A body oscillating in an elastic medium creates a perturbation that is transmitted from one point to another and is called a wave. This happens at a certain speed, which is considered the speed of its spread. That is, it is a quantity characterizing the distance traveled by any point of the wave in a single period of time.
Let the wave move along one of the axes (for example, horizontal). Its shape is repeated in space after a certain time, i.e., the wave profile moves along the axis of propagation with a speed of constant value. During the time corresponding to the period of oscillations, its front will shift by a distance called the wavelength.
It turns out that the wavelength is the very distance that its front "runs" over a period of time equal to the period of oscillations. For clarity, imagine a wave in the form in which it is usually depicted in the drawings. We all remember how, for example, sea waves look . The wind drives them along the sea, and each wave has a crest (maximum point) and the lowest point (minimum), both of which are constantly moving and replacing each other. Points lying in the same phase are the vertices of two adjacent ridges (we assume that the ridges have the same height and movement occurs at a constant speed) or the two lowest points of neighboring waves. The wavelength is precisely the distance between such points (two adjacent ridges).
In the form of waves can spread all types of energy - thermal, light, sound. They all have different lengths. For example, passing through the atmosphere, sound waves slightly change the air pressure. The areas of maximum pressure correspond to the maximums of sound waves. Due to its structure, the human ear picks up these pressure changes and sends signals to the brain. So we hear the sound.
The length of a sound wave determines its properties. To find it, it is necessary to divide the wave velocity (measured in m / s) by the frequency in Hz. Example: at a frequency of 688 Hz, a sound wave moves at a speed of 344 m / s. The wavelength in this case will be equal to 344: 688 = 0.5 m. It is known that the wave propagation velocity in the same medium does not change, therefore, its length will depend on the frequency. Low-frequency sound waves have a wavelength longer than high-frequency ones.
An example of another type of electromagnetic radiation is a light wave. Light is part of the electromagnetic spectrum visible to our eye. The wavelength of light that human vision can perceive ranges from 400 to 700 nm (nanometers). On both sides of the visible range of the spectrum lie areas that are not perceived by our eye.
Ultraviolet waves have a length less than the length of the visible part of the spectrum. Although the human eye is not able to see them, but, nevertheless, they are capable of causing considerable harm to our vision.
The wavelength of infrared radiation is greater than the maximum length that we are able to see. These waves are captured by special equipment and are used, for example, in night vision cameras.
Among the rays available to our vision, the shortest ray has a violet ray, the largest - red. Between them lies the entire spectrum available to the eye (remember the rainbow!)
How do we perceive colors? Rays of light having a certain length fall on the retina of the eye, which has photosensitive receptors. These receptors transmit signals directly to our brain, where a sensation of a certain color forms. What colors we see depends on the wavelengths of the incident rays, and the brightness of the color sensation is determined by the intensity of the radiation.
All objects surrounding us have the ability to reflect, transmit or absorb incident light (in whole or in part). For example, green color of foliage means that from the entire range mainly green rays are reflected, the rest are absorbed. Transparent objects tend to retain radiation of a certain length, which is used, for example, in photography (the use of photo filters).
Thus, the color of the subject tells us about the ability to reflect the waves of a certain part of the spectrum. We see objects reflecting the whole spectrum as white, absorbing all the rays as black.