Science Questions

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

Sun, 13th Jun 2010

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Question

Oilinky, Twitter asked:

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.  Its 400,000 kilometres to the moon.  Youll probably get something about 40 metres across [on the moon].  As a football metaphor, wed be looking at something about twice the size of the 18 yard box.

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I always understood that the ranging laser that utilises the reflectors placed on the moon by the Apollo astronauts has a spot size of about 4 Km but I am sure this is not the ultimate that can be achieved. syhprum, Tue, 15th Jun 2010

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