Complex sentence: The photoelectric effect helped propel the then-emerging concept of the dualistic nature of light, that light simultaneously possesses the characteristics of both waves and particles, each being manifested according to the circumstances.
Split into simpler sentences: The photoelectric effect helped propel the then-emerging concept of the wave--particle_duality in the nature of light. Light simultaneously possesses the characteristics of both waves and particles, each being manifested according to the circumstances.