## The long thin line

By selecting a very dim source of light, and attenuating the beam by means of filters, we may obtain a beam of separated photons and study them essentially 'one at a time'. For example, consider a 1 mW helium-neon laser emitting a thin beam of light of wavelength 632 nm.

Let us calculate how many photons are emitted per second, and the average distance by which they are separated in space.

For X = 632 nm, photon energy hf = 3.14 x 10-20 Joules 1 mW = 10-3 Joules per sec = 3.2 x 1016 photons per sec

If the beam passes through a filter which attenuates it by a factor 1012 this leaves 32,000 photons per second.

Thirty thousand photons per second may sound like a lot of photons, but they are spread over a distance which light travels in one second, i.e. 30 million metres. The line of photons emitted in one second would wrap around the earth more than seven times. This means that in the attenuated beam the photons will be an average distance of 10,000 metres apart.

filter

10 mW laser line of photons

10,000 m

Figure 14.2 The filtered beam photons are about 10,000 m apart.

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