محاضرة 1: معلومات اساسية الجزء 1
Things you need to know
Before studying about lasers, you must be familiar with basic terms used to describe electromagnetic waves:
- Wavelength (l)
- Frequency (n)
- Period (T)
- Velocity of light ( c )
- Index of refraction (n)
We will briefly review these terms, but it is much better if the reader will be familiar with:
- Some terms from geometric optics such as: refraction, reflection, thin lenses etc.
- Some terms from "Modern Physics" such as photons, Models of atoms, etc.
Electromagnetic Radiation in vacuum Electromagnetic Radiation is a transverse wave, advancing in vacuum at a constant speed which is called: velocity of light. All electromagnetic waves have the same velocity in vacuum, and its value is approximately:
c = 300,000 [km/sec] = 3*108 [m/sec] = 186,000 [miles/sec]
One of the most important parameters of a wave is its wavelength.
Wavelength (λ) (Lamda) is the distance between two adjacent points on the wave, which have the same phase. As an example (see figure 1.1 below) the distance between two adjacent peaks of the wave.
In a parallel way it is possible to define a wave by its frequency.
Frequency (ν) (nu) is defined by the number of times that the wave oscillates per second (The number of periods of oscillations per second).
Between these two parameters the relation is:
c = λ * ν
From the physics point of view, all electromagnetic waves are equal (have the same properties) except for their wavelength (or frequency). As an example: the speed of light is the same for visible light, radio waves, or x-rays.
Wave Description A wave can be described in two standard forms:
Displacement as a function of space when time is held constant.
Displacement as a function of time at a specific place in space.
Displacement as a function of space, when time is "frozen" (held constant), as described in figure 1.1. In this description, the minimum distance between two adjacent points with the same phase is wavelength (l). Note that the horizontal (x) axis is space coordinate !
Fig 1.1: Displacement as a function of space coordinate (at fixed time)
A = Amplitude = Maximum displacement from equilibrium.
Displacement as a function of time, in a specific place in space, as described in figure 1.2. In this description, the minimum distance between two adjacent points with the same phase is period (T). Note that the horizontal (x) axis is time coordinate !
Figure 1.2: Displacement as a function of time (at a fixed point in space)
Wavelengths Comparison Figure 1.3 describes how two different waves (with different wavelengths) look at a specific moment in time. Each of these waves can be uniquely described by its wavelength.
Figure 1.3: Short wavelength (l1 ) compared to longer wavelength (l2 )
For electromagnetic waves, this wavelength is related to the type of radiation of the wave.
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