Final answer:
A beam of light spreads due to diffraction, and this effect cannot be eliminated by lenses, mirrors, or apertures. Parallel rays cannot prevent spreading due to the wave nature of light. Increasing the beam diameter can reduce spreading, as seen in laser transmissions to the Moon.
Step-by-step explanation:
A beam of light always spreads out due to a phenomenon known as diffraction. Diffraction occurs because light is a wave, and when it is confined to a beam of finite diameter, it will spread out. This spreading is described by an equation θ = 1.22 λ/D, where θ is the diffraction angle, λ is the wavelength of the light, and D is the diameter of the beam. While it might be intuitive to think that creating a beam with parallel rays could prevent spreading, in practice, this is not possible as the rays will always exhibit some degree of divergence due to diffraction. Lenses, mirrors, and apertures also cannot eliminate diffraction because it is an inherent property of waves passing through any opening or around edges.
One method to reduce diffraction spreading is by increasing the diameter of the beam, D. This is often done in applications such as transmitting laser light to the Moon, where the beam is expanded through a telescope to reduce the diffraction angle and maintain a narrower beam over long distances.