The location where the beam diameter is the narrowest is called the

• A. End of near zone
• B. Focus
• C. Far zone
• D. Focal length

Answer: (B)

When a beam is focused, the rays converge and the cross-sectional diameter becomes smallest at a single point along its path. This point is called the focus (often referred to as the beam waist in Gaussian-beam terms). At the focus the beam achieves its minimum diameter; before the focus it continues to narrow, and after the focus it begins to spread out again. The focal length is a distance describing how far the focus lies from the lens, not the location itself. The near and far zones describe diffraction regions of the beam during propagation and do not designate where the diameter is smallest.