experiment on why the sky is blue

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Why Is the Sky Blue?

Try this easy science experiment

• Ph.D., Biomedical Sciences, University of Tennessee at Knoxville
• B.A., Physics and Mathematics, Hastings College

The sky is blue on a sunny day, yet red or orange at sunrise and sunset. The different colors are caused by scattering of light in the Earth's atmosphere . Here is a simple experiment you can do to see how this works:

Blue Sky - Red Sunset Materials

You only need a few simple materials for this weather project :

• Transparent container with flat parallel sides
• Flashlight or cell phone light

A small rectangular aquarium works well for this experiment. Try a 2-1/2-gallon or 5-gallon tank. Any other square or rectangular clear glass or plastic container will work.

Conduct the Experiment

• Fill the container with about 3/4 full of water. Turn on the flashlight and hold it flat against the side of the container. You probably won't be able to see the beam of the flashlight, although you may see bright sparkles where the light strikes dust, air bubbles, or other small particles in the water. This is much like how sunlight travels through space.
• Add about 1/4 cup of milk (for a 2-1/2 gallon container—increase the amount of milk for a larger container). Stir the milk into the container to mix it with water. Now, if you shine the flashlight against the side of the tank, you can see the beam of light in the water. Particles from the milk are scattering light. Examine the container from all sides. Notice if you look at the container from the side, the flashlight beam looks slightly blue, while the end of the flashlight appears slightly yellow.
• Stir more milk into the water. As you increase the number of particles in the water, the light from the flashlight is more strongly scattered. The beam appears even bluer, while the path of the beam furthest from the flashlight goes from yellow to orange. If you look into the flashlight from across the tank, it looks like it is orange or red, rather than white. The beam also appears to spread out as it crosses the container. The blue end, where there are some particles scattering light, is like the sky on a clear day. The orange end is like the sky near sunrise or sunset.

How It Works

Light travels in a straight line until it encounters particles, which deflect or scatter it . In pure air or water, you can't see a beam of light and it travels along a straight path. When there are particles in the air or water, like dust, ash, ice , or water droplets, light is scattered by the edges of the particles.

Milk is a colloid , which contains tiny particles of fat and protein. Mixed with water, the particles scatter light much as dust scatters light in the atmosphere. Light is scattered differently, depending on its color or wavelength. Blue light is scattered the most, while the orange and red light is scattered the least. Looking at the daytime sky is like viewing a flashlight beam from the side -- you see the scattered blue light. Looking at sunrise or sunset is like looking directly into the beam of the flashlight -- you see the light that isn't scattered, which is orange and red.

What makes sunrise and sunset different from the daytime sky? It's the amount of atmosphere the sunlight has to cross before it reaches your eyes. If you think of the atmosphere as a coating covering the Earth, sunlight at noon passes through the thinnest part of the coating (which has the least number of particles). Sunlight at sunrise and sunset has to take a sideways path to the same point, through a lot more "coating", which means there are a lot more particles that can scatter light.

While multiple types of scattering occur in the Earth's atmosphere, Rayleigh scattering is primarily responsible for the blue of the daytime sky and reddish hue of the rising and setting sun. The Tyndall effect also comes into play, but it is not the cause of blue sky color because molecules in air are smaller than the wavelengths of visible light.

• Smith, Glenn S. (2005). "Human color vision and the unsaturated blue color of the daytime sky". American Journal of Physics . 73 (7): 590–97. doi: 10.1119/1.1858479
• Young, Andrew T. (1981). "Rayleigh scattering". Applied Optics . 20 (4): 533–5. doi: 10.1364/AO.20.000533
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Why Is the Sky Blue: Experiment for Kids

The next time your kids inevitably ask, “Why is the sky blue?,” you can pull up this experiment for an at-home science lesson.         

Time: 10 minutes  

Materials    

• 1 clear drinking glass or small glass bowl  
• Eyedropper (optional)  
• Bright flashlight    

       Safety Tips and Setup Hints  

• A white LED flashlight works well with this lab, such as the flashlight in many camera phones.       
• It helps to see the blue color in the water if the room is darkened as much as possible.  

           Instructions  

• Fill the glass or bowl with water until it is mostly full.      

• Shine the flashlight through the water. What color do you see? (The color will be fairly light.)     

