People fire off lasers at night. It’s also nighttime for airplane pilots, so the cockpit is dark except for the lights of the instrument panel. Therefore, their pupils are open, up to 4 millimeters wide, increasing their vulnerability to sudden glares and flashes. It’s the same effect as what your eyes feel when they’ve been in the dark for a long time, but a dazzling bright light suddenly glares into them. That ruins their night vision. Even if the beam does not hit the cockpit, the pilot sees it in flashes, and so it distracts the pilot. Worse, that must arouse their suspicions someone hostile is targeting them.

Although laser light beams are extremely coherent and focused, unlike ordinary light, they do spread out as it travels. Therefore, a point that creates just a narrow dot at a close distance is inches wide when it reaches the cockpit. When it hits the windscreen or bubble of a helicopter, imperfections in that material spread the light out further, up to a meter, causing such a glare the pilot cannot avoid it. Such flashes might last only a thousandth of a second. When the light’s strong enough, the laser beam causes flashblindness and afterimages. This pamphlet from the Federal Aviation Authority goes into more detail about the danger. The FAA reports that 7,500 laser strikes on aircraft occurred in 2015. They estimate there’ll be 12,000 in 2016.

This article in the British Journal of Ophthalmology points out that the small, red pointers of 1 or less milliwatts of the past did not cause a significant threat to eyes unless someone stared into them for a long time. These are properly categorized as Class 2. However, stores and websites around the world now sell much stronger laser devices, both red and green, with power up to 6000 Mw, usually treated as Class 3B or Class 4, too dangerous to use without proper safeguards.