Allow me to explain (sorry couldn't type chinese in this computer)
childk wrote: 他這Laser 應該是不可見光吧。肉眼看不見的波長
A laser is a tightly focused beam of light, and that means all photons are headed in one direction. They go forward, not sideways, so you can't see the beam. You'll only be able to see the spot of light when it hits the wall.
But more importantly, the reason you cannot see anything is that the mouse turns off the laser automatically when it is lifted off from surface.
childk wrote: 學一下美國 CSI 抽支菸,吐出煙霧看一下可不可以看到光.
In clear air the photons are able to travel in a straight line without any interference but if you beam a laser into smoke, the smoke particles scatter the photons so that now some photons travel sideways, which is why you will be able to see the laser.