# More about frequency and wavelength

Frequency is a measure of how often an event repeats itself. In physics, it is generally used to describe waves. A "wave event" is measured between the two points of a completion. Frequency is the number of completions (or oscillations) during a given period of time. The SI unit for frequency is hertz, where one hertz is equal to a single vibration per second.

### Wavelength

On the earth there are different kinds of waves, from the waves of the ocean, which are produced by the wind, to the electromagnetic waves. The properties of electromagnetic waves depend on the wave length. In particular:

• Have Gamma rays Wavelengths up to 0.01 nanometers (nm).
• Lie X-rays between wavelengths of 0.01 nm and 10 nm.
• Are the wavelengths of ultraviolet light invisible to the human eye between 10 nm and 380 nm.
• Is the visible spectrum of colored light between 380 nm and 700 nm.
• Is the wavelength range of infrared light invisible to the human eye between 700 nanometers and 1 millimeter.
• Follows Microwave radiation with 1 millimeter to 1 meter.
• Have Radio waves A wavelength range of 1 meter and higher.

In this article we focus mainly on electromagnetic radiation and light. We will consider mainly the spectrum from UV light to infrared light.

Electromagnetic radiation is energy that has the properties of both waves and particles, which is called wave-particle duality. Its wave component is a composite wave of the magnetic and electric waves that oscillate in space perpendicular to each other.

The particles that carry electromagnetic energy are called photons. They are most active at higher frequencies. The higher the frequencies (and the smaller the wavelengths), the more damage the photons can do to cells of living organisms. The reason is that at higher frequency, photons have more energy and force particles more to change the molecular structure of tissue and other matter. In particular, ultraviolet, X-ray and gamma rays are very harmful. Some of the high frequency cosmic electromagnetic radiation is blocked by the ozone layer, but still occurs in the environment.

### Electromagnetic radiation and the atmosphere

Only a part of the electromagnetic radiation can penetrate the atmosphere of the Earth. Most of the gamma and X-rays, as well as UV, infrared light and radio waves with long wavelengths are blocked. More precisely, they are absorbed by the atmosphere. Some of the electromagnetic radiation, especially short-wave radiation, is reflected by the Earth’s ionosphere. The rest of the radiation can penetrate the atmosphere. Therefore, at high altitudes, for example in the higher atmospheric layers or above the Earth’s atmosphere, exposure to harmful radiation is much greater than on the Earth’s surface.

The ultraviolet light, which can penetrate to the earth’s surface, causes skin damage (sunburn and skin cancer). On the other hand, the infrared light that passes through the atmosphere is useful to astronomers. They use it when observing space with infrared telescopes. The higher the altitude, the more infrared light is present. Therefore, observatories that use infrared equipment are built as high as possible, for example, on top of a mountain. Some telescopes are brought above the atmosphere and into space to allow better detection of infrared radiation.

### The relationship between wavelength and frequency

Wavelength and frequency form a proportional inverse. This means that as the wavelength increases, the frequency decreases. Conversely, the lower the wavelength is, the higher the frequency is. This makes sense because if a wave oscillates frequently (its frequency is high), there must be more peaks in the time period. Therefore the time between waves is shorter.

If you multiply the frequency by the wavelength, you get the wave speed. Electromagnetic waves always travel at the same speed in a vacuum, called the speed of light. It corresponds to 299.792.458 meters per second.

## Light

Light is an electromagnetic wave and therefore has a frequency and a wavelength. The wavelength determines the color of light, as described below.

### Wavelength and color

The shortest wavelength for visible light is 380 nanometers for violet light. The spectrum goes through indigo and blue to green and yellow, orange and finally red. Visible light can be divided into its components with the help of a prism. This is possible because the wavelengths are different for each color. When the light in the prism is deflected, it exits the prism at different angles depending on that wavelength. This phenomenon is called dispersion. Regular white light projects an image of colors in the same order as they appear in a rainbow.

A rainbow is created in a similar way. Water drops work like a prism and cause light to split into the waves of its components. The colors of a rainbow have played a very important role in human culture and we use them frequently in everyday life, so there are mnemonic devices in many languages to teach children the colors of the rainbow at an early age. In English, for example, there is the fictional character Roy G. Biv. Each letter represents the first letter of the colors of a rainbow. The letters are in the order they appear in the rainbow. There is also the saying "Richard of York gave battle in vain". Some people also create their own mnemonic bridges.

