The nuclear energy is released at the Sun's center as high - energy gamma radiation, a form. of electro- magnetic radiation like light and radio waves only of very much shorter wavelength. This gamma radiation is absorbed by atoms inside the Sun, to be reemitted at slightly longer wavelengths. This radiation, in its turn, is absorbed and reemitted. At the energy filters through the layers of the solar interior, it passes through the X- ray part of the spectrum, eventually becoming light. At this stage, it has reached what we call the solar surface, and can escape into space, without being absorbed Farther by solar atoms. A very small fraction of the Sun's light and heat is emitted in such directions that, after passing unhindered through interplanetary space, it hits the Earth.
A simple magnifying glass, focusing the Sun's rays, can scoarch, a piece of wood or set a scrap of paper on fire. Solar radiation can also be concentrated on a much larger scale. It can burn a hole through thick steel plate, for example, or simulate the thermal shock of a nuclear blast. It can, that is, with the help of a super reflector of the sort that has been set up by French scientists high in the Pyroness. The world’s largest solar furnace is a complex of nearly 20,000 mirrors. It can concentrate enough sunlight to create’temperatures in excess of 6000 degrees Fahrenheit.
The furnace's appearance is as spectacular as its power. Its glittering eight - story - high reflector towers over very old houses. Anchored against a concrete office and laboratory building, the huge reflector consists of nearly 9000 separate mirrors. For the furnace to operate, these small mirrors must be adjusted so that their light will meet exactly at a focal point 59 feet in front of the giant reflector.
What does the passage mainly discuss？

A. The production of solar light and heat.
B. The physical and chemical nature of life.
C. The conversion of Hydrogen to helium.
D. Radiation in the X - ray part of the spectrum.