Oh there is so much bull:
"If the light source were a coherent laser, firing into a vacuum, all the light would stay straight". No, even if were single TEM00 transverse mode it would propagate as a Gaussian beam with a half divergence angle of lambda/pi x w0 where w0 is the 1/e^2 waist radius. Typically a divergence full angle of a milliradian, so at 10 metres a 1mm waist diameter visible beam will have expanded to 1cm.
Of course this phenomenon is not limited to light - any freely propagating electromagnetic wave will behave similarly, whether it is microwaves or X-rays
More optical BS from
https://www.futureshop.co.uk/audioqu...-optical-cable
Craig