How can bats hit the window glass?

  Have you noticed? At dusk in summer, when bats are infested, they often hit the windowpanes in flight.
  It is a bit strange to think about this: it is understandable for birds to hit the glass, because after all, birds flying rely on vision, and they may treat transparent glass as empty holes. But for bats, they don’t rely on vision, but ultrasound. What is the relationship between glass transparency and ultrasound?
  In fact, the degree of transparency of glass has nothing to do with ultrasonic waves, but another property of glass has much to do with ultrasonic waves, that is, smoothness. This is why bats hit the wall.
  We all have experience in life: in the dark, when you use a flashlight to illuminate the mirror at a certain angle, unless you are very close to the mirror, it is not easy to see the light in the mirror. Because the reflection of light follows the reflection law of “the angle of reflection equals the angle of incidence”, and a mirror is a smooth surface. Light is incident at a certain angle and can only be reflected from a certain direction, and your eyes may not be exactly in this direction. You can’t see the reflected light. But if you are illuminating a rough wall, it is different. In this case, light will be reflected from all angles, and some will always enter your field of vision.
  This is the difference between specular reflection and diffuse reflection in optics. Although ultrasound is a wave whose properties are completely different from light waves, it follows the same laws as light waves in terms of reflection.
  We know that bats use their mouths to emit ultrasound and then receive echoes through their ears to locate them. The ultrasonic wave it emits, like the beam of a flashlight, has a certain directionality (in physics terms, it is “focus”). When ultrasonic waves encounter a smooth surface like glass, the reflected waves will also be concentrated in a fixed direction. If the bats are not in this direction, it will be difficult for them to hear the echo. Without receiving an echo, they mistakenly thought that there was nothing in front of them, so they bumped their heads.
  Furthermore, for bats, an upright smooth surface like glass is something they have never encountered in nature, and they obviously have not evolved the ability to deal with such things.
  Therefore, window panes on tall buildings are not only dangerous death traps for birds, but also for bats.