The Core Reason: Limited Exposure Time
Fast-moving objects appear blurry because both human eyes and cameras have a finite 'exposure time' or response speed. During this brief period, the object travels a significant distance. Instead of capturing a single, sharp image, the light from the moving object is spread across multiple points on the retina or sensor, resulting in a smeared or indistinct appearance.
Visual Persistence in the Human Eye
For the human eye, this blur is partly due to retinal persistence. The image of a fast-moving object temporarily remains on the retina even after the object has moved. As the eye's gaze attempts to track the object or shifts, these lingering images overlap, causing a blurry perception. The brain struggles to process the rapid changes in position into a single, clear form.
Camera Shutter Speed and Motion Blur
In photography, motion blur is a direct result of the camera's shutter speed being too slow relative to the object's speed. A longer shutter speed allows more light to hit the sensor, but if the object is moving, it will move across the sensor during that time. The solution is to use a faster shutter speed to 'freeze' the motion, effectively shortening the exposure time to capture the object at a single point in its trajectory.
Relative Motion and Perceived Blur
The perception of blur is also relative. If you are moving at the same speed and direction as an object (e.g., in a car parallel to a train), the object might appear less blurry because its relative speed to your eye is reduced. Conversely, if you are stationary and an object whizzes past, the blur is more pronounced. This highlights that blur is not an inherent property of the object, but rather a function of its relative velocity and the observer's or sensor's integration time.