Colorful Cars

Today's article is a bit different. Usually, we talk about a confusing physics scenario that you've probably experienced before. Instead, today we'll talk about something that you've definitely experienced before, but also definitely never noticed. For example, we see cars driving around every day. But did you know that you see a different color when it's moving towards you versus away from you?

Before I can tell you why that's true, let's quickly review how light works. All the types of light that we can see are actually electromagnetic (EM) waves. Technically, they are a result of vibrations between an electric and magnetic field. Today, let's simplify the EM wave to being a simple 2-dimensional wave for an easier understanding. A central characteristic for all EM waves is frequency. Frequency tells us how quickly a complete cycle of a wave happens. In other words, frequency quantifies how squished together the peaks (the highest point of a wave) of the waves are.

Take a look at Figure 1. Both the top and the bottom graphs are occurring in the same time period. However, the top graph has many more peaks and they are squished together. This tells us the top wave has a high frequency. On the other hand, the bottom wave has peaks spread apart, signifying a low frequency.

Let's start coming back to our scenario with the car. Let's suppose that this car is green, a color that's right in the middle of the visible light spectrum. If the car is outside, white light from the sun will travel towards the car. Since the car is green, every other color in the white light is absorbed and only green light is reflected off the car. When this EM wave of green light reaches our eyes, we can understand it as being green being on the car's surface. The key concept to understand is that we only see the car as green because the EM wave that's reflected off the car has green's frequency. If this frequency were to somehow change between the point where the wave reflects off the car and the point where it reaches our eyes, we would perceive the car to be a different color.

So how is it possible for that to happen? Imagine that the car is no longer stationary. You're standing in front of the car as it drives towards you. As the car approaches, light is continually being reflected off the car in your direction. This causes the EM waves to get an extra "push" in the direction of its motion because the car itself is moving. This is easier to understand with an image (Figure 3). Though this image shows the effect of a moving car on sound waves, the same principle applies: the Doppler Effect.

For an observer standing on the left in Figure 3, when the car is moving towards them, the peaks are squished together, signifying a higher frequency relative to the stationary car's waves. Similarly, the car moving away will reflect light with a perceived lower frequency than green. Using the EM spectrum, we can deduce that since a car moving towards the observer has higher frequency waves reflected, the color gets shifted towards blue. On the other hand, the car moving away reflects lower frequency waves, shifting the perceived color towards red. The only reason we don't notice this happening every time a car drives by is that the effect is only happening to a very small extent. In fact, this concept of redshift is mostly used by astronomers to determine how the universe is expanding. Though we can't actually see redshift with objects on Earth like a car, you can trust the physics: It's definitely still happening.