Why is the sky blue? When transmitted light such as sunlight enters our atmosphere it collides with the oxygen and nitrogen atoms. The color with the shorter wavelength is scattered more by this collision. Because violet and blue are the shortest wavelengths the sky appears to be violet / blue. But because our eyes are more sensitive to blue light than they are violet light, we perceive the sky as blue. Our eyes contain thousand of rods and cones, which are the receptors for light. Whenever one of the 3 Stooges pokes you in the eye you see a giant blue spot. This is because the blue receptors have been activated. Blue is one of the primary colors and thus more easily activated and seen by our eyes. Blue is also how I feel when my baby leaves and my hound dog dies. Also, how I feel when the cops pull me over and I see their blue lights flashing in my rear view mirror. Then, again, blue is the color of the K-mart special, so this color isn't all bad. Why is the sky blue: Summary So, why is the sky blue? It is because blue light from the sun strikes the air molecules and scatters and our eyes perceive it as blue. Why is the sky blue: Short Summary Why is the sky blue, you ask? Blue in sunlight collides with air molecules and our eyes see it as blue. http://science.howstuffworks.com/question39.htm Here is something interesting to think about: When you look at the sky at night, it is black, with the stars and the moon forming points of light on that black background. So why is it that, during the day, the sky does not remain black with the sun acting as another point of light? Why does the daytime sky turn a bright blue and the stars disappear? The first thing to recognize is that the sun is an extremely bright source of light -- much brighter than the moon. The second thing to recognize is that the atoms of nitrogen and oxygen in the atmosphere have an effect on the sunlight that passes through them. There is a physical phenomenon called Rayleigh scattering that causes light to scatter when it passes through particles that have a diameter one-tenth that of the wavelength (color) of the light. Sunlight is made up of all different colors of light, but because of the elements in the atmosphere the color blue is scattered much more efficiently than the other colors. So when you look at the sky on a clear day, you can see the sun as a bright disk. The blueness you see everywhere else is all of the atoms in the atmosphere scattering blue light toward you. (Because red light, yellow light, green light and the other colors aren't scattered nearly as well, you see the sky as blue.) Here are some interesting links: How Light Works How the Sun Works How Stars Work The Physics Classroom: Blue Skies and Red Sunsets Blue Sky and Rayleigh Scattering Weather World 2010: Sunsets Why isn't the Martian sky blue like the Earth's? http://www.sky-watch.com/articles/skyblue.html Home Articles Guide to Observing Tour of the Constellations Book Reviews Why Is The Sky Blue? High Noon “Why is the sky blue?” may seem to be a simple question, but it actually touches some of the deepest aspects of astronomy and skygazing because it deals with light. “Seeing” something whether it’s a table across the room or a galaxy millions of light years away consists of light entering our eyes and the brain taking the light and creating a picture within our brain. So before asking the question “Why is the sky blue?”, it’s necessary to know the answer to the question, “What is light?” What Is Light? Light consists of weightless particles (that contain no mass) traveling at about 186,000 miles or 300,000 kilometers per second. These particles of light are called photons. But light also exhibits the characteristics of a wave. Traditionally, wave activity is explained by imagining a pebble being dropped into a lake, but that imaginary lake has been filled with imaginary pebbles years ago. So instead, imagine a long rope tied to the side of a wall and a girl shaking rhythmically one end of the rope up and down. Each up and down shake of the rope is a cycle and the number of cycles per second is the wave’s frequency. If you observe the rope from the side, you will see a wave pattern of alternating crests and troughs. The distance from one crest to another is the wavelength. When the girl shakes the ropes more rapidly, the frequency will naturally increase, but the wavelength will shorten. Light acts in many ways similarly to the shaking rope. It also has a frequency and a wavelength. As the frequency increases, the wavelength decreases and as the frequency decreases, the wavelength increases. The frequency of light determines its color. Blue light has a higher frequency and a shorter wavelength than red light. Our eyes are sensitive to only a small part of the spectrum of frequencies. There is ultra- violet and infra-red light just beyond our eyes’ range, radio waves with much lower frequencies than light, and gamma rays with much higher frequencies. Sunlight contains many different colors which can be separated through a prism or a rainbow into different colors. In short, keep in mind the following three facts: Light must enter our eyes in order for us to see it The Sun