Summer is coming, it is a good time to enjoy the sun

May 16th of every year is the International Day of Light established by UNESCO.

This international commemoration day is very young, having been established in 2018. Its original intention was to commemorate the birth of the laser on May 16, 1960.

The previous article introduced the principle of laser. It is because of the existence of laser that our nights become dazzling, and high-tech industrial products such as CDs, chips, optical fibers, and laser engraving have come into being, greatly improving the convenience of life.

laser

Image source: sztu.edu.cn

However, although laser is beautiful, it becomes a bit monotonous after looking at it for a long time.

Image source: gifer

The light and shadow of nature are truly unpredictable and fascinating. The passionate summer is coming soon. Today we will take stock of the strange optical phenomena in summer!

1. Shadow of the Air

It is common knowledge that opaque objects will cast shadows under sunlight. This is a classic example of "light travels in straight lines".

But did you know that the invisible and colorless air actually has a shadow ?

On a sunny morning with good lighting conditions, we can find a white wall with direct sunlight and observe it carefully.

Image source: Internet

We will most likely see some vague shadows on the white wall. They are like water flows, with erratic shapes and rising continuously.

This is the "shadow" of the air, which allows us to indirectly observe the flow of air.

Air always flows from low-temperature areas (high pressure) to high-temperature areas (low pressure), and during the flow, the density and temperature of the air change constantly.

Because air of different density and temperature has different refractive index for light, when a beam of light passes through flowing air, it cannot maintain straight-line motion due to the change in refractive index. Finally, some parts of the light projected onto the wall are bright, while others are slightly darker, thus forming patterns similar to ripples on the water surface.

This phenomenon can be seen on sunny days all year round, but the summer sun is more intense, which can heat the air near the ground in a short period of time, causing it to flow upward quickly, so it is easier to observe . After noon, the air temperature near the ground has reached a stable state, and violent convection no longer occurs, so it is no longer easy to see.

The air "shadow" above the candle flame

Image source: Sohu

The shadow of the air can also be observed above a high temperature heat source. For example, a raging fire can rapidly increase the temperature of the surrounding air, causing obvious light distortion. Asphalt can absorb a lot of heat under the scorching sun. Not only can the violent evaporation of air be observed on the road, but also a special optical phenomenon - mirage can be seen.

2. Mirage on the road

When talking about mirages, people may think of illusory oases in the desert, majestic skyscrapers on the sea, shadowy temples in the sea of ​​clouds, etc.

Although these mirages are spectacular, they are rarely seen.

In contrast, mirages on the road are more common and down-to-earth .

Mirage on the road

See watermark for source

On a hot and sunny summer day, if we are driving on the road, we will find a large pool of water on the road ahead. But when the car approaches, the water suddenly disappears and reappears not far ahead.

When I was a kid, I went cycling in the summer vacation. I was thirsty and tired on the hot road. I kept trying to ride towards the puddle of my dreams, but I always came up empty-handed. I felt very frustrated and confused. Later, when I grew up, I realized that this was a mirage phenomenon caused by the refraction of light.

Refraction of light as it passes through the interface of cold air (blue) and warm air (red)

Image source: Sohu

When light moves from hot air into cold air, it refracts toward the low temperature area; when light passes through multiple temperature layers of air in succession, it propagates along a curve.

Mirages can be divided into two types: superior mirage and inferior mirage. Superior mirages appear in the sky, when the air temperature near the ground (sea surface) is lower than that in the sky, and the light travels along an upward convex curve; while inferior mirages appear on the ground, when the air temperature near the ground surface (road surface, desert, etc.) is higher than that in the sky, and the light travels along a downward convex curve.

Superior mirage (left) and inferior mirage (right). Note that in an inferior mirage, the ground appears to be a mirror reflecting the sky.

Image source: Reference [1]

Therefore, the scorching hot air forms a mirror-like reflective interface on the surface of the road and the desert, and the so-called puddles are actually just reflections of the sky.

Mirage in Qaidam Basin

3. Beautiful Rainbow

The weather in summer is changeable, and short-term rainstorms often occur. When the rain stops and the sky clears, we can wait for the rare rainbow.

Seeing a huge colorful arch standing between heaven and earth, the shocking and pleasing feeling is hard to describe in words.

The rainbow is also caused by the refraction of light. Sunlight is composed of colorful monochromatic light, and these monochromatic light has different refractive indices, so it is easy to be separated by the dense water droplets in the air after rain in the form of refraction-reflection-refraction. The prism rainbow experiment in physics class is a typical example, which is also very familiar to us.

