• Question: What causes rainbows?

    Asked by msummers to Drew, Amy on 21 Jun 2011. This question was also asked by stary.
    • Photo: Drew Rae

      Drew Rae answered on 17 Jun 2011:

      Drops of water acting in the air act as prisms separating the colours of light. Your eye gets hit with light of different colour from different raindrops depending on the angle between the sun, the raindrop and you.

    • Photo: Amy MacQueen

      Amy MacQueen answered on 21 Jun 2011:

      Hi msummers!

      Light is made up of a collection of many colors: red, orange, yellow, green, blue, indigo, violet. That is why a prism (a triangular piece of glass or plastic) can take in white light on one side and produce its own mini-rainbow on the other side. To understand rainbows, you have to start by understanding what is happening inside a prism to let it separate white light into its colors.

      To produce a mini-rainbow, you allow a narrow strip of white light to fall on one face of the triangular glass – it then separates out the other side.

      This separation of the colours occurs because of something called the refractive index of the glass. Every material has a different refractive index. When light enters a material (for example, when light traveling through the air enters the glass of a prism), the difference in the refractive index of air and glass causes the light to bend. The angle of bending is different for different wavelengths of light. As the white light moves through the two faces of the prism, the different colors bend different amounts and in doing so spread out into a rainbow.

      In a rainbow, raindrops in the air act as tiny prisms. Light enters the raindrop, reflects off of the side of the drop and exits. In the process, it is broken into a spectrum just like it is in a triangular glass prism. The angles cause different colors from different drops to reach your eye, forming a circular rim of color in the sky – a rainbow!

      🙂 🙂