Final answer:
The perpendicular illumination pattern in question is typically observed in the double-slit interference experiment, where a laser beam creates a pattern of bright and dark fringes on a screen after passing through slits.
Step-by-step explanation:
The illumination pattern that is fired perpendicular to the observer-target line is observed in experiments showing the behavior of light and its wave-particle duality. An example of such an experiment is the double-slit interference, where a laser beam passing through a grid of vertical slits produces an interference pattern. This pattern is created when light waves emerge from the slits and spread out in all directions, even though the original light source is aligned perpendicularly to the observer-target line. Observers can see this effect in experiments where a light source emits rays which then pass through a double slit, causing a distinct pattern of bright and dark fringes to appear on a screen. This pattern can also be observed in simplified setups with a single slit or in cases where a lens is used to focus light and produce parallel rays, as seen in lighthouses or traffic lights.