Electromagnetic waves: what they are, simple explanation, the main characteristics and types of waves. The complete physics review.
An electromagnetic wave is a disturbance that propagates without transport of matter, that is, energy that propagates through matter without the bodies moving appreciably from their position.Wave motion is a form of energy transmission .
Electromagnetic waves: characteristics
The physical quantities that characterize a wave are: the speed of propagation (v), the wavelength and the frequency.
The wavelength is the distance between two successive crests (troughs). It depends on the medium of propagation and the frequency of the source that pulsates within the medium.
The wave frequency (f) is the number of crests (troughs) that pass a given point on the surface in one second. It is a scalar quantity that is measured in Hz. It depends on the pulsation rate of the source, i.e. it is the frequency of the source and does not depend on the transport medium.
The period (T) of a wave is the time interval between the passage of two crests from the same point on the surface and is the reciprocal of the frequency.
The relationship between period and frequency is: T= 1/f
Or, it is the time interval required for the wave to travel a distance equal to the wavelength.
V = distance traveled (λ) / T = distance traveled (λ) * f (v depends on the propagation medium).
λ = v / f = v * T
The wave amplitude represents the maximum displacement of the molecules of the medium from their equilibrium position. It depends proportionally on the energy transported by the wave.
With respect to the direction of vibration of the particles and that of propagation of the wave, waves are distinguished into longitudinal waves , whose vibrations occur along the direction of propagation of the wave, and transverse waves , whose vibrations occur in a direction perpendicular to that of propagation of the wave.
With respect to their geometric shape, we distinguish waves into:
- spherical : characterized by wave fronts that coincide with concentric spherical surfaces around the source;
- circular : wave frequencies that coincide with concentric circles around the source;
- flat : characterized by wave frequencies that coincide with planes that are parallel to each other;
- rectilinear : characterized by wave frequencies that are parallel lines.
Wavefront: set of points reached simultaneously by the perturbation. The geometric shape of the wave depends on the type of source that produces it.
Types of waves
There are two types of waves:
- Mechanical waves , which require a medium of propagation;
- Electromagnetic waves : they propagate in the absence of a medium and can only be transverse.
Reflection . Reflection of a wave occurs when a wave encounters an obstacle and changes the direction or sense of its propagation movement. The more we change the surface from flat to curved, the more the waves take on a different direction from the incident ones.
The laws of reflection:
- the radius of the incident wave, the normal to the reflecting surface and the radius of the reflected wave lie in the same plane;
- the angle of incidence and the angle of reflection are equal.
Refraction . The refraction of a wave occurs when a wave crosses in an oblique direction the surface separating two media in which it propagates with different speeds (e.g. water with a step inside: the v is greater when the water is deeper, that is, the wavelength is proportional to the speed).
Laws of refraction:
- the radius of the incident wave, the normal to the surface separating the two media and the radius of the refracted wave lie on the same plane;
- the ratio between the speed of propagation of the wave in the two media is constant and equal to the relative refractive index: v1/v2=n1,2.
Diffraction . Diffraction is the property of waves to bypass obstacles and is more noticeable the closer the slit the wave passes through has dimensions similar to the wavelength.
Interference . Interference is the ability of waves to pass through each other. We are talking about:
- constructive interference : concordant waves that meet, adding the effects and then resuming the initial characteristics;
- destructive interference : opposite waves reduce the effects and then resume their initial characteristics.
The principle of superposition suggests the possibility of creating two stationary waves, that is, constant in time. It is obtained by superimposing wave motions, that is, by adding two waves of equal frequency that propagate along the same direction in opposite directions.
The light
Light is a “special” physical agent of which we have intuitive knowledge and which forms the basis of optical studies.
Light sources : These are the various phenomena that give rise to the production of light.
Natural sources : for example the sun .
Artificial sources : generally exploit the property of bodies to emit light if heated to a sufficiently high temperature.
Luminous bodies : all those that under normal conditions emit light. If light encounters opaque bodies it forms shadows (a region without light that an object forms behind itself); if it passes through transparent bodies (which are in turn divided into “diaphanous” and “translucent”) the property of a body to let itself pass or not also depends on its thickness.
If an opaque body is illuminated, it diffuses the light and reflects the light in front of itself (diffusion is important, otherwise we would have no image of the bodies around us except the luminous ones).
It is the dust that allows the diffusion of light rays in all directions.
A body, in addition to partially diffusing the light that illuminates it, absorbs part of the light, transforming it into heat.
In most practical cases, light travels in straight lines, hence the ray model.
In the construction of the shadow model in the “shadow cone” there is no absolute darkness due to the light reflected from the walls and surrounding objects. If what illuminates an object is not a point source, you do not get a shadow with clear edges but a gradual passage from a darker central area to a lighter one, that is, “penumbra”.
A darkroom is a box where all the walls are black except for one translucent one, where there is a small hole through which external light comes.
Light travels at 300,000 km/s.
Incident ray : The ray coming from the light source.
Reflected ray : the ray that does not pass through the surface, it comes back along a direction usually different from the incident one.
Refracted ray : passes through the surface propagating in a direction different from the incident one.
Reflection of light : occurs when light passes from a transparent medium to an opaque one.
Diffusion : the rays are reflected but since the points on the surface are not aligned their directions vary in a disorderly manner.
Laws of reflection of light :
- the incident ray, the surface normal and the reflected ray lie on the same plane;
- the angle of incidence is equal to the angle of reflection.
Refraction : for example when we look at a body through water. It occurs when light passes through the separation surface between two transparent media of different intensity (e.g. air and water). The light continues its path in the two media but changing direction.
Laws of refraction:
- the incident ray, the surface normal and the refracted ray lie in the same plane;
- the angle of incidence and the angle of refraction are such as to keep the ratio constant: n1,2=sin0i/sin0r
The principle of reversibility : if light can travel a given optical path, then it can also travel the reverse path.
The relative refractive index : is the ratio between two homogeneous quantities, and allows us to characterise one medium with respect to another in relation to their capacity to refract, that is, to deviate, the path of a light ray by bringing it closer to the normal or moving away from it.
Total reflection occurs when light passes from a more refractive medium to a less refractive one.
Light scattering occurs when a beam of white light is passed through a transparent prism. The beam undergoes double refraction, from air to glass and then from glass to air.
In the first case the angle of refraction is smaller than the angle of incidence (glass is more refractive than air); in the second the angle of refraction is larger than the angle of incidence.
The angle of deviation is the angle formed by the entry ray and the exit ray. White light is broken down into chromatic components.
Corpuscular model ( Newton ): light is a flow of particles that are continuously emitted by luminous bodies and generate visual sensations when they enter the eye.
Wave model (Huygens): light is a wave, a transfer of energy without propagation of matter.
In wave theory, color is associated with wavelength. The wavelengths of light are between 10 to the minus seventh and 10 to the minus sixth. F=c/distance traveled (λ) where c is the v of light in a vacuum.
Frequencies between 10 to the thirteenth and 10 to the fourteenth Hz. The color of a body depends on the light that illuminates it, it is not an intrinsic characteristic of bodies.