Blue Skies All Around! (9/7/16)
Amaze your friends and family with this super simple science experiment which actually explains one of the greatest mysteries on Earth: Why is the sky blue?
- Clear container
- Glass of Milk
- Plastic drinking straw
- Flashlight (with batteries!)
- Start by pouring water into your clear container until it is nearly full. Be sure to leave some room because we will need to add more liquid later.
- Turn on your flashlight and shine the light into the water from the top of the container. Can you see the beam of light traveling through the clear water? If you look really closely, can you see any tiny dust particles floating in the water?
- Now grab your glass of milk. Dip the drinking straw all the way to the bottom of the milk glass, and then seal the open end of the straw with your thumb. By sealing the top end, you will create a vacuum that can hold the milk inside of the straw until you take your thumb off!
- With your thumb still firmly sealing the top end of the straw, hold the milk-filled straw directly above your container of water. Remove your thumb to let the milk pour into the water!
- With a spoon, stir the milk into the water until the mixture is evenly cloudy.
- Once again, shine your flashlight into the water from the top of the container. Can you see the beam of light now? What do you notice about the glowing light throughout the container? You may notice a slightly blue color in the milk. You have created a blue ‘sky’ inside of the container!
- Now hold your flashlight against the far side of the clear container so the light shines directly toward you through the solution. From the other side of the container, does the light still glow blue? What color do you see this time? Now you have created a yellow-orange ‘sunset’ inside of the container!
HOW DOES IT WORK:
When you add milk to the container of water, you are actually mixing tiny particles of solid milk fat into the solution. These tiny particles will scatter the light coming from the flashlight, just as gas molecules and dust particles in our atmosphere will scatter light coming from the Sun!
Both the white light that comes from our flashlight and the light coming from the Sun contain all of the colors of the rainbow (red, orange, yellow, green, blue, indigo, and violet). But these individual colors will scatter differently, depending on their wavelength.
Blue light has a much shorter wavelength than other colors of light, so it is more likely to bump into these particles and bounce away in different directions. When you hold the flashlight above the container and observe the light beam from the side, the milk solution appears blue because you are actually seeing the blue light being scattered outward to the sides! Similarly, the sky is blue during the day because we see the blue light scattering in all directions. The unscattered light makes the Sun appear slightly yellowish.
When you shine the flashlight into the opposite side of the container and observe the light directly through the container, it looks red because the blue light was removed by the scattering. This is also why the sky looks yellow during a sunrise or a sunset, and the Sun appears red!
YOU CAN ALSO TRY:
- Use containers of different sizes and shapes. Does it make a difference if the container has round sides or flat sides?
- More milk! With more particles of milk fat in the solution, does the blue light scatter more or less? Are the colors as bright as before? Why or why not?
- Try out other milk products to see what works the best. Milk comes in different concentrations of milk fat. Skim milk has a lot less fat than 2%, which has even less than whole milk does! Why not try a dollop of whipped cream?
- Turn out the lights! If you are in a very well-lit room, too much ambient light might ‘wash out’ your results. Try turning down any lights in the room for a better effect!