The difference in visibility and reaction to a jet flying at 1300 mph (Mach ~1.7) versus a bullet traveling at the same speed is primarily due to their respective sizes, distances, and contrasts with the background.
Size and Contrast: A jet is much larger and more visible than a bullet. Jets have substantial surface areas, often with reflective paint and lights, making them easier to spot against the sky or clouds. In contrast, a bullet is tiny, especially at a distance, and has little surface area to reflect light. This makes it much harder to see with the naked eye, even if it's moving at high speeds.
Distance: Jets flying at high speeds are typically at a considerable distance from the observer, often several thousand feet above the ground. While their absolute speed is fast, their apparent speed from the observer's perspective may not be as extreme. On the other hand, a bullet's speed is also remarkable, but it usually travels at a much closer distance to the observer. The combination of high speed and proximity makes bullets challenging to detect and react to in real-time.
Visual Acuity and Time to React: Human eyes have a limited frame rate and visual acuity. While a jet might be moving rapidly, it covers a more extensive visible distance over a short period, making it easier for humans to track and respond to its movement. However, a bullet is much smaller and traverses a short distance in the blink of an eye, making it difficult for our eyes and brain to process the information and react quickly enough.
Trajectory: The trajectory of a bullet is typically more linear and harder to anticipate, especially when it is shot directly towards or away from the observer. A jet, in contrast, usually follows more predictable flight paths and is often moving in more controlled airspace, making it easier to track and react to its movements.
It's essential to note that while humans may not visually see a bullet traveling at high speed, they can still be affected by its impact or hear its sonic crack if it breaks the sound barrier. Additionally, when it comes to bullets fired from firearms, their velocities can vary widely depending on the type of firearm and ammunition used, with some bullets exceeding the speed of sound (supersonic) and others remaining subsonic.