After analyzing where, when, and how much solar energy reaches us, the frequency analysis of electromagnetic radiation was addressed. The spectra of solar radiation as a function of wavelength are shown in Fig. 1. The solar radiation spectra shown are those reaching the boundaries of the atmosphere (AM 0) and the solar radiation spectrum reaching the Earth's surface after the solar ray has passed through the Earth's atmosphere entering at an angle of \( 42^{o} \) above the horizon denoted by AM 1.5. The letter d next to the spectrum designation indicates that it is incident radiation directly from the Sun, while the letter g indicates that it is the sum of direct radiation from the Sun plus Rayleigh scattering and radiation reflected from clouds.
The energy percentages of the different areas of the spectrum are as follows:

• \( 49\% \) – visible range and near infrared,
• \( 44\% \) – wavelengths greater than 800 nm,
• \( 7\% \) – near ultraviolet (120-300 nm),
• \( 0.001\% \) – x-ray and far ultraviolet.

The energy radiated by the Sun in the form of electromagnetic waves before it reaches the Earth's surface is reflected (about \( 30\% \)) and absorbed by the Earth's atmosphere (about \( 20\% \)).
Approximately \( 175 \cdot 10^{15 } \) \( W ^{} \) reaches the surface of the Earth's atmosphere, and approximately \( 89 \cdot 10^{15 } \) \( W ^{} \) reaches the surface of the Earth. Absorption in the Earth's atmosphere occurs due to the presence of oxygen molecules, ozone, carbon dioxide, water vapor, etc. In addition, Rayleigh scattering on dust particles occurs in the troposphere.
The intensity of solar radiation at the distance of the Earth from the Sun is at 1362 \( \frac{W}{m^{2}} \). This quantity was called the solar constant and it is the energy of solar radiation that falls on the surface of one square meter per second at a distance of the Earth from the Sun (the average distance is 150 million km). Based on this, the energy emitted per second by the Sun was calculated and it amounts to \( 9.65 \cdot 10^{25 } \) \( W ^{} \). The total energy that reaches the Earth's surface throughout the year ranges from 600 \( kWh/m^{2}/year \) in Scandinavian countries, and up to over 2200 \( kWh/m^{2}/year \) in central Africa. In Poland it is about 990 \( kWh/m^{2}/year \) [1].
Solar radiation originates from the photosphere, the outer surface of the gaseous layer of the Sun, which temperature is about 5770 \( K ^{} \). The maximum of the blackbody continuous spectrum electromagnetic radiation, as for solar radiation, is in the visible spectral region with a wavelength of about 500 nm ( Fig. 1 ).

The visible region of the spectrum is from \( 4 \cdot 10^{-7 } \) \( m ^{} \) to \( 7.8 \cdot 10^{-7} \) \( m^{} \). Comparing the blackbody spectrum ( Fig. 2 ) with that of the Sun, it can be assumed to a first approximation that they are similar ( Fig. 1 ).
The operating range of the solar panels (green) against the solar radiation spectrum is shown in Fig. 3.
The radiation from the Sun above the Earth's atmosphere corresponds approximately to the radiation from a blackbody with a temperature of 5762 \( K^{} \) (~6000 \( K^{} \)). The solar radiation that reaches the Earth's surface is even more distinctly different from the blackbody spectrum. This is due to the absorption, scattering and reflection of radiation into space, which causes a significant modification in the distribution of its spectrum [2].

Fig. 3 shows the spectrum of solar radiation at the Earth's surface (AM1) after passing through the atmosphere [3].

As solar radiation passes through the atmosphere, there are losses associated with:

• Rayleigh scattering (proportional to \( \lambda ^{-4} \) ),
• absorption by the electron shell of gases \( O_{2} , N_{2}, O_{3} \),
• molecular absorption (rotations and oscillations) of molecules \( H_{2}O \) and \( CO_{2} \),
• dispersion on dust and aerosol particles,
• change in refractive index due to temperature, pressure, turbulence,
• change in the humidity of the atmosphere.

In Poland, about 1000 \( \frac{W}{m^{2}} \) of solar radiation reaches the Earth's surface. The use of a portion of solar radiation for conversion to electricity is related to the physical properties of semiconductors.
The energy region that is used to convert solar energy in photovoltaic devices is highlighted in Fig. 3. This region is almost \( 50\% \) solar energy, the rest of the spectrum is not used.
Research is being conducted on phenomena and technologies to extend the range of frequencies used, such as the thermoelectric effect [4]; nevertheless, in practice, of the solar installations available only the photovoltaic effect is widely used commercially.