Transum Software

Mathematical Optical Illusions

Don't let your brain be fooled by these geometric optical illusions in this online quiz.

Instructions Midarc Trapezia Triangle Inequalities Dumbbells More

Arrow Arrow Arrow Arrow Arrow

There are five mathematical optical illusions that you can see by clicking on the tabs above.

Each illusion comes with a question which you can answer using the dropdown boxes below.

When you think you have answered all five questions correctly click on the Check button.

You need to get all five answers correct first time to win a trophy.

If you get one or more answers wrong the diagrams will change to show the correct answer.

What colour is the arc that passes through the centre of the black circle?

Trapezium Trapezium

Which line is longer: red or blue?

Triangle Triangle Lines

What is the colour of the longest line: red, blue or green?

Inequalities Inequalities Lines

Which line is longest: red or blue?

Dumbbells Dumbbells Lines

Which line is longest: red or blue?

More mathematical optical illusions.

Christmas Tables

Christmas Tables

Missing Square

Missing Square

Chess Board Paradox

Chess Board Paradox

Parallel or not?

Parallel or not?

Click an image above to see the details of the optical illusion.


You got them all correct and have won a trophy!








Take your time! Do not click the Check button until you are absolutely sure you have all the answers correct. You only get one chance!

Only after you have made a decision on all five illusions should you click the Check button. You can go back to each diagram as many times as you want by clicking on the tabs above. You can change your answers as many times as you want too.

The answers to this and other Transum puzzles, exercises and activities are available here when you are signed in to your Transum subscription account. If you do not yet have an account and you are a teacher, tutor or parent you can apply for one here.

A Transum subscription also gives you access to the 'Class Admin' student management system, downloadable worksheets many more teaching resources and opens up ad-free access to the Transum website for you and your pupils.


Try your best to answer the questions above. Type your answers into the boxes provided leaving no spaces. As you work through the exercise regularly click the "check" button. If you have any wrong answers, do your best to do corrections but if there is anything you don't understand, please ask your teacher for help.

When you have got all of the questions correct you may want to print out this page and paste it into your exercise book. If you keep your work in an ePortfolio you could take a screen shot of your answers and paste that into your Maths file.

Why am I learning this?

Mathematicians are not the people who find Maths easy; they are the people who enjoy how mystifying, puzzling and hard it is. Are you a mathematician?

Comment recorded on the 12 July 'Starter of the Day' page by Miss J Key, Farlingaye High School, Suffolk:

"Thanks very much for this one. We developed it into a whole lesson and I borrowed some hats from the drama department to add to the fun!"

Comment recorded on the 10 April 'Starter of the Day' page by Mike Sendrove, Salt Grammar School, UK.:

"A really useful set of resources - thanks. Is the collection available on CD? Are solutions available?"

Each month a newsletter is published containing details of the new additions to the Transum website and a new puzzle of the month.

The newsletter is then duplicated as a podcast which is available on the major delivery networks. You can listen to the podcast while you are commuting, exercising or relaxing.

Transum breaking news is available on Twitter @Transum and if that's not enough there is also a Transum Facebook page.

Featured Activity

Three Ways

Three Ways

Can you get to the target number by multiplying together four different one digit numbers? Can you do it in three different ways? There are nine levels to this online challenge and a virtual Transum Trophy available for each level.


"Numeracy is a proficiency which is developed mainly in Mathematics but also in other subjects. It is more than an ability to do basic arithmetic. It involves developing confidence and competence with numbers and measures. It requires understanding of the number system, a repertoire of mathematical techniques, and an inclination and ability to solve quantitative or spatial problems in a range of contexts. Numeracy also demands understanding of the ways in which data are gathered by counting and measuring, and presented in graphs, diagrams, charts and tables."

Secondary National Strategy, Mathematics at key stage 3

Go Maths

Learning and understanding Mathematics, at every level, requires learner engagement. Mathematics is not a spectator sport. Sometimes traditional teaching fails to actively involve students. One way to address the problem is through the use of interactive activities and this web site provides many of those. The Go Maths page is an alphabetical list of free activities designed for students in Secondary/High school.

Maths Map

Are you looking for something specific? An exercise to supplement the topic you are studying at school at the moment perhaps. Navigate using our Maths Map to find exercises, puzzles and Maths lesson starters grouped by topic.


If you found this activity useful don't forget to record it in your scheme of work or learning management system. The short URL, ready to be copied and pasted, is as follows:

Alternatively, if you use Google Classroom, all you have to do is click on the green icon below in order to add this activity to one of your classes.

It may be worth remembering that if should go offline for whatever reason, there is a mirror site at that contains most of the resources that are available here on

When planning to use technology in your lesson always have a plan B!

Steve Rosse,

Monday, February 13, 2023

"The Buddha says that what we see of the material world is an illusion, a shadow of reality brought to us by a mere five woefully inefficient senses. Cats see more than us, dogs smell more, bats hear more, snakes taste more. The universe that we cannot see, hear, feel, taste or touch is exponentially larger than the one we can.

Quote from Leaving Thailand - A Memoir by Steve Rosse."

Do you have any comments? It is always useful to receive feedback and helps make this free resource even more useful for those learning Mathematics anywhere in the world. Click here to enter your comments.


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