The human eye is most sensitive to light when it is at a wavelength of 555 nm and bright and when it is at a wavelength of 505 nm and dim. Not all animals can recognize colors. Cats, for example, do not have color vision. Other animals, on the other hand, can distinguish colors much better than humans can. They can even detect ultraviolet and infrared light.

### Reflect color

If an object has a certain color, it means that light of a certain wavelength is reflected (or emitted) from that object. Objects that appear white reflect all colors, while objects we perceive as black absorb all colors and reflect nothing.

A diamond is an example of an object that has a very high dispersion. A well-cut diamond resembles a prism. Light enters the diamond, reflects off its many sides, and exits again. This gives it an excellent sparkle. Glass cut in a similar way also sparkles. However, the chemical composition of the diamond makes it reflect the light better and it appears more brilliant. However, the cut is very important. If the edges are not correct and the cut is not wide enough or too deep, the light that entered the surface does not exit the surface again and is "lost". In a properly cut diamond, light enters, reflects once or twice from the sides, and then exits where we can see it. The diagram illustrates this.

### Spectroscopy

Spectral analysis or spectroscopy is used to understand the chemical composition of objects. This is especially useful when direct chemical analysis is not possible, such as with stars. One branch of spectroscopy is called absorption spectroscopy and measures the type of radiation an object absorbs. The chemical structure of the material determines what kind of light it will absorb, based on the wavelength. This is a useful tool when analyzing what materials the object is made of. One can perform such an analysis from a distance. This is advantageous not only in astronomy, but also in the case of dangerous, fragile or very small objects.

## Discovering Electromagnetic Energy

Electromagnetic radiation is energy, just like light. That is, its recognition depends on the amount of energy emitted. The longer the wavelength, the less energy is emitted. The ability of animals to detect this energy and their sensitivity to certain amounts of light makes vision a reality. This ability allows animals to differentiate between different types of electromagnetic radiation, especially colors in visible light. The abilities of man-made technology to detect this radiation is based on the same principles.

### Visible light

Animals and humans can detect a range of electromagnetic energy. Many animals and people recognize in some way visible light. In some cases, animals can detect a range of colors; in others, they can only distinguish between light and dark areas. Photons enter the eye through the retina and are absorbed by the chemical components of the photoreceptors, called cones. The eye has another type of photoreceptor, but this one can’t distinguish between colors. It’s the rods. You determine how strong the light is.

There are usually different types of cones in the eye. People have three types of cones. They absorb photons of specific wavelengths that match visible light with a range of specific colors. This triggers a chemical reaction which in turn sends a neural signal through the nervous system to the visual cortex in the brain. It is the area that processes color information. The combination of information about how much each type of cone was stimulated is then used to determine the color seen.

While humans have three types of cones, other animals such as some species of birds or fish have four and five types of cones, respectively. In some species the females have more cone species than the males. Gulls that forage or dive for food at the water surface have red or yellow oil droplets in the cones of their retinas, as do many other birds. This oil works as a filter and allows birds to see more colors. Reptiles also have this property.

### infrared light

Snakes do not only have photoreceptors, they can also Infrared light recognize. Your sensors absorb the energy emitted by infrared light in the form of heat. Infrared can also be detected as heat by special devices like infrared goggles. This is a technology that is used in combat and in security. Also some bats can see infrared light, as well as some insects. Animals and devices that detect light based on temperature can usually determine if an area has been recently disturbed, for example, if a rodent has dug a hole in the ground or if a criminal has hidden something in the ground. Infrared is used in telescopes to find distant astronomical bodies. Other uses for infrared radiation include detecting temperature changes when looking for leaks in a building or enclosure, in art history, in meteorology, and in medicine.

### Ultraviolet light

Unlike humans, some fish species can ultraviolet light by absorbing it. Its visual system contains pigments that react to UV light. It is believed that this ability is useful for eating and choosing a mate as well as other social behaviors. Some birds can also detect ultraviolet light, and much like fish, this is used in courtship displays. Some plants and animal tissues reflect UV light. Birds use their UV sensitivity to find food. Some species of lizards, turtles and rodents also have this ability. The green iguana (picture) is an example of this.

