Question

I saw in the video that in collidation the particles cannot be seen but in rays of light we are able to see the particles. Why?

Answer 1

The collision of particles cannot be seen with our eyes because the size of particle is very small. But this is true in case of solution only. In case of suspension the particle size is relatively big (more than 1000nm). Further they will scatter the light in the perpendicular direction and their path is illuminated. (This is called as Tyndal effect). So in the illuminated path we can see that the dust particles are colliding in the sunlight.
For your information the size of salt particles in water is 0.5nm and the particles in the dust is in the order of 0.5 to 1 micro metre and they are visible to our eyes