Why is there dust in the air and on appliances when the sun shine’s through the window?

The sun beams reflect off the dust and make it easily visible. That dust is always there but usually not visible due to human retina acuity. The sun coming through the window means much more light rays are present and so add more reflection, thus reaching the threshold for your eye to detect them; and cause a perception of the dust in a sun beam or on a metal surface. Additionally, appliances tend to be made of sheets of metal and so electrostatically attract dust.