R-ATPHY-26

EMR!?!?!

• Propagation of energy through space (not necessarily through matter)
• Coincidentally, most light is visible light. (Blackbody radiation fr)

Nature of light

• Light is electromagnetic radiation (EMR).
• It is a transfer of energy, i.e. no medium(matter) is required.
  • Some guys thought it had a medium to travel through, called 'Luminiferous aether'. They were wrong.
• Wave-particle duality
  • Sometimes acts as wave, sometimes acts as particle.

Wave behaviour

• Reflection ($\theta_{i}=\theta_{r}$)
• Refraction, i.e. Snell's law
• Key idea: velocity changes as moving throughout different media. $v=\lambda f,\therefore \lambda \downarrow$,
• If it slows down it bends toward the normal.

(top ten norrie analogies)

• Diffraction

  • Bending of wave around obstacle.
  • For wave to diffract, $\lambda \geq d$, where $d$ is the size of the aperture.
  • $\lambda \uparrow$ means wave diffracts to greater extent.

• Interference(superposition)

  • Waves arrive at same point in space and interfere (essentially combine).
  • If waves in phase, then constructive interference occurs, resulting in maximum intensity.
  • If waves are out of phase, then destructive interference occurs, resulting in minimum(no) intensity.
    • If half a wavelength, then usually is maximum destructive interference (no intensity).
  • This is important for later (Young's double slit experiment)

More on light

• The light wave exists through the oscillations of an electric and magnetic field, which propagate each other and move orthogonal.
  • This allows it to be self propagating!
  • (Side note, this is called a poynting vector)
• Transverse - The net wave is oscillating at right angles to direction of propagation, i.e to the direction of the component electric, magnetic fields.

Polarisation

• Electric field oscillating in a particular direction - Evidence for transverse nature.
• Imagine polarizer 2 was turned 90$\degree$. This would let in more light as the slits are aligned. This is how you have the polarising trick (rotating polarised lens makes it go light and dark. )

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