Light and colors

1987/02/01 Barrenetxea, Tere - Elhuyar Fundazioa Iturria: Elhuyar aldizkaria

• Fisika

The cockpit faucet drips. Drop a drop per second, forming a small wave in the bath water. Once created, the wave expands towards the walls of the bath forming an increasingly larger circle, and once the wall sounds, it turns back towards the center. But on the return it has less force, the wave is weakened. After making this trip on two or three occasions, the wave will disappear completely. While this happens, the new drops will create new waves, with the same path.

The frequency of the waves is given by the number of waves passing by a point per unit of time. Think that in our bathtub only drops one drop per second, so the frequency will be 1 wave/second. The wavelength is the distance between the peaks of the two consecutive waves. In the example of bath we assume 45 cm. In our example, therefore, a single wave is generated per second and its distance with the previous one is 45 cm. From here we can conclude that these waves take a step of 45 cm in a second and is their speed. The speed of a wave is, therefore, the product of its wavelength by its frequency.

Marine waves, like bathing waves, are two-dimensional, that is, they propagate in two dimensions, that is, on the surface of the water. The sound waves, on the other hand, depart from the source and extend through space, so they are three-dimensional. In the sound waves the air is compressed at the top of the wave and between the two summits the density of the air is lower. When these waves come to our ears, we hear the sound and the tone of that sound will appear to us higher the frequency of the waves. Musical tones are an example of waves of different frequency. The basic tone of the central DO, for example, has a frequency of 265 waves/second. What is, therefore, its wavelength?. Or asking the same in another way, what distance would there be between two peaks if those waves were visible?

To answer this question it is necessary to know previously the speed of sound: the speed of sound in the air is 340 meters per second at sea level. As in the case of the bathroom, the wavelength is dividing the speed by the frequency, obtaining a wavelength of 1.3 meters of the basic tone of the central DO.

On the other hand, the human ear is not the perfect device to receive sound waves. There are frequencies too small (less than 20 waves per second) and too high (over 20,000 waves per second) that we cannot receive. Therefore, our ear, despite being perfectly adapted, has its limitations.

The waves of light are similar to those of sound. Both are three-dimensional and can be measured frequency, length and speed. But the waves of light have a curious aspect: they do not need any kind of support to propagate. The light that comes to us from the sun and the stars comes after a long journey through the space in which there is nothing. In this space, astronauts cannot be heard if the radio is not used, but they can be seen without any obstacles.

The frequency of light we see people is very high: About 600 billion waves reach our eyes in just a second. The wavelength of the visible light is 0.00005 cm.

We mentioned earlier that we heard the sound waves of different frequencies as a different tone. Something similar happens with light: the light waves of different frequencies create different colors. The frequency of the red light is 460 billion waves per second, while the purple light is 710. Each frequency will give a different color.

But human vision is also limited. Just as we do not hear sounds with too much or too little frequency, there are frequencies of light that we cannot see, that is, colors that we cannot see. Some have more frequency than the waves we can see (e.g., gamma rays, 100 trilions per second) and others are smaller (e.g., radio waves). Passing the spectrum of light from the highest to the smallest frequencies, we find gamma rays, X-rays, ultraviolet light, visible light, infrared light and radio waves. All these waves propagate in the vacuum and each is a different type of light, as is the normal visible light.

Visible light is the only visible light for people. If our bodies were able to transmit and receive radio waves or X-rays, we could communicate over long distances and explore very small things. Why have our eyes not evolved towards that direction? Then we will try to explain it.

Any material can absorb light from certain frequencies, but not from others. Each substance has its own hobbies. Some frequencies, such as gamma rays, are absorbed by all kinds of materials. As a result, gamma light cannot make long trips: all objects are absorbed by the air, so it disappears after traveling a few meters. The gamma rays emitted by the Sun do not have access to the Earth, since in the atmosphere they find on their way they are absorbed. The Earth, therefore, is totally dark for gamma rays.

Something similar occurs with X-rays and most frequencies of ultraviolet and infrared light. On the contrary, the absorption of visible light is much lower in most materials. For example, air is generally transparent for visible light. In the case of "smog" or pollution of this type, these small particles in the air absorb part of the visible light and reflect another, so we see the colored air in these cases. This is the phenomenon that gives Bilbao a pardo-known color.

The light that will serve us, must necessarily be able to expand through the atmosphere without being absorbed. Gamma rays present a low usability due to their rapid absorption. A large proportion of the energy emitted by the Sun is of the type of visible light. Therefore, this would be another reason to adapt our eyes to this light and not to that of other frequencies.

Let's see now what colors are. When the light reaches any green plant, the red and blue frequencies are absorbed, while the green is reflected. That is why it seems to us that this plant is green.

Graphs can be made by analyzing the proportion of light that reflects each color. Any object that absorbs the red light and reflects the blue, will be blue for us. We see something white when it reflects all the colors approximately in the same measure. But this also serves for the gray and black color. The difference between black and white does not lie in the reflected light frequency, but in the luminous proportion that is reflected.

The brightest object we know is, without a doubt, a precious snow. But it only reflects 75% of the light. At the other end would be the black balus or velvet, which only reflects a small proportion of the light that attacks it.

Therefore, to say that two things are as different as white and black, at least from this point of view does not make much sense, because white and black are the same. The difference is in the proportion of light that does not absorb or reflect, and not in color.

The living use the colors to absorb sunlight and produce energy through photosynthesis, to remind their parents where they have the mouth, to attract the attention of insects, to hide them, etc. Long. All this is due to the nature of light, the physics of stars, the chemistry of the air and the evolutionary process.