Hi, I am a new member here : ) I have always wondered about this. It seems that the answer is easy but I missed an important lecture. De Broglie proposed that just as light has both wave-like and particle-like properties, electrons also have wave-like properties. Why is that? I know the relation between matter and energy but I don't Understand what made him think that photons and localized particles should be the same in this regard. Please explain this point to me. Thank you all and please excuse my poor writing skills.