China
Africa
Explore chapters and articles related to this topic
Foundations of electromagnetism
Published in Riadh Habash, BioElectroMagnetics, 2020
If EM waves were radiated equally in all directions from a point source in free space, a spherical wavefront should result. A wavefront may be defined as a plane joining all points of equal phase. The wave travels at the speed of light so that at some point in time the energy will reach the area indicated by wavefront 1 in Figure 1.6. The PD at wavefront 1 is inversely proportional to the square of its distance from its source r in meters, with respect to the originally transmitted power. If wavefront 2 in Figure 1.6 is twice the distance of wavefront 1 from the source, then its PD in watts per unit area is just one-fourth that of wavefront 1. This is according to the inverse-square law, which states that power received is inversely proportional to the square of the distance from the source.
Electromagnetic Waves
Published in Myeongkyu Lee, Optics for Materials Scientists, 2019
Wavefront means an imaginary surface joining all points of equal phase in a wave. The wavefronts of a plane wave are planes. Any wave propagates in the direction normal to its wavefront. A plane wave thus has a straight propagation direction, as shown in Figure 1.6. A small stone vertically dropped into a tranquil lake will generate a two-dimensional circular wave on the water surface, in which the wavefronts are in the form of concentric circles. Similarly, the radiation emanating from a point source of light can be considered as a spherical wave spreading out in all radial directions. The resulting wavefronts, that is, the surfaces of constant phase, are spherical surfaces centered at the source. The surface of a sphere becomes more flattened with increasing radius. Thus, a spherical wave will behave like a plane wave when it is far away from the source. As illustrated in Figure 1.6a, the wavefronts are usually drawn along the crests of a wave, with the spacing between two adjacent wavefronts equal to one wavelength. When a planewave beam is focused by a lens, the beam size decreases and then increases after being minimized at focus. Since the wave propagates normal to its wavefront, planar wavefronts become spherical after going through the lens, as shown in Figure 1.7.
Introductory Topics
Published in Riadh W. Y. Habash, Electromagnetic Fields and Radiation, 2018
If EM waves were radiated equally in all directions from a point source in free space, a spherical wavefront should result. A wavefront may be defined as a plane joining all points of equal phase. The wave travels at the speed of light so that at some point in time the energy will reach the area indicated by wavefront 1 in Figure 1-6. The power density at wavefront 1 is inversely proportional to the square of its distance from its source r in meters, with respect to the originally transmitted power. If wavefront 2 in Figure 1-6 is twice the distance of wavefront 1 from the source, then its power density in watts per unit area is just one-fourth that of the wavefront 1. This is according to the inverse-square law, which states that power received is inversely proportional to the square of the distance from the source.
A high-resolution wavefront sensing method to investigate the annular Zernike polynomials behaviour in the indoor convective air turbulence in the presence of a 2D temperature gradient
Published in Journal of Modern Optics, 2021
E. Mohammadi Razi, Saifollah Rasouli, M. Dashti, J. J. Niemela
In this work, we observe for the first time the effect of a 2D TG on the change of the Zernike coefficients of a light beam wavefront after propagating through a controlled turbulent medium. Due to the limited width of heater's surface, a horizontal component of the TG also appeared. Using a second telescope and a two-channel moiré-based wavefront sensor, the wavefront of the light beam was reconstructed. The reconstructed wavefront was then expanded versus the Zernike annular polynomials. The experiments were performed at different heater temperatures, from room temperature to C. Results show that the variance of the Zernike coefficients increase as the heater temperature increases. Finally, we find that the X- and Y-tilt aberrations coefficient variances are not equal when the heater's temperature exceeds C, suggesting that the convective air turbulence is not isotropic; in particular, plots of the difference in coefficient variance between X- and Y-tilts show non-monotonic increase with heater temperature and can be divided into three distinct regions with different slopes.
Super-resolution pupil filtering for visual performance enhancement using adaptive optics
Published in Journal of Modern Optics, 2018
Lina Zhao, Yun Dai, Junlei Zhao, Xiaojun Zhou
where ρ is the normalized radius over the circular pupil. The filter is characterized by the amplitude transmittance A(ρ) and the phase function φ(ρ). Super-resolution techniques differ in the way the functions A(ρ) and φ(ρ) are defined and optimized. Our strategy is based on the control of continuous phase function φ(ρ) so that they can be reproduced by a deformable mirror. The general modification procedure is to expand the pupil function in some complete set of functions and adjust the coefficients to approximate the pre-specified point spread functions (PSF). Because Zernike polynomials are orthogonal over a unit circle, they are usually used as a representation of wave-front phase. The phase function φ(ρ) can be mathematically described as the weighted sum of Zernike basis functions:
On propagation of harmonic plane waves under the Moore–Gibson–Thompson thermoelasticity theory
Published in Waves in Random and Complex Media, 2021
Komal Jangid, Manushi Gupta, Santwana Mukhopadhyay
In physics, when we talk about a plane wave, the wavefronts are planes as shown in Figures 1 and 2. Here the wavefronts, which are the locus of all points where the wave has the same phase, are planes perpendicular to the direction of propagation (see Figure 1), that move in that direction together with the wave.