Optical Illusions

Here are some optical illusions:

Red Filter Glasses

Enjoying this summer...
Now I made another tool to teach light and color in my Physics class.
Take a look:

Refraction

Snell's law (also known as the Snell–Descartes law and the law of refraction) is a formula used to describe the relationship between the angles of incidence and refraction, when referring to light or other waves passing through a boundary between two different isotropic media, such as water and glass.

Refraction of light at the interface between two media of different refractive indices, with n₂ > n₁. Since the velocity is lower in the second medium (v₂ < v₁), the angle of refraction θ₂ is less than the angle of incidence θ₁; that is, the ray in the higher-index medium is closer to the normal.

In optics, the law is used in ray tracing to compute the angles of incidence or refraction, and in experimental optics and gemology to find therefractive index of a material. The law is also satisfied in metamaterials, which allow light to be bent "backward" at a negative angle of refraction (negative refractive index).

Although named after Dutch astronomer Willebrord Snellius (1580–1626), the law was first accurately described by the Arab scientist Ibn Sahlat Baghdad court, when in 984 he used the law to derive lens shapes that focus light with no geometric aberrations in the manuscript On Burning Mirrors and Lenses (984).^[1][2]

Snell's law states that the ratio of the sines of the angles of incidence and refraction is equivalent to the ratio of phase velocities in the two media, or equivalent to the opposite ratio of the indices of refraction:

with each  as the angle measured from the normal,  as the velocity of light in the respective medium (SI units are meters per second, or m/s) and  as the refractive index (which is unitless) of the respective medium.

The law follows from Fermat's principle of least time, which in turn follows from the propagation of light as waves.

in Wikipedia

Learn more about the mathematics of refraction in physicsclassrom.

Watch this video about refraction in gases (MIT):

Electromagnetic momentum density in matter solved

Optical Force Measurement. Credit: eecs.northwestern.edu

"(PhysOrg.com) -- Researchers from the NIST Center for Nanoscale Science and Technology and the University of British Columbia have shown that the interaction between a light pulse and a light-absorbing object, including the momentum transfer and resulting movement of the object, can be calculated for any positive index of refraction using a few, well-established physical principles combined with a new model for mass transfer from light to matter."

Electromagnetic radiation, like light, carries momentum and can transfer its momentum to matter via radiation pressure. In the past century, there has been a controversy over the correct form of the electromagnetic momentum density in matter.

There was two forms:

•  In the “Minkowski formulation,” the momentum density is proportional to the index of refraction;
• in direct contrast, the “Abraham formulation” finds it to be inversely proportional. 

And the two equations for the momentum in a dielectric with refractive index n are:

• The Minkowski version:

• The Abraham version:

where h is the Planck constant, ν is the frequency of the light and c is the speed of light in vacuum.

In 2011, an optical experiment was performed to study this problem based on first principles. It tested the velocity-addition formula of light in a reversed Fizeau experiment. The result was that the light speed c= 299,792,458m/s in vacuum of Lorentz transformation should be replaced by c/n to describe electrodynamic phenomena in a dielectric medium^[1] Consequently, the momentum of a photon in vacuum is p=E/c and the value should be p=E/(c/n)=nE/c in media, although it is not measured directly.^[2] It is asserted, that this confirms Minkowski's formulation.

[1] Wang Zhong-Yue, Wang Pin-Yu, Xu Yan-Rong (2011). "Crucial experiment to resolve Abraham-Minkowski Controversy". Optik 122 (22): 1994–1996. doi:10.1016/j.ijleo.2010.12.018

[2] Wang, Zhong-Yue. Graphene, neutrino mass and oscillation. arXiv:/0909.1856

The recently published in Applied Physics support the Abraham formulation: Revisiting the Balazs thought experiment in the presence of loss: electromagnetic-pulse-induced displacement of a positive-index slab having arbitrary complex permittivity and permeability, K. J. Chau and H. J. Lezec, Applied Physics A 105, 267-281 (2011).

"The researchers propose a set of postulates for light-matter interaction that encompass: a) the Maxwell equations, which govern classical electromagnetic behavior; b) a generalized Lorentz force law, which describes the force felt by matter in the presence of an electromagnetic field; c) a model for electromagnetic mass density transfer to an absorbing medium; and d) the Abraham formulation of momentum density. Using both closed-form calculations and numerical simulations of the interaction between an electromagnetic pulse and a test slab, the researchers demonstrated that their postulates yield results that are consistent with conservation of energy, mass, momentum, and center-of-mass velocity at all times." ( PhysOrg)

Read entire article in PhysOrg

Hurricane Balls

Amazing!
We only need a concave mirror, some iron's balls and led's lights.