The human eye can also absorb UV rays, but cannot detect them. Instead, cells of the retina, cornea and lens are damaged by prolonged exposure. It can cause a number of eye diseases as well as blindness. Similar to infrared light, UV light is used in a number of fields, such as medicine, disinfection, curing, chemical imaging, observatories, detecting counterfeit money and fake IDs if they have inks detectable with UV light. The latter does not always work, as some fake IDs are created from real ones by, for example, just replacing the photo. In this case, they have the markings that can be seen with UV light, just like the real badges. Small amounts of UV light are also needed by humans and some animals to produce vitamin D.

### Colorblindness

Defects in color vision cause inability to distinguish between colors. This can show at a certain wavelength or at all colors. The reason may be found in damaged or underdeveloped photoreceptors, but may also be associated with problems elsewhere in the neural pathway to the brain, including brain damage to the visual cortex where information is processed. In most cases, this is a disadvantage, but since many animals are colorblind, some scientists believe it is a trait that happened through natural selection and gave some species an advantage in evolution. For example, color-blind animals and humans can detect camouflaged animals better than those with intact color vision.

Despite some advantages, color blindness is considered a disadvantage in society. Some professions are reserved only for people with so-called normal color vision. In some countries, an unrestricted driver’s license is dependent on color vision. Professions that rely on processing color information, such as graphic design, or in which colors serve as warnings or directional indicators, generally cannot be performed by color-blind individuals.

To handle the problem with colorblindness, a number of tools are being developed, such as color code tables, where characters represent a color. These characters are sometimes used together with color coding in public places in several countries. Some graphic designers do not use color coding at all, or prefer a combination of colors and other visual information (such as brightness) to ensure that color-blind people can use the design. Since in most cases color blindness expresses itself as red-green deficiency, some designers are calling for abandoning the signaling function of "red = danger, green = ok" and using a red-blue combination instead. Some computer user interfaces also take color blindness into account and allow settings to be made for this under the operating aids.

## Color and Computer Vision

Computer vision is a rapidly developing field of artificial intelligence and color recognition is a sub-field. Until recently, a significant amount of research and development in computer vision was done without consideration of color. More laboratories are now incorporating color vision into their projects. Some algorithms that work with monochrome images are being adapted to color images.

### Applications

Applications for computer vision include robot navigation, self-driving cars and drones, security (facial recognition etc.), examining image databases, tracking objects based on their color, and many more. Following up can be very useful. This allows a computer to recognize the direction of a person’s gaze and follow the movement of various objects (cars, people, hands), etc.

For unfamiliar objects, other characteristics, such as shape, are more important for successful recognition. However, when there is repeated interaction with the same object, color can be useful in identification. Color does not depend on the resolution of an image, unlike shape. Therefore, processing based on color may be faster and use fewer resources. Colors also allow us to distinguish between objects of the same shape and, as with warnings, are a faster signal (z. B. red = danger) than allow the processing of shapes or writing in the case of warnings. More examples of the use of color vision by computers can be found on YouTube.

### Processing

The image to be processed is either taken by an integrated camera of the device or provided by the user. It is then analyzed by the computer system. While capturing images is a common process, there are many challenges with color processing because the way humans perceive color is not easily replicated. As with hearing, where we respond to frequencies, sound pressure levels and duration of sound, so with vision we gather information about color from frequency and wavelength in combination with other complex factors. Colors of surrounding objects affect the perception of a color, for example.

From an evolutionary point of view, this alignment is necessary so that we can adapt to the environment and not ignore important aspects of the environment but notice salient things. Our senses sometimes deceive us because of this adaptability. For example, we can perceive two objects that reflect light at the same frequency as being different colors because other objects surround them, as in the illustration of the famous optical illusion. Here, we perceive the brown square in the top half of the image (second row, second column) as brighter than the square in the second half of the image (fifth row, second column). In fact, both squares have the same color, but are perceived differently because the first is surrounded by darker colors, while the second is surrounded by lighter colors. For computer scientists it is difficult to develop algorithms that take these factors into account. Despite the difficulties, there is progress in this field.