Have you ever wondered why the sky is blue? Or why the sunset is red? Or what color our sunset would be if we had a blue giant instead of a white star? This lab will answer those questions by showing how light is scattered by the atmosphere.


Particles in the atmosphere determine the color of the planet and the colors we see on its surface. The color of the star also affects the color of the sunset and of the planet.


Materials


  • Glass jar
  • Flashlight
  • Fingernail polish (red, yellow, green, blue)
  • Clear tape
  • Water
  • Dark room
  • Few drops of milk

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Download Student Worksheet & Exercises


  1. Make your room as dark as possible for this experiment to work.
  2. Make sure your label is removed from the glass jar or you won’t be able to see what’s going on.
  3. Fill the clear glass jar with water.
  4. Add a teaspoon or two of milk (or cornstarch) and swirl.
  5. Shine the flashlight down from the top and look from the side – the water should have a bluish hue. The small milk droplets scatter the light the same way our atmosphere’s dust particles scatter sunlight.
  6. Try shining the light up from the base – where do you need to look in order to see a faint red/pink tint? If not, it’s because you are looking for hues that match our real atmosphere, and the jar just isn’t that big, nor is your flashlight strong enough! Instead, look for a very slight color shift. If you do this experiment after being in the dark for about 10 minutes (letting your eyes adjust to the lack of light), it is easier to see the subtle color changes. Just be careful that you don’t let the brilliant flashlight ruin your newly acquired night-vision, or you’ll have to start the 10 minutes all over again.
  7. If you are still having trouble seeing the color changes, shine your light through the jar and onto an index card on the other side. You should see slight color changes on the white card.
  8. Cover the flashlight lens with clear tape.
  9. Paint on the tape (not the lens) the fingernail polish you need to complete the table.
  10. Repeat steps 7-9 and record your data.

What’s Going On?

Why is the sunset red? The colors you see in the sky depends on how light bounces around. The red/orange colors of sunset and sunrise happen because of the low angle the Sun makes with the atmosphere, skipping the light off dust and dirt (not to mention solid aerosols, soot, and smog). Sunsets are usually more spectacular than sunrises, as more “stuff” floats around at the end of the day (there are less particles present in the mornings). Sometimes just after sunset, a green flash can be seen ejecting from the setting Sun.


The Earth appears blue to the astronauts in space because the shorter, faster wavelengths are reflected off the upper atmosphere. The sunsets appear red because the slower, longer wavelengths bounce off the clouds.Sunsets on other planets are different because they are farther (or closer) to the Sun, and also because they have a different atmosphere than planet Earth. The image shown here is a sunset on Mars. Uranus and Neptune appear blue because the methane in the upper atmosphere reflects the Sun’s light and the methane absorbs the red light, allowing blue to bounce back out.


sunset-mars


Exercises


  1. What colors does the sunset go through?
  2. Does the color of the light source matter?

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Comments

19 Responses to “Sky in a Jar”

  1. You need something that allows light to pass and imparts a certain color to that light. Nail polish works best and there really isn’t a good alternative.

  2. khansamahmood1 says:

    i don’t have nail polish can i use something else?

  3. Putting clear tape over the plastic bezel and then coloring the tape with sharpies is an extra experiment suggested by another student in the comments. So we can’t really give details on that.

    It is best to first try the experiment how it is shown in the video. When doing this experiment, you need the flashlight to be as bright as possible.

    Not, that you want to use milk OR cornstarch, not both. It sounds like you may be adding too much milk or cornstarch.

    Instead of a couple teaspoons, add a only a couple drops of milk and then try the experiment. Then if it isn’t cloudy enough, add a couple more drops and try again.

  4. anonakris says:

    We used flashlights with tape over the light, colored with sharpies and did not see the colors separate out as mentioned in another comment. We had jars of water/milk and water/cornstarch. About 2 cups liquid to 1 teaspoon water/cornstarch. What did we do wrong?

    Also, the light did not penetrate through to the other side of the jar through either water/milk or water/corn starch, which was surprising.

  5. chaterbox86 says:

    thank you for making this project!

  6. emilyannejon says:

    It totally confused my younger siblings for a while! 🙂 I was such a magician 😉

  7. That sounds really neat! Thanks for sharing your results 🙂

  8. Melanie Gonzales says:

    We colored the flashlight tape with areas of blue, red, orange, and green sharpie and when we shined it through the bottom of the glass, the colors separated out in the water. Blue, then green, then orangeish yellow and red towards the top!

  9. Try it and tell me how it works! 🙂 (I think it should be fine…)

  10. Mikkelyn Adams says:

    Can I use colored Sharpies instead of fingernail polish?

  11. No, it just means that the LED has more blue than red wavelengths (which is normal). LEDs and Fluorescent lights look “cooler” (green-blue-purple) than incandescent or sunlight.

  12. I did the experiment using a torch that had an LED bulb. Instead of showing red, it was blue with a tinge of yellow! Is that normal?

  13. Jonathan Keller says:

    Thanks for the video but one correction. The speed of light is constant. It is the multiple of its frequency and wavelength that, when multiplied, gives the speed of light constant. Light bends or refracts when it travels from one medium to another. It is this temporary change of speed that causes a bending as the light in front slows down causing the faster light to “catch up” resulting in a bend. And since blue light bends the most at the junction of the two different mediums, blue light must have the greatest change in speed or the greatest reduction in speed.

    Anyway, keep up the great work. All the best.

  14. Crystal Burling says:

    i think inside maybe outside if its night time?

  15. You’re right – and there’s a whole section on this in Unit 9 Lesson 1 as well as in the experiments section of Unit 9 to help. This particular experiment is listed in a couple of different units, so if you aren’t looking at it from Unit 9 (Light), then it might be confusing and look incomplete! Let me know if you still need more info after reading up about Light from the Topic section.

  16. Kiirsi Hellewell says:

    This is a really fun experiment. But: we still don’t have our questions answered, even though we watched the video and read everything. This doesn’t talk at all about different wavelengths and speeds of light, so we still don’t understand the colors of sunsets and sunrises.very well. It would be helpful to have a video, animation or diagram or more explanation about light waves and speeds and how they affect colors.

  17. kalei jacks says:

    this was cool but then we dropped an orange colored straw in the jar to see what would happen and all the liquid glowed orange!

  18. It needs to be DARK, so if you live in a dark area (not in the city), it doesn’t really matter.

  19. sevy keble says:

    This is a great expirement! Question: Is this supposed to be done inside or outside?