Schematic diagram of water droplet spectrometry

Image source: globalweatherclimatecenter.com

Next, let’s continue to explore the blind spots of knowledge: Why are rainbows always round?

The answer is simple:

Although a large number of small water droplets refract the sunlight in all directions, only the rainbow light in a specific area can be seen by us.

This "specific area" is a circular ring area, and its range is determined by the refractive index and reflectivity of light at the air-water droplet interface.

Draw two virtual cones with the human eye as the vertex. The circle formed by the base of the two cones is the rainbow that the human eye can see at this time. Only the 42° virtual cone is drawn in the figure.

Image source: blogspot.com

Light of different colors has different refractive index and reflectivity when passing through the air-water droplet interface, so they eventually propagate outward at a specific angle, ranging from 40° (purple light) to 42° (red light).

With the human eye as the vertex, draw two huge virtual cones in the air with semi-vertex angles of 40° and 42° respectively. The circular rings surrounded by the bases of these two cones are the circular rainbows we see.

The current angle of sunlight causes the center of the rainbow ring to be below the ground, so only part of the rainbow can be seen.

Image source: globalweatherclimatecenter.com

A complete rainbow can be seen from the plane

Image source: dailymail.co.uk

The position of this virtual ring will be different depending on the observer's location.

So when Xiao Ming sees a rainbow at position A, Xiao Hong can also see another rainbow at position B, and Xiao Liang at position C on the plane even has the chance to see a complete circular rainbow.

We can borrow Shakespeare's famous saying: There are a thousand rainbows in the eyes of a thousand people. So, cherish the rainbows you see, because each rainbow is a landscape that belongs to you.

4. Powerful Lightning

Summer storms also feature a unique natural phenomenon: lightning.

Strictly speaking, lightning is not an optical phenomenon, but an electrostatic phenomenon. However, this does not prevent us from appreciating the huge energy contained in lightning.

Lightning comes from the discharge of charged matter in the air.

Due to the mutual friction between water droplets, dust and other particles in the air, a cumulonimbus cloud with abundant water vapor can easily accumulate a large number of positive and negative charges. Generally, the upper part of the cloud is positively charged and the lower part is negatively charged. When the potential difference between the two is large enough, it will discharge through various channels, thus forming lightning. According to the different discharge methods, lightning can be divided into three types: cloud-to-cloud lightning, cloud-to-cloud lightning and cloud-to-ground lightning.

Lightning in a cloud (left), between clouds (center), and cloud-to-ground lightning (right)

Image source: Wikipedia

Before lightning is born, the discharge poles will first discharge at the tip, forming a path through the flow of electric charge. Because the atmosphere is always in a state of intense convection, the electric charge will spontaneously seek the route with the lowest resistance when flowing, thus forming a curved path like a river or tree roots. Once the path is formed, a huge current will pass through, forming a dazzling lightning.

Pine tree struck by lightning in the mountains

Photographed by the author

Due to the huge potential difference between clouds in the sky, the discharge voltage is as high as one million volts, and the temperature on the discharge path can reach tens of thousands of degrees Celsius. A lightning bolt lasts only a few microseconds, so even though its discharge power is high, it cannot release too much energy, "only" enough to chop down a tree.

But this is also very dangerous to the human body, so never go to open places to attract lightning during thunderstorms .

5. Other optical phenomena

In addition to the examples mentioned above, we can observe many other interesting optical phenomena in summer:

For example, the rainbows surrounding the sun and the moon (solar halos and lunar halos), the mottled light and shadows under the shade of trees are actually small suns (pinhole imaging), the bright stars in the night sky are constantly blinking (light refraction), the sea of ​​stars composed of fireflies (luminescence from chemical fluorescent substances), etc.

This summer, let us bring our curiosity and start our summer "light-enjoying" journey!

References:

[1] Basdevant, JL. (2023). Variational Principles. In: Variational Principles in Physics. Springer, Cham. https://doi.org/10.1007/978-3-031-21692-3_2

[2] What Atmospheric Conditions Cause the Appearance of Rainbows?. Global Weather & Climate Center. https://www.globalweatherclimatecenter.com/

[3] Lightning. Wikipedia. https://zh.wikipedia.org/wiki/%E9%97%AA%E7%94%B5

Author: Liuhuo

Engineer at a research institute of weapons industry