Has a little kid ever asked you why the sky is blue? Have you ever been able to answer this little kid? Well, whether you have or have not, Wednesday Science is coming to the rescue!
First, we need to talk about what makes something appear a certain color. White light, which shines on most objects, is actually the combination of all different colors of light. You can see this if you shine white light through a clear prism, as prisms split white light into its components. When an object appears as a certain color, that is because it reflects that color of light to your eyes, and absorbs all other colors. For example, a leaf absorbs every single color of light except green, which it reflects. This makes your eyes perceive the leaf as green, as they can only see the green light that is being reflected off the leaf. The colors that are absorbed by the leaf cannot be seen. A bit perplexing at first glance, but it makes sense once you think about it!
Another concept important to explain is wavelength and frequency of light waves. As the name suggests, wavelength is… the length of the light wave! Well, more specifically, it is the length of the distance over which the wave repeats (see diagram). The frequency of a wave is the number of times the wavelength repeats over a certain time frame, like a second. The general trend with light waves is the more energy the wave carries, the greater the frequency and the shorter the wavelength. Red, orange, yellow, green, blue, indigo, and violet are ordered by increasing frequency and energy, and decreasing wavelength. These properties are what help them separate when they travel through a prism!
Source: Wikimedia Commons
A nice real-world connection of wavelength and frequency is the Doppler effect. When something is moving towards you, the waves get squishy squished, resulting in a shorter wavelength and higher frequency. When something is moving away from you, the waves get stretchy stretched, resulting in a longer wavelength and smaller frequency. Ever been scared of that police car that came towards you but hopefully wasn’t coming for you? The sound waves (still waves!!) might have seemed higher-pitched (higher frequency) as it came towards you because of the Doppler effect! The same goes for stars moving away from us; they might seem reddish, a color associated with low frequency and long wavelength.
So, going back to the little kid from before, why is the sky blue? It's because of how sunlight scatters in the atmosphere. Blue light has a high frequency and thus makes particles it passes through move back and forth really fast. The faster the back-and-forth movement, the better the light scatters, so blue light scatters very well. Violet light actually has a higher frequency, but there is more blue light than violet light in sunlight and blue light is picked up by our eyes better. Another super duper cool thing is that the reason why the sky is blue is actually why some people’s eyes appear blue! The light is scattered in the person's iris because there is no melanin protein to absorb the light!
Photo credits: Chris Lawton, Unsplash, https://unsplash.com/photos/5IHz5WhosQE
Post By: Roshni P. M. and Hannah W.