ME 542 · Materials Structures · Diffraction primer
What Is Light? A 2,500 year speed run
A three minute cartoon history of how people figured out what light is, from Empedocles' "laser eyes" to the Braggs reading atomic spacings with X-rays. It sets up our discussion of diffraction: how can a wave reveal the spacing of atoms you cannot see?
The story in one line per stop
- ~450 BC Empedocles: the eyes send out fire. Euclid: light travels in straight lines and reflects at equal angles.
- ~1021 Ibn al-Haytham (Alhazen): light travels from objects into the eye; the camera obscura.
- 1665 Grimaldi notices fringed shadow edges and names the effect diffraction.
- 1666 to 1704 Newton splits white light with a prism and argues for particles.
- 1678 Huygens: every point on a wavefront launches new wavelets.
- 1801 Young's double slit: bright and dark fringes, so light interferes like a wave.
- 1818 Fresnel's diffraction theory, Poisson's objection, and Arago's bright spot.
- 1865 Maxwell: light is an electromagnetic wave, c = 1/√(μ₀ε₀).
- 1905 Einstein: light also comes in packets, called photons.
- 1912 and 1913 von Laue diffracts X-rays with a crystal; the Braggs write nλ = 2d sin θ.
- 1924 to 1927 de Broglie: matter is wavy too; electrons diffract.
Questions to bring to class
Why does a crystal act like a grating?
What sets the spacing of the "slits," and why do X-rays (not visible light) work for atoms?
Why does Bragg's law have a 2?
Trace the two rays reflected from neighboring planes and find the extra path length.
Why was the Poisson spot so convincing?
Which prediction can a wave theory make that a particle theory cannot?