Visual Illusions


Visual illusions. A subset of the more general category of all illusions. Visual illusions can be fun at times, like when we enjoy them at magic shows. At other times, they can be perplexing: how can squares A and B below be the same color?

And yet they are:
Then there is the famous ‘inferior mirage’, one that shows us things that don’t exist. Like the beaten to death oasis in a desert.

If there’s such a thing as an ‘inferior mirage’, does that mean there’s a ‘superior mirage’, you ask? Yes. And it wrecked a man’s reputation.

Kathryn Schulz describes it in her terrific book, Being Wrong. In 1818, John Ross set out to find a (faster) sea route from Europe to Asia via the Arctic. At one potential point of progress, he was disappointed when the fog cleared: a huge chain of mountains appeared about 27 miles. Ross gave up and returned. But his second-in-command, William Parry, who was following in a different ship, hadn’t seen any mountains blocking the way. Parry told as much when they got back home. The sponsors of the voyage “naturally preferred the idea of the mountains not existing to the idea of their existing”. Ross’ reputation was now tarnished. But worse was to come that would ruin his reputation.

A year later, Parry was sent back to the same place:
“This time, Parry did see the Crocker range (the mountains)- and then sailed right through them. The mountains were a mirage.”
This was a ‘superior mirage’, something understood only much later. It is called ‘superior’ because it shows us something that does exist, but at the wrong location: The mountains that Ross saw did exist, but not 27 miles away but (hold your breath) 2oo miles away!

A superior mirage is caused when the air near the ground is colder than the air above. This is the opposite of what would happen in most places. Except near the poles where it is freezing cold at the surface:
“This inverted situation dramatically increases the degree to which light can bend… light sometimes bends so much that… (it) can be reflected from objects up to several hundred miles away.”

In 1939, Robert Bartlett was sailing between Greenland and Iceland. He knew his ship was around 350 miles from Iceland when he suddenly saw the Icelandic coast so close that he could make out its landmarks. It seemed to be just 25-30 miles away! Unlike Ross though, “125 years’ worth of improvements in navigational tools and geographic knowledge prevented Bartlett from making the same mistake as Ross”.

Seeing isn’t always believing. It’s why they keep telling pilots to trust their instruments, not their eyes…


Comments

  1. Ramiah KumarNovember 21, 2018 at 11:43 AM

    Very interesting.

    This business of light refracting through air has given rise many phenomena on land too, apart from the famous desert oasis thing. I saw pictures of double sun happening in USA and even triple sun in some eastern country. There is a place in South America where an electric pole looks considerably bent, when seen from a distance at an angle, but perfectly straight when seen in the perpendicular direction.

    People get floored when such things happen, but, even if we know optics well and know about the science of optical illusions, when you actually encounter them, they take the breath away! :-) I suppose Nature also enjoys playing the magician (i.e. doing honest cheating) sometimes!

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