What is the wavelength? Does it need a medium?
The wavelength of visible light varies from 4×10-7m (violet) to 7×10-7m. Visible light does not require a medium.
What is it?
It is the only part of the electromagnetic spectrum that the eyes can detect.
When we see objects, it’s because they’re being illuminated by visible light. When we see that the sky is blue, or the grass is green, or hair black, or that an apple is red, that’s because we’re seeing different wavelengths within the 400nm-700nm band. The color of an object that we see is the color of light reflected. All other colors are absorbed.
We see these waves as the colors of the rainbow. Each color has a different wavelength. Red has the longest wavelength and violet has the shortest wavelength. When all the waves are seen together, they make white light. The visible region extends between the infrared and ultraviolet regions in the following order: red, orange, yellow, green, blue, indigo, violet
When white light shines through a prism, the white light is broken apart into the colors of the visible light spectrum. Water vapor in the atmosphere can also break apart wavelengths creating a rainbow.
Cones in our eyes are receivers for these tiny visible light waves. The Sun is a natural source for visible light waves and our eyes see the reflection of this sunlight off the objects around us.
Light bulbs are a common source of visible light waves.
What are the uses / advantages and dangers / disadvantages?
Too much visible light can damage the retina in your eye. This can happen when you look at something very bright, such as the Sun. The damage could be so bad that it is permanent.
Today, with the help of photothermal and photovoltaic technologies, solar energy is being widely used. Whereas in photothermal devices, sunlight is used to heat some substance, photovoltaic devices directly convert solar energy into other forms with the help of solar panels, which are being used quite commonly throughout the world today.
Visible light is also made use in many other machines today such as photoelectric copying machines, laser printers, CD players, FAX machines, optical recording media, and optical disc mass-storage systems of exceedingly high bit density.