Do Visible light have an effect on people? If so what. Are they harmful? How?

At some level of power, all EM wavelengths can be harmful, including visible light. They are all radiant energy. And if that energy is absorbed by your cells, that can be damaging in a number of ways. Visible light can be harmful in high intensity. A laser or sunlight focused with a magnifying glass can cause a burn. In low intensity it is not harmful. On the other hand gamma rays and x rays can be harmful even at low intensity.