Final answer:
The type of multiplexer that divides the circuit horizontally for transmitting different light frequencies across multiple channels is known as wavelength division multiplexing (WDM).
Step-by-step explanation:
The type of multiplexer described in the question sounds like a wavelength division multiplexing (WDM) system. This type of technology is utilized in fiber-optic communication networks to allow for the simultaneous transmission of multiple signals across the same optical fiber by using different wavelengths (or frequencies) of light. A common analogy is that of a television antenna which has cross wires of various lengths to more efficiently receive different wavelengths, as mentioned in the given reference.
In a similar fashion, WDM divides the optical transmission bandwidth into several channels based on different light frequencies. Each channel carries a separate data stream, and all channels are combined into a single optical signal for transmission. At the receiving end, another device called a demultiplexer splits the light into the original channels/wavelengths.