Introduction: Optical Illusion - Two Lines

Picture of Optical Illusion - Two Lines

This Instructable will show you how to make a simple optical illusion. This illusion is called the Poggendorff Illusion and has been around for a while.

So what's happening in the image below? What is that red line doing there?

Go to the next step to see what's going on inside the black rectangle.

Step 1: Inside the Box

Picture of Inside the Box

Here you can see that the blue line actually leads into the red line. Sure, the color contributes to the illusion, but it can be confusing even if the lines are all black.

Step 2: Black Lines

Picture of Black Lines

Here we can see two black lines going in and out of the black rectangle. I'm sure you know what's coming, but go to the next step to see what's happening inside the box.

Step 3: Inside the Box

Picture of Inside the Box

As you can see, one of the lines is continuous, but it shifts from being the top line to the bottom lines.

Step 4: 2 Lines With Color

Picture of 2 Lines With Color

If we go back to the colored lines, we can see the same effect.

Step 5: How to Make It

Picture of How to Make It

You can use many different graphics programs to recreate these images. Photoshop, Illustrator, GIMP, Inkscape, or any other. As long as it can make lines and rectangles, and they all can, you're fine.

Start by making one diagonal line.

Step 6: Copy the Line

Picture of Copy the Line

Copy the line and paste another one just a little bit below it.

Step 7: Copy Both Lines

Picture of Copy Both Lines

Now copy both of those lines and move the copy so that the ends of the two lines come together like in the image below.

Step 8: Put a Rectangle on Top

Picture of Put a Rectangle on Top

Just drop a rectangle on top and you're done! Now you can play with making the lines different colors to recreate the images from the beginning of the Instructable.


madhops0620 (author)2009-02-18

I don't understand what the illusion is. Anyone care to explain?

slayer04 (author)madhops06202009-02-23

it makes it appear as if the both lines are connected but in reality only one is to the other on the opposite side, it's a trick of the eye

Sarah86 (author)slayer042009-03-04

Well... don't really work with me cause I got geometric vision. Since birth, I have problem to see the distance, so my deep vision rely on geometric form.

schlenbea (author)Sarah862009-03-09

It's too bad. You should really see how fascinating the illusion is. You're definitely missing out. I wish I could help you see it.

Sarah86 (author)schlenbea2009-03-11

Oh don't worry, I can still see how the illusion work, but I don't fall.

Allonsy (author)madhops06202009-02-19

basically u see the blue line comin into the black rectangle, and u see a read and blue one coming out the other side. ur brain thinks that the blue lines are connected, when actually its the read and the blue line connected. get it?

KWHCoaster (author)Allonsy2009-02-20

To me the blue line on the left looks like it is between the lines on the right.

madhops0620 (author)Allonsy2009-02-19

yeah I figured it out right after I posted this

Allonsy (author)madhops06202009-02-20

lol :D

madhops0620 (author)madhops06202009-02-18

nevermind, haha now I feel stupid

sharlston (author)2009-03-09

this is so cool any more cool ideas then please email them to

imonsei (author)2009-02-20

heh. I actually thought it was a trick but i guessed that the line above went between the two at the bottom :P

amakerguy (author)2009-02-19

I fell for the trick to. Brains are so cool, I like them because there always a mystery.

ChrysN (author)2009-02-17

Nice illusion, it is scary how easy it is to trick our brain.

BeanGolem (author)2009-02-17

Classic, but WHY does our mind do that?

PizzaPlanet (author)2009-02-17

nice sweet blows ur mind a little that what we see is not reality... excuse me i just finished reading the watchmen comics deep stuff

hg341 (author)2009-02-17

well done i liked it all lot you fooled me too

hg341 (author)hg3412009-02-17

im sry i sayed that worng

well done i liked it a lotyou fooled me too

thats what i ment to say

the_burrito_master (author)2009-02-17

Very cool! First comment! nice ible. It fooled me!

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