Thursday, July 7, 2011

Visible Light

Visible light waves are the only electromagnetic waves we can see. 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. 

Visible spectrum (in order from the shortest wavelength to the longest)
- Red
- Orange
- Yellow
- Green
- Blue
- Indigo
- Violet

- Allows us to see.
- Are readily absorbed or reflected, giving us color

- Wavelengths are too long to permeate skin, hence the use of shorter X-rays instead to examine bone structure of a living being.

- Are readily absorbed/reflected, which is why we discern the colors of objects; this is not a good thing for transmitting information over long distances, hence the use of much longer wave length radio waves

- Not easily discernible at low levels (i.e., in the dark). Switching to the slightly longer infrared allows tracking of heat sources such as machinery and living things in the dark.

Q1) Why of all colours, only blue is reflected off the sky and the sea?

The blue sky colour is due to Rayleigh scattering (It refers to the scattering of light off of the molecules of the air, and can be extended to scattering from particles up to about a tenth of the wavelength of the light). As light moves through the atmosphere, most of the longer wavelengths pass straight through. Little of the red, orange and yellow light is affected by the air.

However, much of the shorter wavelength light is absorbed by gas molecules. The absorbed blue light is then radiated in different directions and gets scattered all around the sky. Whichever direction you look, some of this scattered blue light reaches you. Since you see the blue light from everywhere overhead, the sky looks blue. The sea is coloured due to the same reason.

Q2) Why is the sunset red?

As the sun begins to set, the light must travel farther through the atmosphere before it reaches you. More of the light is reflected and scattered. As less reaches you directly, the sun appears less bright.

The color of the sun itself appears to change, first to orange and then to red. This is because even more of the short wavelength blues and greens are now scattered. Only the longer wavelengths are left in the direct beam that reaches your eyes. 

Q3) How do we see colour?
What we actually see as color is known as its colour effect. When an object is hit with light rays, the object absorbs certain waves and reflects others, this determines the color effect. For example, when we observe a red fluorescent lamp, it appears red because it reflects only red light and absorbs all other light.

The lamp does not have color in itself. The light generates the color. What we see as color is the reflection of specific wavelength of light rays off an object.

The color white: If all light waves are reflected from a surface the surface will appear to be white.
The color black: Similarly, when all light waves are absorbed by a surface the surface will appear to be black.



  1. Excellent. Good information on visible light. I did not know most of what was presented here!

  2. Well done. Much information is presented.