The Science Behind the Fun

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Earth’s air is made of several gases. Most of it is nitrogen, about 78 percent, about 21 percent is oxygen, there is a little bit of a gas called argon, a little water vapor, and very small amounts of gases, such as carbon dioxide and others. While our eyes can’t see the individual bits of gases, called atoms or molecules, we can feel them when the wind blows. The amounts of these gases change depending on the seasons, the altitude, the weather, whether there has been a volcanic eruption, or other reasons, but these are generally the amounts we measure.  

The light from the Sun is made of many colors, and we can see those colors when we split the Sun’s white light using a prism. When white light hits the gas molecules, most of the light—such as the reds, oranges, and yellows—is able to move its way through the molecules and pass to the ground to your eyes. However, the blue light is scattered around by the gases. It is this scattered blue light we see as a blue sky. In this lab, the big particles of milk in the water acted like the particles of air, scattering the blue light from the flashlight. You can try playing with the amount of milk in the water and the type of flashlight until you get the easiest blue color to see.     

    RELATED: Find STEM Classes for Kids in Your Area

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Why is the Sky Blue Science Experiment

In this Why is the Sky Blue science experiment, little learners can see for themselves how the blue wavelength is scattered faster than the red light from the sun. The simple experiment needs only a few things, but it can be a fascinating launch pad for discussions about the sky, the sun, the solar system, and even the weather. In fact, we will be featuring this experiment in our  Science Sleuths: Wild Weather  class!

Why is the Sky Blue? Science Experiment

Thought Question:   Why is the sky blue?

• Measuring cup filled with 2 cups of water
• A clear glass
• Safety goggles
• Paper towels

Directions:

• Put on your safety goggles and place a paper towel down on your work area in case the water spills.
• Fill the empty glass halfway with water from the measuring cup.
• Add one tablespoon of milk to the water in the glass.
• Shine the flashlight through the milky water* and watch as the water begins to turn blue.

*You may need to shine the flashlight up from the bottom of the glass to get the best results.

Conclusion:  The sky looks blue, because the blue wavelength scatters faster than the other wavelengths in the atmosphere. At sunset, we see the red and orange sky, because the blue has already been scattered out throughout the day.

PRINTABLE COPY:  YOU CAN DOWNLOAD A PRINTABLE COPY OF THE SCIENCE EXPERIMENT HERE.

If you are studying weather, you might also want to check out these weather-related resources: National Geographic Kids Everything Weather Book WeatherBug App Weather Station Experiment Kit The Kids’ Book of Weather Forecasting All About the Weather Coloring Book Is It Hot or Cold?: Learning to Use a Thermometer Book

WHY IS THE SKY BLUE? SCIENCE EXPERIMENT VIDEO

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Our experiment didn’t work! The milk didn’t turn blue. We tried several clear glasses/containers. Maybe too thick? Not sure.

That’s a very interesting experiment for kids

Why is the Sky Blue: Science Projects by Science Made Simple

Why is the sky blue - easy science projects.

These 3 fun and easy science projects will help you understand why the sky is blue and sunsets are red.

SAFETY NOTE: Please read all instructions completely before starting. Observe all safety precautions.

Table of Contents

• The Atmosphere
• Light Waves and Colors
• Why is the sky blue?
• Blackness of Space
• Red Sunsets
• Learn More: The Atmosphere
• Easy Reading

Project 1 - Split light into a spectrum

What you need:

• a small mirror, a piece of white paper or cardboard, water
• a large shallow bowl, pan, or plastic shoebox
• a window with direct sunlight coming in, or a sunny day outdoors

What to do:

• Fill the bowl or pan about 2/3 full of water. Place it on a table or the floor, directly in the sunlight. (Note: the direct sunlight is important for this experiment to work right.)
• Hold the mirror under water, facing towards the sun. Hold the paper above and in front of the mirror. Adjust the positions of the paper and mirror until the reflected light shines on the paper. Observe the colored spectrum.

What happened:

The water and mirror acted like a prism, splitting the light into the colors of the spectrum . (When light passes from one medium to another, for example from air to water, its speed and direction change. [This is called refraction, and will be discussed in a future issue.] The different colors of light are affected differently. Violet light slows the most, and bends the most. Red light slows and bends the least. The different colors of light are spread out and separated, and we can see the spectrum.)

