Light demonstration (Britanya Fields)

Title: Why is the sky Blue but the sunset Red?

Principle(s) Investigated: Spectrum of sunlight; Red and Blue wavelengths. Diffusion. Scattering.

Standards : CA- California K-12 Academic Content Standards

Subject: Science

Grade: Grade Seven

Area: Focus on Life Science

Area: Investigation and Experimentation

Sub-Strand 7: Scientific progress is made by asking meaningful questions and conducting careful investigations. As a basis for understanding this concept and addressing the content in the other three strands, students should develop their own questions and perform investigations. Students will:

Standard a.: Select and use appropriate tools and technology (including calculators, computers, balances, spring scales, microscopes, and binoculars) to perform tests, collect data, and display data.

Materials:

* A Clear Tall Glass or Glass Beaker (obtained from home or the classroom)

* A flashlight (Hardware store)

* Skim Milk (Grocery store)

* Blank white card or Paper (Stationary store)

* Something to mix with. This can be anything from a spoon, to a stick, to a straw. (Home or grocery store)

Procedure: 1. Write a hypothesis to explain why you think that they sky is blue and sunsets

are red.

2. Fill the tall clear glass with water. Turn off the lights. Place the flashlight so that the

beam shines through the glass. Add a tiny amount of skim milk (about a tablespoon at a time); stir

until you can clearly see the beam shining through the liquid.

3. Look at the beam from the side of the glass and then from the end

of the glass. Write down your observations.

4. Let the light project onto a white card, which you hold at the end of the glass. Write down your observations.

5. What do you predict would happen if we added more and more milk to the glass?

Student prior knowledge: This activity would come at the end of a lesson explaining the principles of the light spectrum, wavelengths, diffusion, and scattering. Prior knowledge would just be the students awareness of the colors of the sky.

Explanation: We are using the flashlight, water, and milk as a model for what's happening in the sky. The flashlight is the sun and the milk is the molecules of gas in the atmosphere. Students should understand that molecules scatter the sun's light in the atmosphere and that is what the milk is doing to our flashlight's light.

The sun produces white light, which is made up of light of all colors on the spectrum; red, orange, yellow, green, blue, indigo, and violet. Light is a wave and each of these colors corresponds to a different frequency, and therefore wavelength, of light. The colors in the rainbow spectrum are arranged according to their frequency: violet, indigo, and blue light have a higher frequency than red, orange, and yellow light.

Sunlight passes through the earth atmosphere before we see it. The atmosphere has gas molecules in it like Nitrogen N2, and Oxygen O2. Lights runs into these molecules and is scattered in all directions which is called diffusion. The shorter the wavelength, the greater diffusion is. Because blue has a lower frequency or wavelength than red, it is diffused at a higher rate and the sky looks blue. In the evening when the sun is setting, it is further from us in the horizon. Because it is further away, the light has to pass through more atmosphere than it does in the daytime. The blue light, which was diffused more quickly and further away isn't visible to us anymore. Red and yellow, which isn't diffused so easily further out in the atmosphere is now visible

Questions & Answers: 1. One of the questions my students asked was, "is that the same reason why the ocean is blue?" The answer to that is no. The ocean is blue because water absorbs colors in the red part of the light spectrum. So all that is left to see is the blue light. When other colors are seen in the ocean it has to do with what particles are found in that area. For example, When there are a lot of algae they contain chlorophyll and other substances that absorb blue and red light which leaves the green light to be reflected back and that's what we see.

2. Another question my students asked was, "Why are clouds white?". First it is important to know that Clouds are made up of water droplets and ice crystals. Their water droplets or ice crystals are large enough to scatter the light of ALL the seven wavelengths. Since the light is not separated it remains white light and that is why clouds look white.

3. "Why can we see all the colors in a rainbow and not just Blue or red?" Isaac Newton discovered in 1666 that when he passed regular sunlight through a prism, the prism would split the light up into a band of colors which is all of the colors on the spectrum. When it rains, the air is filled with raindrops. These raindrops act like a prism. If sunlight passes through the raindrops at just the right angle it is split into its spectrum, which is made up of the colors of the rainbow.

Applications to Everyday Life: The questions my students asked were all applications to everyday life; why are clouds white, rainbows colorful, and the ocean blue. Colors of light can be seen all around us. Almost all objects scatter light. If the things around us would reflect light perfectly without scattering it would be a mirror. One thing that my students did not bring up is things that are colored without light, like fabric. If they would have I could have explained to them that it is all light. The absence of light is black. There is no "color" until we see it. Things like fabric or flowers or anything else around us is showing us color because it is absorbing all other wavelengths of light except the one we see, like the ocean.

Photographs:

Videos: Why is the sky blue? Why is the sunset red?

More advanced video