An optical illusion as a lesson starter

I'm always on the look out for an image or video clip to act as an inspiring lesson starter. This is one of my favourites. Look at the image below. Which square is lighter in shade, A or B?

(Image is ©1995, Edward H. Adelson, but may be redistributed freely, as detailed here).

Square B is clearly lighter, isn't it? But watch my short video clip:

Yes, A and B are actually the same shade. No matter how many times I look at the image, I still can't see it that way though. (The reason the illusion works is that our brains naturally compensate for the perception of a shadow falling across square B i.e. our brains have evolved to perceive the shade of a colour in a real world full of variable lighting conditions, not in a world of optical illusions). This beautiful demonstration was published by Ted Adelson in 1995; the original figure is found here, and his explanation here.

You can also download the video file from the Resources section.

How might we use this illusion in lessons? I can think of a few possibilities:

• When discussing racial / cultural / religious tolerance - the metaphor being that 'Different races, cultures or religion may look or behave differently, but underneath, we are all much the same'
• In science - "What we observe in an experiment may not be a true indication of what is actually going on. We have to test our observations, make predictions, and test them". (In this case, the test is sampling the two colours and comparing them side-by-side)
• In history - "The motivations of a person or government may not be what they appear to be. We need to look deeper".
• Discussing thinking skills / skeptical thought - "Seeing is not necessarily believing. Just because we think we have seen something (a UFO, ghost) does not mean that the object we are seeing is what you think it is, or that it is real". The human brain is constantly turning the signals from our eyes into an interpretation of the physical world around us, using incredibly complex simulation "software" that has evolved over millions of years. Sometimes that software produces inaccurate interpretations - for example, falsely compensating for shadow in the scene above.

If you are interested more generally in optical illusions visit Michael Bach's website.

Optical illusions stimulate the cognitive conflict that teachers should seek to engender in our students, challenging them to look beyond the obvious to the underlying principles and themes of a topic.

Do you have any further suggestions for using the optical illusion above? What are your favourite images for lesson starters?