Complex sentence: This ray of light often contains of a variety of different wavelengths, and is called the transverse mode, because its electromagnetic vibrations occur perpendicular (transverse) to the length of the fiber.
Split into simpler sentences: This ray of light often contains of a variety of different wavelengths. Although the ray travels parallel to the length of the fiber, it is often called the transverse mode since its electromagnetic vibrations occur perpendicular (transverse) to the length of the fiber.