Bendy Light
Why do we see heat hazes and mirages when it is hot?
Figure 1
Both of these effects can be attributed to a phenomenon called refraction. Refraction is when light changes mediums (the substance it travels through), causing its path to change. Light does this because it changes speeds as it travels through different mediums. In Figure 1, the light changes mediums from the air to the glass. The light goes from fast to slow because it enters a denser medium. It makes sense if you think about it. We run slower in water than we do in the air. The reason for a speed change resulting in a path change can be explained by a more advanced topic that we won't cover in this article, called Fermat's principle. However, we can predict how the path of the light will change using a formula called Snell's Law.
Figure 2
Refer to Figure 2. Let's break down what's going on here. The ray of light is going from the air into the water. The light will move slower in the water because it is denser. To analyze this, we can draw an imaginary line perpendicular to the boundary of the substances called the normal (dotted line in Figure 2). The ray in the air makes an angle with the normal called the angle of incidence. Once the ray passes into the water, it makes another angle with the normal called the angle of refraction. Snell's law tells us that when a light ray goes from high to low speeds, the angle of refraction is less than the angle of incidence. This explains why the angle of refraction is less than the angle of incidence when the light goes from air to water. Similarly, when a light ray goes from low to high speeds, the angle of incidence is less than the angle of refraction. Take a second to look at the figure and understand what these statements mean.
Figure 3
Let's talk about heat hazes now. An example of a heat haze is in Figure 3. The region behind the jet's exhaust looks blurry. It's obvious that heat hazes occur in regions of high temperature. When air has a high temperature, it means that, on average, its particles have greater speeds and there is more space between the particles. As a result, we can generalize that warm air is less dense than cold air. This means that light will travel faster in warm air as compared to normal air. You may have already inferred that this change of speed causes refraction.
Figure 4
For example, let's look at Figure 4. The road absorbs a lot of heat from the sun because it is black and made from asphalt. As a result, the region of air directly over the road is warmer than the air surrounding it. As light rays travel from the road to the camera's lens it passes through the medium of hot air and the medium of cooler air around it. As the rays move between hot and cold, they fluctuate in speed, causing refraction that makes the blurry spot in Figure 4.
Mirages are phenomena when a liquid appears to be spilled on the surface some distance away. This happens because of the path that light rays from the sun take to reach the viewpoint. Refer to Figure 5. As the rays from the blue sky come down towards the ground, it starts curving as it refracts through the warm air. The reason for this curved shape can be explained by snell's law. As the ray goes to higher speeds, its angle of incidence is less than its angle of refraction, making a shape resembling an upward-facing parabola. This ray continues to the person's eyes. The person's mind sees only blue rays that seem to be coming from a spot on the ground. This reflection of the blue sky is what is perceived as the liquid body in a mirage.
Figure 5