Positively-charged particles consisting of two protons and two neutrons emitted by radioactive materials is called

Alpha particle

Explanation:

Alpha decay is defined as the decay process in which a heavier nucleus decays into a lighter nuclei. This decay process releases alpha particle which is also known as helium nucleus.

Alpha particle released carries a mass of 4 units and a charge of +2 units. The alpha particle is also known as helium atom.

In this decay, the atomic number of the atom released gets reduced by 2 units and atomic mass is reduced by 4 units as helium contains 2 protons and 2 neutrons as atomic number is 2 and mass number is 4.

The chemical equation for the alpha decay process is given as:

Alpha particle

Explanation:

electromagnetic shielding is the practice of reducing the electromagnetic field in a space by blocking the field with barriers made of conductive or magnetic materials. shielding is typically applied to enclosures to isolate electrical devices from their surroundings, and to cables to isolate wires from the environment through which the cable runs. electromagnetic shielding that blocks radio frequency electromagnetic radiation is also known as rf shielding.

the shielding can reduce the coupling of radio waves, electromagnetic fields and electrostatic fields. a conductive enclosure used to block electrostatic fields is also known as a faraday cage. the amount of reduction depends very much upon the material used, its thickness, the size of the shielded volume and the frequency of the fields of interest and the size, shape and orientation of apertures in a shield to an incident electromagnetic

from wikipedia

snell's law

refractve index = sin incidence / sin refraction