News in Science

Carrots & Night Vision

Parents have lots of tricks to get their kids to eat their vegies. One often-used persuasive tale is that if they don't eat their carrots, they'll go blind, or at the very least, not be able to see in the dark. The parents are telling a little white lie, but there is in this little fib, as there is in all good mythconceptions, a small seed of truth.

Carrots came from around Afghanistan. They had reached the Greeks and the Romans before the birth of Christ, and by 1300 AD had reached northwestern Europe, India, Japan and China. In the Middle Age in Europe, they were used as a medicine to cure most maladies, including sexually transmitted diseases and snakebites.

Carrots are involved in vision, thanks to their link to Vitamin A. You can get Vitamin A from some animal products (such as fish and liver), but not from vegies. However, many of the yellow vegetables and fruit are yellow because of coloured chemicals called "carotenes" - as are the feathers of canaries, and the shells of lobsters. Once you eat these yellow vegies, your liver converts the carotenes to a family of chemicals that go under the name of Vitamin A.

If you don't have enough Vitamin A, you won't have healthy skin, a healthy immune system - and good vision. The Vitamin A travels in the blood to the retina (in the eye) where it is converted into a chemical called "retinal". When light hits this retinal chemical, it changes shape. This starts off a process, which ends with electricity travelling to the visual centres at the back of the brain - which turn the electricity into the wall-to-wall sensation that we call vision. No Vitamin A means no retinal in your retina, which means no electrical signals travelling to your visual centres.

So, if you don't get enough carotenes or Vitamin A in your diet, eventually you will suffer problems in your vision. This was the basis of the myth started by the Royal Air Force, the RAF.

In the Battle of Britain, in 1940, the British fighter pilot, John Cunningham, became the first person to shoot down an enemy plane with the help of radar. In fact, in WW II, he was the RAF's top-scoring night fighter pilot, with a total of 20 kills. Some pilots were better flying in daylight, while others, like Cunningham, were better at night. His nickname was "Cats' Eyes". The RAF put out the story in the British newspapers that he, and his fellow night pilots, owed their exceptional night vision to carrots. People believed this to the extent that they started growing and eating more carrots, so that they could better navigate at night during the blackouts that were compulsory during WW II.

But this story was a myth invented by the RAF to hide their use of radar, which was what really located the Luftwaffe bombers at night - not human carrot-assisted super-vision.

On one hand, not enough Vitamin A will make you unable to see at night. But lots of Vitamin A can be poisonous - and it definitely will not give you super-human vision at night.

The device that told the RAF night fighter pilots where the incoming bombers would be was the invention called "radar" (from RAdio Detection And Ranging). In a radar unit, an antenna sends out a brief burst of radio waves (say with a peak power of one million watts), then stops transmitting. It switches off, and then listens for the very weak echo, as some of the radio waves bounce off the metal skin of the plane and return to the antenna. They can be as weak as one millionth of a watt. The time interval between the transmitted and received signals gives the distance to the plane.

At the beginning of WW II, France, the UK, the USA, the Soviet Union, Italy, Japan and Germany each had explored and researched radar - but only the UK had developed a fully-functioning network. This network was called "Chain Home", and operated 24 hours per day from September 1938 until the end of the war.

But why did the German Air Force see pass the obvious radar towers on the English coast, and fall for this blatant "carrot-super-vision" myth? Because this myth, that carrots would make their eyes better, already existed in German folklore.

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