Solar Radiation and Measurement Questions and Answers

Answer: B

The key gives \(10^{8}\) K.

Hydrogen nuclei repel each other strongly, so fusion needs extremely high kinetic energies. Textbooks give the temperature needed for thermonuclear fusion as of the order of \(10^{7}\) to \(10^{8}\) K; the Sun's core is measured at about \(1.5 \times 10^{7}\) K, and fusion of heavier nuclei, or of hydrogen in a laboratory reactor, needs around \(10^{8}\) K.

Among the options only \(10^{8}\) K lies in that range, and it is the nearest to the solar core figure. \(10^{6}\) K and \(10^{4}\) K are far too cool (the Sun's visible surface is only about 5,800 K), and \(10^{10}\) K is a hundred times hotter than any stellar core burning hydrogen.

Answer: B

The answer is Irradiance.

Irradiance is the radiant power falling on a surface per unit area, in watts per square metre: a rate, energy per unit time. That matches 'incident on a surface per unit time'. The solar constant, about 1,361 W/m², is the irradiance at the top of the atmosphere.

Insolation (incoming solar radiation) is the closest distractor; it usually means the total solar energy received over a period, such as kWh/m² per day, rather than the instantaneous rate. 'Radiation' is the general process, and conduction is heat transfer through matter, not solar radiation at all.

Answer: C

The correct order is X-rays \(\lt\) UV \(\lt\) IR \(\lt\) Microwave.

In the electromagnetic spectrum wavelength increases from X-rays (about 0.01 to 10 nm) through ultraviolet (10 to 400 nm), visible light, infrared (700 nm to 1 mm) and then microwaves (1 mm to 1 m).

Option A lists the order backwards, and options B and D put X-rays and ultraviolet, or microwave and infrared, the wrong way round.

Answer: B

Solar energy reaches the Earth's surface by radiation.

Space between the Sun and the Earth is nearly a vacuum, and only electromagnetic radiation can cross a vacuum. Sunlight travels as shortwave radiation through space and through the atmosphere, which is largely transparent to it, before being absorbed at the surface.

Conduction needs direct contact between molecules and only transfers heat from the surface to the air in contact with it. Convection is transfer by vertical movement of a heated fluid, and advection is horizontal transfer by moving air or water. These redistribute heat within the atmosphere but cannot carry energy from the Sun.