Light, which allows us to see the world around us. We gather first-hand information about the physical world through the perception of light and observe how they change. From this perspective, this ability of light to carry and transmit information is perhaps its most important and distinguishing characteristic.
Light gives life to all things. It is no exaggeration to say that light is tied to the biological and chemical reaction processes that allow us humans and even the Earth to exist. It can be said that light shapes our perception of the things around us. This importance of light can also be understood in terms of our everyday experience. For example, we use light for illumination, either natural light from the sun or the moon or artificial light.
To discuss what light is, one can start with some very common properties of light: brightness, intensity, color, and temperature. All of these perceptible properties indicate that light is a material entity. But what exactly is light? We can start by talking about the brightness of light, using a household light bulb as an example. The power of a household light bulb is usually several tens of watts (power is measured in watts, abbreviated as watts and expressed as W, representing the energy consumed per second), with the exact value varying depending on the bulb model. A 50W bulb is enough to illuminate an entire room, while car headlights are generally slightly more powerful, between 60W and 100W. The floodlights used to illuminate soccer fields are even more powerful, up to several kilowatts.
I will discuss in more detail how light is produced by these different sources, but by the amount of power, we already have a concrete idea of the brightness of light. There is no doubt that the sun is one of the brightest light sources. It radiates a tremendous amount of energy, with a power of over 1025 W. (The number 1 is followed by 25 zeros!) Because the brightness of the sun is so great, even though it is very far away from us, we still cannot look directly at the sun.
The light discussed above looks dimmer the further away it is from us. Therefore, power is not the only indicator of brightness. To some extent, brightness is related to the proportion of energy we receive from a light source. For example, a laser pointer has much less power than a light bulb, usually only a few thousandths of a watt (less than 10-2W or 10mW), but it looks very bright when it shines on the screen. This leads us to the next concept related to the brightness of light. This important concept is the intensity of light (more accurately expressed as “irradiance,” but people may be more familiar with the term “intensity”), that is, the energy of light received by the receiver per unit area.
The intensity of light depends on the light source’s ability to concentrate light. The light emitted by a laser pointer looks bright because its beam is concentrated in a small spot, compared to the sun’s light, which spreads over a large area. Therefore, even though the sun outputs a lot of energy, the light it emits is not as intense as a laser pointer. The basic characteristic that describes a light source’s ability to concentrate light is called “light source coherence”. This has to do with the tendency of a light source to emit light in a particular direction. For example, the sun and light bulbs always radiate light in all directions, which is why we can see the sun from any corner of the earth and light bulbs from any part of the room. But a laser pointer only emits light in one direction, the direction in which the laser pointer is pointed. If the laser does not shine on a surface, people can not see the laser, this is because the laser beam has a clear pointing.
This experiment performed by Newton is world famous. The first part of the experiment is similar to what Descartes (Descartes) and others did earlier: let the sun’s light pass through a small hole located on a dark screen, only a small beam of light can pass through the small hole. Let this small beam of light pass through the prism and shine on the screen. At this point, we will see a rainbow-like band of colors on the screen. Newton believed that this series of colors is the color of white light is broken down, and these colors have universal. Goethe was so fascinated by this phenomenon that he borrowed some prisms from a local nobleman and did his own experiments. He soon came to the conclusion that Newton’s experiment was completely wrong, because Goethe himself had discovered a completely different set of colors.
In Goethe’s experiment, he looked through the prism to the window frame. He did the exact opposite of what Newton did, he observed a black line against a bright background, so he saw a completely different color than what Newton observed. In contrast to the red, green, and blue observed by Newton, Goethe observed cyan, magenta, and yellow. This set of colors is the complementary colors to the color spectrum observed by Newton. Combining the colors Newton saw together yields white, and combining the colors Goethe saw together yields black. While Goethe believed that color is a thing we perceive, Newton believed that it is an inherent property of light. Both of their views were actually correct. Today, we have separated the physical properties of color from its physiological properties (the perception of color).
Individuals respond differently to color. In fact, based on this principle, colored light can even be used for medical treatment. From an artistic point of view, our consciousness is born from the perception of a particular color of light, and this interpretation is important and can be understood simply as the perception of color is very important. However, from a physical point of view, we can explicitly assign a fundamental physical characteristic to the label “color” – frequency – at least until we enter the field of quantum optics. The range of light extends well beyond the visible spectrum. It extends from the blue visible end to the invisible region, passing in turn through the ultraviolet and far-ultraviolet spectral regions, and then to the X-ray and gamma-ray spectral regions. From the red visible end of the spectrum in the opposite direction, it passes through the infrared, microwave, radio wave and T-ray [1] spectral regions.
To “see” them, it is not enough to use the naked eye, we also need to use a variety of other tools, but at least we already know that these “colors” of light exist. For example, we can feel the sun’s temperature because our skin absorbs the infrared radiation from the sun; low-frequency microwaves are often used for cell phone communication, and can also be used to cook food by heating the water in it. Invisible light of shorter wavelengths is also common, such as ultraviolet radiation from the sun that causes sunburn, and X-rays are often used for medical imaging. X-rays are also used in many non-medical applications, for example, X-ray diffraction patterns can be used to reveal the structure of molecules or solids. When X-rays are directed into a molecule or solid, the X-rays scattered through these atoms will form a certain pattern if their constituent atoms are regularly arranged.
Even if the distance between the atoms is one ten thousandth of the diameter of a human hair, we can infer the structure of the arrangement of the atoms from that pattern. Perhaps the most famous example is that of James Watson and Francis Crick, who more than half a century ago determined, from X-ray diffraction patterns taken by Rosalind Franklin (Figure 3, right) and Maurice Wilkins, the the molecular structure of DNA. This discovery led to an understanding of the mechanism of molecular replication and brought about a great change in the field of biomedicine.
