• Question: what makes water to be colourless

    Asked by 4 east to Kamal on 16 Jan 2017.
    • Photo: Kamal Bhattacharya

      Kamal Bhattacharya answered on 16 Jan 2017:

      Water looks colorless, but it actually isn’t really. Let me first explain what color means. When you look at a banana, why does it appear yellow? First, light or white light consists of many colors. If you think of light as a wave, it consists of a whole spectrum of wavelengths. The so called visible spectrum are all the wavelengths, from violet at 350nm to red at 850nm, that the human eye can see. Light interacts with matter, so if you think of the banana peel of a billions of little particle, some of them absorb a set of colors, meaning they hit the banana and you’ll never see them again. Some of the spectrum, in this case the yellow spectrum, gets reflected. So you take all the colors and shine it on the peel and only the yellow color gets reflected.

      In the case of water almost all the colors get reflected back by the H2O molecules. So white light in, white light out and hence it appears colorless. But in reality what happens is that H2O molecules absorb a bit of the red spectrum and thereby you will see that water actually has a slight blueish tint. Can you think of an experiment on how to test this?