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To achieve a particular goal in physics, we usually conducted together with the measurement. Observation of a phenomenon in general is not complete if not accompanied by quantitative data obtained from the measurement results. Lord Kelvin, a physicist said, if we can measure that we are discussing with them and the numbers, we know what we're talking about it. 

What do you do when making measurements? For example you measure the length of a table by using the study span, and long tables that span is 6. Thus, the measure is to compare something that is measured with something that is defined as a kind of unit. In the measurement of the above you have to take as a long span. 

Something that can be measured and expressed with a number called the scale. An example is the scale length, mass, and time. The unit has in general. Have a meter long, has a mass of kg, and has a second time. But later there will be some who do not have the scale unit, such as light refraction index and type of relative mass. 

Before the existence of international standards, almost every state set its own system satuannya. The use of various units of a magnitude to cause this difficulty. The first difficulty is requesting a variety of measuring instruments in accordance with the unit is used. Second difficulty is the complexity of conversion from one unit to another unit, for example, to the foot of span. This is not the regularity of the set-conversion unit is. 

Due to hardship caused by the use of a different system then the ideas appear to menggunkan only one type of unit only for the scale-scale in science and technology. An international agreement has set a single international system (International System of Units) unit abbreviated SI. SI unit is taken from the metric system that has been used in France.

wave (physic)

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In physics, oscillation that is propagated from a source. Mechanical waves require a medium through which to travel. Electromagnetic waves do not; they can travel through a vacuum. Waves carry energy but they do not transfer matter. The medium (for example the Earth, for seismic waves) is not permanently displaced by the passage of a wave. The model of waves as a pattern is used to help understand the properties of light and sound. Experiments conducted in a ripple tank with water waves can explain how waves slow down as water becomes shallower, how waves change direction when travelling through another medium, and how waves are reflected from different surfaces. See also standing wave.

Types of wave
There are various ways of classifying wave types. One of these is based on the way the wave travels. In a transverse wave, the displacement of the medium is perpendicular to the direction in which the wave travels. An example of this type of wave is a mechanical wave projected along a tight string. The string moves at right angles to the wave motion. Electromagnetic waves are another example of transverse waves. The directions of the electric and magnetic fields are perpendicular to the wave motion. In a longitudinal wave the disturbance takes place parallel to the wave motion. A longitudinal wave consists of a series of compressions and rarefactions (states of maximum and minimum density and pressure, respectively). Such waves are always mechanical in nature and thus require a medium through which to travel.Sound waves are an example of longitudinal waves. Waves that result from a stone being dropped into water appear as a series of circles. These are called circular waves and can be generated in a ripple tank for study. Waves on water that appear as a series of parallel lines are called plane waves.

Characteristics of waves
All waves have a wavelength. This is measured as the distance between successive crests (or successive troughs) of the wave. It is given the Greek symbol λ. The frequency of a wave is the number of vibrations per second. It is expressed in hertz, symbol Hz (1 Hz = 1 cycle per second). The reciprocal of this is the wave period. This is the time taken for one complete cycle of the wave oscillation. The speed of the wave is measured by multiplying wave frequency by the wavelength.

Properties of waves
When a wave moves from one medium to another (for example a light wave moving from air to glass) it moves with a different speed in the second medium. This change in speed causes it to change direction. This property is called refraction. The angle of refraction depends on whether the wave is speeding up or slowing down as it changes medium. Reflection occurs whenever a wave hits a barrier. The wave is sent back, or reflected, into the medium. The angle of incidence (the angle between the ray and a perpendicular line drawn to the surface) is equal to the angle of reflection (the angle between the reflected ray and a perpendicular to the surface). See also total internal reflection. An echo is the repetition of a sound wave by reflection from a surface. All waves spread slightly as they travel. This is called diffraction and it occurs chiefly when a wave interacts with a solid object. The degree of diffraction depends on the relationship between the wavelength and the size of the object (or gap through which the wave travels). If the two are similar in size, diffraction occurs and the wave can be seen to spread out. Large objects cast shadows because the difference between their size and the wavelength is so large that light waves are not diffracted around the object. A dark shadow results. When two or more waves meet at a point, they interact and combine to produce a resultant wave of larger or smaller amplitude (depending on whether the combining waves are in or out of phase with each other). This is called interference. Transverse waves can exhibit polarization. If the oscillations of the wave take place in many different directions (all at right angles to the directions of the wave) the wave is unpolarized. If the oscillations occur in one plane only, the wave is polarized. Light, which consists of transverse waves, can be polarized.