Why sky is blue?

Lisaura Maldonado Pereira

 

Every day we see strange things everywhere and we ask ourselves, how these can happen. How can we hear sound, how can a magnet serves, how electricity is made, are some of the common questions that anyone can make. Fortunately, we had discussed in class almost all these rear nature’s behavior, helping us to expand our knowledge in the physics area. 

But now I ask; how the sky gets its color? This is a particular theme that I consider interesting. The sky color is the product of light’s conduct for being a mix of all wave colors that make white. We had seen in class some of its characteristic behaviors as the interference and diffraction, but answering this question I found another behavior that is the responsible of this event, the refraction. As many people know, the refraction is the change in direction of a wave due to a change in its speed. In other words, it can be said that the light waves (or the boundary between the media and light waves) suffer a change in direction when it travel from a medium with a given refractive index to a medium with another at an angle. Also the wavelengths are affected, they increases or decreases as the speed of light change. For example, a light ray will refract as it enters and leaves glass, assuming there is a change in refractive index. This phenomenon is known as light scattering.

Figure 1: Refraction of light rays: a soda straw sticking out of a glass of water. It looks broken.

This effect explains the blue color of the sky. Imagine that we let a ray of sunlight through a prism of glass (in this case would be the water molecules in the air). The light is opened in a range of colors (scattered) by refraction and as a result of this dispersion, we see a range of colors: violet, blue, green, yellow and red. The violet ray is the one that has spread over the direction of white lightning and that is precisely the explanation of the color of the sky. The deviation is maximum for rays of short wavelength (violet and blue), and lowest for long-wavelength (yellow and red), which are hardly deflected. Violet and blue rays, once diverted, collide with other particles of air and change its course again, and so on. When, finally, reach our eyes, do not seem to come directly from the Sun, but we come from all regions of the sky, and in the form of fine rain. That is why the sky appears blue to us, while the sun appears yellow, as yellow and red rays are slightly deflected and go almost directly in a straight line from the Sun to our eyes. 

Figure 2: Sunbeam passes through a prism of glass.

This can be also applied to the rest of color of nature. As we seen, everything has a reason and almost everything can be explained with physics. I consider it a very interesting process about our daily life and it demonstrates how fabulous can be nature, if we pay attention to all its details.