Still air is just about the best insulator there is. At the molecular level, heat transfer is easier when atoms are packed closely together. So the actual point of most insulations is to get rid of as much solid material and trap still air inside little pockets.
A plane angle is one that can be measured completely within a plane. This may be because the angle occurs in a 2-D space, or it is an angle that is contained completely within a plane that is in a 3-D (or higher-D) space. So the angles in a triangle are plane angles, because they are in a plane. Similarly, the angles in a spherical triangle are plane angles, because even though they measure things in a 3-D space, they can always be shown to be wholly within a plane. Another way to consider a plane angle is to think of angles within a circle. You can divide the circle up into 2π radians, for example, or 360°, or 400 gon, or whatever units you choose. However, the radian as a unit is defined by the subtended arc length that is equal to the radius of the circle. A steradian, the basic unit, is solid angle is one that is subtended by a portion of the surface area of a sphere that is equal to r^2, where r is the radius of the sphere. It can be considered to be a cone subtended at...
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