Night-time Shortwave radio broadcast
"Medium wave" generally refers to the band of frequencies between 300&nbsp;kHz and 3&nbsp;Mhz (100 to 1000&nbsp;m wavelength), and this includes the AM broadcast band (530 - 1600&nbsp;kHz in the US).

"Long wave" refers to lower frequencies, 30 to 300&nbsp;kHz (1 - 10&nbsp;km), while "short wave" (also called "HF") refers to higher frequencies in the range 3 to 30&nbsp;MHz (10 - 100&nbsp;m).

All of these frequencies can be reflected by the ionosphere under the right conditions, with the effect tapering off toward the upper end of the SW band.

The ionosphere, created mainly by UV radiation from the sun, is lower and denser during the day, and higher and thinner at night, which is why it allows longer propagation distances at night.



Here's a more complete listing of the commonly-named bands:
$$
\begin{matrix}
\text{Band Name} &amp; \text{Frequencies} &amp; \text{Wavelengths} \\
\hline
\text{ELF} &amp; 3  - 30 \text{ Hz} &amp; 100,000 - 10,000 \text{ km} \\
\text{SLF} &amp; 30 - 300 \text{ Hz} &amp; 10000 - 1000 \text{ km} \\
\text{ULF} &amp; 300 \text{ Hz} - 3 \text{ kHz} &amp; 1000 - 100 \text{ km} \\
\text{VLF} &amp; 3 - 30 \text{ kHz} &amp; 100 - 10 \text{ km} \\
\text{LF (also LW)} &amp; 30 - 300 \text{ kHz} &amp; 10 - 1 \text{ km} \\
\text{MF (also MW)} &amp; 300 \text{ kHz} - 3 \text{ MHz} &amp; 1000 - 100 \text{ m} \\
\text{HF (also SW)} &amp; 3 - 30 \text{ MHz} &amp; 100 - 10 \text{ m} \\
\text{VHF} &amp; 30 - 300 \text{ MHz} &amp; 10 - 1 \text{ m} \\
\text{UHF} &amp; 300 \text{ MHz} - 3 \text{ GHz} &amp; 1 \text{ m} - 10 \text{ cm} \\
\text{SHF} &amp; 3 - 30 \text{ GHz} &amp; 10 - 1 \text{ cm} \\
\text{EHF} &amp; 30 - 300 \text{ GHz} &amp; 1 \text{ cm} - 1 \text{ mm} \\
\end{matrix}
$$

ELF is interesting because the "resonant frequency of the Earth" falls into this band. If you think of the space between the Earth's surface and the ionosphere as a waveguide, the path length (circumference) is roughly 40,000&nbsp;km, corresponding to a frequency of about 7.5&nbsp; Hz. This frequency also penetrates fairly well into seawater, which is why the military has used this band to communicate with submerged vessels.

EHF is also known as "millimeter wave", and at these frequencies (and up), you can start to do useful imaging. This is the kind of wavelength used for security scanners, etc.