Project 2 - Sky in a jar

• a clear, straight-sided drinking glass, or clear plastic or glass jar
• water, milk, measuring spoons, flashlight
• a darkened room
• Fill the glass or jar about 2/3 full of water (about 8 - 12 oz. or 250 - 400 ml).
• Add 1/2 to 1 teaspoon (2 - 5 ml) milk and stir.
• Take the glass and flashlight into a darkened room.
• Hold the flashlight above the surface of the water and observe the water in the glass from the side. It should have a slight bluish tint. Now, hold the flashlight to the side of the glass and look through the water directly at the light. The water should have a slightly reddish tint. Put the flashlight under the glass and look down into the water from the top. It should have a deeper reddish tint.

The small particles of milk suspended in the water scattered the light from the flashlight, like the dust particles and molecules in the air scatter sunlight. When the light shines in the top of the glass, the water looks blue because you see blue light scattered to the side. When you look through the water directly at the light, it appears red (like a sunset or sunrise) because some of the blue was removed by scattering.

Project 3 -Mixing colors

• a pencil, scissors, white cardboard or heavy white paper
• crayons or markers, a ruler
• a small bowl or a large cup (3 - 4 inch, or 7 - 10 cm diameter rim)
• a paper cup
• Use the bowl to trace a circle onto a piece of white cardboard and cut it out. With the ruler, divide it into six approximately equal sections.
• Color the six sections with the colors of the spectrum as shown. Try to color as smoothly and evenly as possible.
• Poke a hole through the middle of the circle and push the pencil part of the way through.
• Poke a hole in the bottom of the paper cup, a little bit larger than the diameter of the pencil. Turn the cup upside down on a piece of paper, and put the pencil through so the point rests on the paper on a table. Adjust the color wheel's position on the pencil so that it is about 1/2 inch (1 - 2 cm) above the cup.
• Spin the pencil quickly and observe the color wheel. Adjust as necessary so that the pencil and wheel spin easily.

The colors on the wheel are the main colors in white light. When the wheel spins fast enough, the colors all appear to blend together, and the wheel looks white. Try experimenting with different color combinations.

Learn more about colors using chromatography

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John Tyndall's blue sky apparatus

Have you ever wondered why the sky is blue? The answer was first discovered over 150 years ago right here at the Royal Institution using this tube.

Date : 1869

Place made : The Royal Institution of Great Britain, Basement Laboratory

Materials : Copper alloy, iron, glass, wax

Measurements : H: 495mm, W: 888mm, D:109mm

Description

John Tyndall  was a keen mountaineer and spent quite a lot of time in the Alps, both climbing and investigating phenomena such as glaciers. This interest in nature can also be seen in many of his other diverse discoveries, including his discovery in the 1860s of why the sky is blue in the day but red at sunset.

Tyndall began to experiment with light, shining beams through various gases and liquids and recording the results. He used this simple glass tube to simulate the sky, with a white light at one end to represent the sun. He discovered that when he gradually filled the tube with smoke the beam of light appeared to be blue from the side but red from the far end.

Tyndall realised that the colour of the sky is a result of light from the sun scattering around particles in the upper atmosphere, in what is now known as the ‘Tyndall effect’. He thought that the light scattered off particles of dust or water vapour in the atmosphere, like the smoke particles in the tube, but it’s now known that the light scatters off the molecules of the air itself.

Tyndall knew that white light was made up of a whole rainbow of coloured light and thought that the blue light appeared because it was more likely to scatter off the particles. We now know that this is because it has a much shorter wavelength than red light and is much more easily scattered, so to our eyes the sky looks blue.

This experiment also explains why the sky often appears to be red in colour as the sun sets. As the sun gets lower in the sky the angle means that the light we see passes through more atmosphere. By the time it reaches us the blue light has already scattered off, leaving the longer wavelength red light to be seen.

Tyndall’s blue sky tube is a very simple but effective way of demonstrating this scattering effect. You can easily create your own version using a glass beaker of water: shine white light through the water and slowly stir in a few drops of milk at a time and see what happens.

Where can I view this?

This object is currently on display on the lower ground floor of the Faraday Museum .

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