With the best laser, how small a spot could you focus from earth to the moon?

13 June 2010

Question

With the best laser, how small a spot could you focus from earth to the moon?

Answer

We posed this question to Graeme Hirst from the STFC Central Laser Facility...

Graeme - You're trying to fight a thing called diffraction and that depends on how big you can make your laser beam down at the earth. A back of the envelope calculation - laser light spreads out about one wavelength in 10 metres. It's 400,000 kilometres to the moon. You'll probably get something about 40 metres across [on the moon]. As a football metaphor, we'd be looking at something about twice the size of the 18 yard box.

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