Light that falls on the object gets reflected
Light that falls on the object gets reflected | Reflected light reach the human eye
kFor the clear vision of an object, the light falling on that object should be reflected in suitable angle and reach the human eye. Or else the object itself should be a light emitting device. Size of the object plays major role in reflecting the light falling on it. In ordinary light illumination, even though some light falls on the dust particles, the light that gets reflected from those particles has very small intensity. So they are not visible. But in the case of a projector which emits huge intensity of light, much light falls on the dust particles and get reflected from them. Reflected light from the dust particles get scattered and reach the human eye. That’s why we are able to see the tiny dust particles.