Question

In the absence of atmosphere, the sky appears,
A. colored.
B. blue
C. indigo
D. black

Answer 1

Hint: We see various different colors due to the wavelength of each color. Each color has a specific wavelength. Each wavelength activates different cells in the retinas. These wavelengths are scattered as they travel a long distance through the atmosphere. We will try to find the relation between wavelength and scattering.

Complete answer:
From our daily observation, we can confirm that sky is usually blue during the day time. Now if we ponder a little over the question that why is sky blue, we can find the answer in a couple of words “scattering of light”. If you have done your homework right, we know that the Sun emits white light. This white light travels to earth and enters the earth’s atmosphere. As soon as it enters the atmosphere the light is scattered by the air molecules and other impurities like the dust particles. We already know that white light is made of 7 different colors or the colors of a rainbow. Newton has proved this in his experiment of prism. Among these colors, Red color has the maximum wavelength of about 750 nanometers and blue color has about 400 nanometers. So, the blue light is scattered the most. Others light stay together in the same spectrum. And as a result, we see the sky is blue. Now what would happen if there is no atmosphere to scatter the light? We can scratch our brains a little and change the perspective of viewing this question. So, let's say there is no light to be scattered instead of no atmosphere to scatter the light. The result would still be the same because there is no scattering in both conditions. So, what is the color of sky then? We very well know there is no light entering the earth's surface on the other side where there is night. And the color of night is black. So, we have our answer: the color of sky without an atmosphere would be black.

So, the correct answer is “Option D”.

Additional Information:
While thinking students might wonder that if the sun is bright and there are stars in every possible direction, why isn't the sky bright? This is something called the Olbers paradox. To answer this we must know that the universe is roughly 150 million years old and the light from distant stars has yet to reach because such stars haven't been around that long. Also, the universe is continuously expanding and this further stretches the wavelength of light making it invisible.

Note:
Students can directly relate the question to what is the color of space and still get the same answer as there is absolutely no atmosphere in space. And also, it is easy to remember that space is black.