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ultrasonic
ultrasonic

Ultrasound . Sound waves with a frequency higher than the human ear can detect (above 20 kilohertz). See sound.
Other animals may be able to hear some ultrasound fields. The sonar system of bats, for example, operates at a frequency of 50 kilohertz or more. Ultrasonics with frequencies that reach megahertz have many uses in the fields of research, industry, and medicine, such as detecting the presence of mines at sea, detecting fetuses in the womb, and cleaning as well.
Compare with infrasound.


ultraviolet
ultraviolet

ultraviolet. A region in the wave spectrum
electromagnetic waves, their wavelength ranges approximately between 10-10 and 10-7 meters, and their frequency range is between 1015 and 1018 Hz. Ultraviolet rays result from discharges of white-hot bodies or certain gases. Ultraviolet rays affect photographic films and cause photo-electric fluorescence and photoelectric effects. Human skin produces vitamin D using these rays (a type of photosynthesis, in fact). Tan dyes the skin with a tan color.
Ultraviolet rays, especially at higher frequencies, pose a danger to the eyes and skin.


unit. unit
unité f

lonliness . The electrical unit is the kilowatt-hour.
See joule - meter.

units
units fpl

Units of standard quantities by which all other quantities are measured or stated. In the past, people created units in the shapes they needed. These units include the day, foot, and pound. However, the correct “system” of units must be:
A) Simple, does not cause problems in its use, and is easy
Learning and studying.
b) Cohesive, with units that are clearly linked to each other.
C) It is widely accepted and used by most people in large areas.

The first attempt to establish such a system of units took place in 1791. The metric system that was established at that time was based on the meter, gram, and second.

This system, through other systems, led to “the system
système International d'Unités (SI).
This system is widely used in the fields of science these days. This system is not perfect, although it is simple, reasonably coherent, and widely accepted around the world.

The base units in the International System of Units (SI) include: the meter (m) for length, the kilogram (kg) for mass. This unit may gain a new name, and the second (s) for time. The units emerging from these include: density (kilograms per cubic meter kg m), momentum (kilograms per second kg/s), and acceleration (meters per second per second m/s2). There are several fairly complex derived units. For example, the derived unit for pressure is the kilogram (meter per second per second per square meter, i.e. kg m/s2/m2). It is natural to give these units special names, and the name in this case is “Pascal”. Thus, we get a set of standard units. If any of these units is too large or too small for a particular use, one can add the correct prefix to it. Each prefix describes how the newly coined unit relates to the standard unit. Thus, a kilometer is 1000 metres, a kilovolt is 1000 volts, and a kilonewton is 1000 newtons. The table below lists the most common unit prefixes, and the prefixes in parentheses are not approved by the International System of Units (SI):

Prefix -- symbol -- meaning

Pico --- 12 10-1 p
nano - nano 10-9 n
Micro - micro 10-6
Milli - milli 10-3 m
( centi ( 10-2 C
(Deci - deci . ( 10-1d
( Dhaka - deka ( 101 da
( hecto ( 102 h
Kilo - kilo 103 k
Mega - mega 106 M
giga - giga 109 G
Tera - tera 1012 T