How does the path of light become visible due to scaterring of light? According to me it will only tell the presence of colloidal particle but the book give the example of cinema hall projector and says that you could tell path of light due to scattering of light. Nevertheless, without scaterring of light also we can tell its path.
The book just means the slight scattering of the light beam from dust gives the chance to see the path of light visually. Of course, you also see the presence of colloidal particles. Without this scattering, you will not "see" the beam, which is travelling in exactly one direction, as your eyes will have nothing to detect.