Q. What is the wavelength?
A. Wavelength of visible light = 400 to 700nm.
Q. Does it need a medium?
A. No. Done.
Q. What is it?
It is the only portion of the electromagnetic spectrum that is visible to our naked eye. The color of an object is the electromagnetic wavelength that is not absorbed but instead reflected by it. All colors of light have varying wavelengths and form white light when combined. This portion of the electromagnetic spectrum lies between the infrared and ultraviolet regions in the order of the colors of the rainbow. With Red having the longest wavelength while Violet has the shortest. So simply, rainbows are formed when white light get screwed up inside a raindrop and emerge as the various wavelengths it is composed of.
What are the uses / advantages and dangers / disadvantages?
Uses include lightbulbs or torchlights to help brighten certain areas and since light travels in a straight line, shadow puppets can be used to entertain people. Advantages, lets you see color. Dangers include permanent or temporal blindness due to over-exposure. Disadvantages are that since light travels in a straight line, people get really angry when you walk past in the middle of a presentation or a movie.
Some from Qi Ren
Some from Kimberly