Chapter 4: Problem 2

What would you expect the sky color to be at an altitude of \(50 \mathrm{~km}(30 \mathrm{mi})\) ? Why? What factors explain the lower atmosphere's blue color?

Short Answer

Expert verified

At an altitude of 50 km, the sky would appear predominantly black because the atmosphere is too thin to scatter sunlight. The blue color of the lower atmosphere is due to Rayleigh scattering that favours shorter wavelengths, such as blue and violet light.

Step by step solution

Understanding Scattering

The color of the sky is primarily determined by the scattering of sunlight in the atmosphere. Scattering is the phenomenon where light collides with tiny molecules and gets sent off in different directions.

Understanding Rayleigh Scattering

Rayleigh scattering is a specific type of scattering that occurs when the particles causing the scattering are very small compared to the wavelength of the light (less than one tenth). It is named after Lord Rayleigh, who studied it in the 19th century. This scattering favours shorter wavelengths, such as blue and violet light. When sunlight passes through the atmosphere, blue and violet wavelengths are scattered in all directions in the sky, giving it a blue appearance to an observer looking from the ground up.

Why sky is not violet

Even though violet light is scattered more than blue light, the sky does not appear violet to us because our eyes are less sensitive to violet light and because sunlight is less intense in the violet part of the spectrum. Moreover, some of the violet light gets absorbed in the upper atmosphere, which is why the blue color dominates.

Understanding color change with altitude

As we go higher in the atmosphere, there are fewer and fewer molecules to scatter the sunlight. Hence, the amount of scattered light decreases and the sky looks darker. At about 50km altitude (30 miles), it will not be enough scattered light to make the sky appear blue. Therefore, it would appear black, as in space.

Why lower atmosphere is blue

On the other hand, the lower atmosphere seems to be blue because of the high density of air molecules which scatter shorter wavelength blue and violet light to a far greater degree than than longer wavelengths of light, such as red, yellow, and green. But because our eyes are less sensitive to violet light and because there is less violet sunlight to begin with, we see the sky as blue.

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What would you expect the sky color to be at an altitude of \(50 \mathrm{~km}(30 \mathrm{mi})\) ? Why? What factors explain the lower atmosphere's blue color?

Short Answer

Step by step solution

Understanding Scattering

Understanding Rayleigh Scattering

Why sky is not violet

Understanding color change with altitude

Why lower atmosphere is blue

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