Solar panels are designed to capture sunlight and convert it into electricity through a process known as the photovoltaic effect. However, when a solar panel becomes dirty, this can't happen to the capacity that it should. Solar panels work by absorbing photons (light particles). 

When dust, dirt, bird droppings, and other debris compact on the surface of the panel blocks some of these photons from reaching the cells. This blockage leads to a reduction in the amount of sunlight absorbed by the panel. 

Photovoltaic cells are sensitive to the intensity of light they receive. When the light is blocked or diffused by dirt, the efficiency of the cell's ability to convert light into electricity is diminished.