I am a bit confused as to what I should use to derive the equations of motions from the lagrange equation. Suppose I have a lagrange function: $$L(x(t), \dot{x}(t)) = \frac{1}{2}m\dot{x}^2-\frac{1}{2}k(\sqrt{x^2+a^2}-a)$$ Method 1: Principle of least action $$\delta L = \delta \dot{x}(m\dot{x...

I am not asking about whether the photon goes through both slits, or why. I am not asking whether the photon is delocalized as it travels in space, or why. I have read this question: Do we really know which slit the photon passed through in Afshar's experiment? Which theory explains the path o...

We were taught at school that EM radiation can be categorised as radio waves, microwaves, infrared waves, visible light, ultraviolet radiation, X-rays, and gamma rays, but how arbitrary is this categorisation? I understand that for humans the 'visible light' category is an obvious, but how did s...