Rotating Squares Illusion

By the end of this lab you'll be able to:

• Draw concentric squares that each rotate slightly from the last
• Understand how a small per-square rotation creates a strong spinning illusion
• Control the illusion strength by varying the rotation step

Concentric squares, each rotated a few degrees from the previous one — but perfectly static. The human visual system interprets the gradual rotation as actual spinning motion. This is the "rotating squares" optical illusion.

Welcome to the Rotating Squares Illusion!

Your brain sees motion where there is none — pure static geometry! This is one of the most famous geometric optical illusions in Op Art. Let's code it together!

How It Works

Draw 20 concentric squares. Each square is rotated 5° more than the previous one. The squares also shrink inward by a fixed amount. The combination of rotation + shrinkage creates the illusion of a spiral/spinning motion.

Sample Code

import turtle
import math
monty = turtle.Turtle()
monty.speed(0)
monty.hideturtle()

n_squares = 20
start_size = 160
rot_step = 5

for i in range(n_squares):
    size = start_size - i * 7
    if size <= 0:
        break
    angle = i * rot_step
    monty.penup()
    monty.goto(0, 0)
    monty.setheading(angle)
    monty.forward(size * math.sqrt(2) / 2)
    monty.setheading(angle + 135)
    monty.pendown()
    monty.pensize(max(1, 3 - i // 7))
    color_val = int(255 * i / n_squares)
    for _ in range(4):
        monty.forward(size)
        monty.left(90)

What Do You Think Will Happen?

Each square is rotated 5° more than the last — they're all stationary. Will this look like a stack of squares, or will it appear to be spinning? Make your guess — then click Run to find out!

Try It Now

▶ Run ↻ Reset

The image appears to spin — but it's completely static. Were you right? The illusion is strongest when you look slightly to the side of the image.

How It Works

Each square's corner is found by going forward from the center at 45° (diagonal) and then turning to draw the square. The illusion works because our visual system extrapolates motion from the gradual rotation gradient — it "reads" the progression of angles as a time sequence rather than a spatial arrangement.

Explanation Table

Line	What it does
angle = i * rot_step	Each square rotated 5° more than the last
setheading(angle + 135)	Start drawing from correct corner
size - i * 7	Squares shrink as they approach center
forward(size * sqrt(2) / 2)	Distance to corner along diagonal

Learning Check

Your Turn — Make It Spin Faster

Change rot_step = 5 to rot_step = 10. Will the illusion of spinning be stronger or weaker? Predict, then run to find out!

▶ Run ↻ Reset

rot_step = 10 creates a more chaotic-looking arrangement that may look like faster spinning to some viewers.

Experiments

1. Try rot_step = 45. The squares alternate between two orientations. You'll know it worked when you see only two overlapping orientations.

2. Use black and white alternating. Set colors to ['black', 'white']. You'll know it worked when the high contrast makes the illusion even stronger.

3. Use hexagons instead of squares. Change the inner loop to draw 6 sides of length size/1.5 with left(60). You'll know it worked when hexagons appear.

4. Draw both clockwise and counter-clockwise. Run the loop twice — once with rot_step = 5 and once with -5. You'll know it worked when two counter-rotating spiral illusions appear.

Your Brain Was Tricked!

You built a genuine optical illusion entirely from static squares! Op Art (Optical Art) uses math and geometry to fool the visual system. Up next: Café Wall Illusion — parallel horizontal lines that look slanted.