emits light with different colors Different colored light has different wavelengths Sunlight and the Atmosphere The atmosphere around the Earth is largely made up of two colorless gases: oxygen and nitrogen. Red and blue light reacts very different from each other to oxygen. Because the wavelength of blue light is roughly the size of an atom of oxygen, blue light interacts with the oxygen and is scattered by it, while red light, with its longer wavelength, goes right pass the oxygen atoms. If the Earth had no atmosphere, the sun’s light would travel directly from the Sun in a straight line towards our eyes and we would see the Sun as a very bright star in sea of blackness. But because the Sun’s blue light is scattered by the oxygen in the atmosphere, blue light from the Sun enters our eyes from all sorts of different angles and we see the entire sky as blue. The atmosphere scatters violet light even more effectively, but our eyes are more sensitive to blue. Wherever we look towards the sky, some light is bouncing off an oxygen atom and entering our eyes, making the sky appear to be blue. Sunrise, Sunset Who hasn’t enjoyed watching a sunset as the Sun’s red disk sinks below the horizon. Why does the Sun now appear red? During sunrise and sunset, the Sun’s light must pass a greater distance through the atmosphere in order to reach our eyes because instead of dropping directly through the atmosphere, it reaches the Earth at an angle. The same scattering effect on the blue light, also takes place, but the blue light is unable to pass through the extra distance and reach our eyes. This leaves only the red light which passes, unhindered through the atmosphere and reaches our eyes in a direct line with little or no scattering. We see the Sun’s disk red because its blue light has been blocked by the atmosphere. We don’t see the entire sky red because there is no scattering and the red light reaches us in a direct line. http://www.csiro.au/resources/ps1ti.html Why is the sky blue? Follow these instructions to find out why the sky is blue and sunsets are red. What you need What to do What's happening On a clear day, the sky looks bright blue. At dusk, a sunset can show reds, pinks and oranges. Why is the sky blue? What makes the sunset red? Try this activity to find out. YO U WIL L NE E D To do this activity you need to collect: a large glass jar or vase water milk or powdered milk a torch a dark room WH AT T O DO 1. Fill the jar with water. 2. Add a few drops of milk or half a teaspoon of powdered milk. 3. Switch on the torch and shine down into the water - it should appear blue. 4. Shine the torch through one side of the jar. The torch should be near the middle. 5. Go to the other side and look at the light through the water. 6. The water should appear pink, while the area directly in line with the torch should appear a yellow-orange colour. WH AT ' S H APPE NI NG? When you look up in the sky it is actually scattered blue light that you are seeing. The sun produces white light, which is made up of light of all colours: red, orange, yellow, green, blue, indigo and violet - these are the colours you find in a rainbow. Light is a wave and each of these colours corresponds to a different frequency, and therefore wavelength, of light. The seven colours of the spectrum all have different wavelengths and they are arranged accordingly. Violet, indigo and blue light have a higher frequency - which means shorter wavelength - than red, orange, and yellow light. When the white light from the sun shines through the Earth's atmosphere, it collides with gas molecules. It is these molecules that scatter the light. Blue light has a high frequency, and it is scattered ten times more than red light. But why does the sky become pink and red at sunset? The sky changes colour because the atmosphere spreads the light in a different way according to the position of the sun. The red light that is not scattered continues on in its original direction. When the sun is on the horizon, its light takes a longer path through the atmosphere to your eyes than when the sun is directly overhead. By the time the light of the setting sun reaches your eyes, most of the blue light has been scattered out. The light you finally see is reddish orange, the colour of white light minus blue. In this activity you changed the colour you saw by changing the position of the beam of light. Similarly, the sky appears a different colour depending on the position of the sun. The water containing milk molecules causes the light to scatter in the same way the molecules in the atmosphere do. The atmosphere is the mixture of gas molecules and other materials surrounding the earth. It is made mostly of the gases nitrogen and oxygen. Argon gas and water are the next most common things. There are also small amounts of other gases, plus many small solid particles like dust, soot and ashes, pollen, and salt from the oceans. It is these molecules and particles that cause the scattering of light. The most brilliant sunsets occur when there are lots dust and smoke particles in the air - so beautiful sunsets often occur over polluted cities. This experiment was featured in Science by Email.