It is late fall. You are driving your car home from work. The sun is setting faster than you thought, so turn on your headlights. And suddenly, without warning, a deer jumps onto the road, just a few feet from your 2-ton car. He brakes and realizes there are no cars in the opposite lane, so he instinctively turns the wheel to the left, narrowly escaping a catastrophic collision with the deer. Your heart is racing, but you are not hurt. And neither does the deer.
Cautiously, you start driving again and wonder what’s wrong with that deer. You think to yourself, “why didn’t it move?” This question keeps you awake most of the night. In fact the answer is very simple.
Deer do most of their tasks in low light. They are made for that. Their eyes let in much more light than human eyes. Thus, when the sun rises or sets, they go out to drill, mate and roam. When a deer is caught in the very intense light from a car’s headlights, its eyes are fully dilated and the deer literally goes blind. Being momentarily blinded by this light, they do not know what to do and therefore do nothing.
Therefore, the expression, a deer caught by headlights.
It should also be noted that researchers now claim that deer have 20/200 vision. A normal, healthy human is 20/20. Yet another reason to make sure you drive safely and avoid any roadside deer encounters.