; second image: 5 cm farther (65 to 70); third: 5 cm nearer (95 to 90). (1½)

Answer: 10, 70, 90 cm; the images alternately move nearer and farther

Example 5: Barber shop · 4 marks · Case-basedHOTS / competency

In a barber shop two parallel mirrors on opposite walls are 3 m apart. A customer sits 1 m from the front mirror.
(a) Find the distances from the front mirror of the first three images behind it.
(b) Which of these three are laterally inverted?
(c) Why can only a few images be seen though the series is endless in theory?
(d) Is the customer’s second image laterally inverted? Explain.

Try it first, then show the solutionSolution

(a) $x = 1$ , $2 d - x = 5$ , $2 d + x = 7$ m. (1)
(b) The image at 1 m has one reflection (inverted); the image at 5 m has two (not inverted); the image at 7 m has three (inverted). (1)
(c) Each reflection absorbs a little light, so later images are too faint to see. (1)
(d) No: the second image involves two reflections, and each flips left and right, so it is flipped back. (1)

Answer: (a) 1, 5, 7 m (b) 1 m and 7 m inverted, 5 m not (c) light lost at every reflection (d) not inverted

Where students lose marks in Lateral Inversion and Multiple Images in Two Parallel Mirrors

The slips examiners see most often, and what to write instead.

Mistake: Hours and minutes reversed separately
The whole time must be subtracted from 11:60.
Wrong working
Mirror shows 4:20, so the actual time is 8:20 (12 − 4 and the same minutes).
Right working
11:60 − 4:20 = 7:40.
Do this: Subtract the full time, with borrowing.
Usually costs: The answer.
Mistake: Image distances measured from the wrong mirror
List the distances from the mirror in which the images are formed.
Wrong working
All distances measured from the object.
Right working
Distances x, 2d − x, 2d + x from M₁.
Do this: Name the mirror in each answer.
Usually costs: The answer.
Mistake: All images called laterally inverted
Each reflection flips left and right.
Wrong working
Every image is laterally inverted.
Right working
Odd number of reflections: inverted; even: not.
Do this: Count reflections.
Usually costs: The reasoning mark.

Exam tips

For lateral inversion give an example (AMBULANCE) and the reason (rear-view mirror).
State the positions of images as a list with units (cm from M₁).
Say "virtual" for all plane mirror images.
Use 11:60 minus the mirror time for clock questions.

Practice questions on Lateral Inversion and Multiple Images in Two Parallel Mirrors

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Your right hand appears in a plane mirror as the image’s:
(a) right hand
(b) left hand
(c) upper hand
(d) lower hand
Hint
Lateral inversion.
Answer
(b) left hand
Q2 · 1 mark · Very short answerBoard level
How many images are formed, in theory, by two parallel plane mirrors facing each other?
Hint
Repeated reflections.
Answer
An infinite number.
Q3 · 2 marks · Short answerHarder board level
A clock facing a plane mirror seems to show 8:25. What is the actual time?
Hint
11:60 − 8:25.
Answer
$11 : 60 - 8 : 25 = 3 : 35$ .
Q4 · 3 marks · Short answerHarder board level
Two parallel mirrors are 60 cm apart and an object lies midway between them. Find the distances of the first four images from the nearer mirror $M_{1}$ .
Hint
x = 30, d = 60.
Answer
$x = 30$ ; $2 d - x = 90$ ; $2 d + x = 150$ ; $4 d - x = 210$ cm.
Q5 · 2 marks · Assertion-reasonHarder board level
Assertion (A): Of the many images formed by two parallel mirrors, only a few can be seen.
Reason (R): Some light is absorbed at each reflection, so the images become fainter.
(a) Both true, R explains A.
(b) Both true, R does not explain A.
(c) A true, R false.
(d) A false, R true.
Hint
Light loss.
Answer
(a)
Q6 · 3 marks · Long answerHOTS / competency
A student says: "A mirror interchanges left and right but not up and down." Explain, with a reason, why the image is nevertheless not inverted vertically and why we describe the effect as lateral inversion.
Hint
Think of what the mirror reverses.
Answer
The image of each point lies on the perpendicular from the point to the mirror, so only the front-to-back direction is reversed; up and down are not changed. Comparing the image with the object by turning it round about a vertical axis exchanges left and right, so the image appears laterally inverted (left-right reversed) but upright.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.What is lateral inversion?Show answer
Interchange of the left and right sides of an object in its image.
2.Positions of images behind M₁?Show answer
x, 2d − x, 2d + x, 4d − x, …
3.Why are later images fainter?Show answer
Light is absorbed at each reflection.
4.Clock in a mirror?Show answer
Actual time = 11:60 − mirror time.
5.Is the second image laterally inverted?Show answer
No.

Lateral Inversion and Multiple Images in Two Parallel Mirrors: frequently asked questions

What is lateral inversion?

In a plane mirror the left side of an object appears as the right side of its image and vice versa, while the image stays upright.

Why are only a few of the images in two parallel mirrors visible?

In theory the images are endless along a line perpendicular to the mirrors, but each reflection absorbs some light, so later images are too faint to see.

Why is AMBULANCE written backwards?

So that drivers ahead, looking in their rear-view mirrors, see the word correctly because of lateral inversion.