Light… and other Electromagnetic Radiation

Radiation A loaded term; That which makes radiation dangerous is primarily its energy; Energy is related to wavelength: low frequency, long wavelength, low energy. Length of exposure, proximity to the source is also important.

The Electromagnetic Spectrum Wavelengths ranging from 10,000 m (and longer) to 0.000,000,000,000,01 m (and shorter).

Danger! Pretty safe up through visible light (unless you stick your head in a microwave oven!). Pretty much dangerous from Ultraviolet and shorter.

Sources of EM Radiation 1.Antennae 2.Atomic orbitals 3.Atomic nucleus

Antennae Electrons (or any charged particle) emit electric fields. Moving charges make magnetic fields, so moving charges emit electromagnetic radiation. Mostly longer wavelength: Marine radio to microwaves.

Atomic Orbitals For all atoms: hydrogen illustrated. When an atom’s electron absorbs the right amount of energy, it will move to a higher energy level, a higher orbit about its nucleus. After a billionth of a second or so it falls back down to the initial or ground state.

Photons As the electron drops back down to the ground state, it emits a photon (particle of light) with a wavelength directly dependent on the energy of the jump. These jumps to ground yield wavelengths too short to be seen (ultraviolet). However, when they land on the second level (n = 2), the result is a discreet spectra.

Emission Spectrum

Atomic Nucleus Very high energy photon from an atomic nucleus breaking apart. Takes some of that E = mc 2 for its energy. Hard X-Rays and Gamma rays. Even higher radiation comes from exploding stars (supernovae): cosmic rays.

The Particle/Wave Duality Huygens ~ 1650: light is a wave; Newton ~ 1700: light is a particle; Young ~1800: light is a wave Planck~1900: light is a particle It’s both! A particle of light (photon) has wave characteristics.

Optics Geometric Optics Physical Optics

Images Real (can be projected) or virtual (“in” the mirror) Erect (right-side up) or inverted Magnified (bigger) or diminished (smaller)

Geometric Optics: Mirrors Plane (flat) mirrors: virtual image, erect, magnification of 1. Concave (curved inward). Like a makeup mirror. Inside the focal length (1/2 the radius of the curve) the image is virtual, erect and bigger. Outside it’s smaller, inverted, and real. Convex (curved outward). Passenger side mirror. Images are virtual, erect, and smaller.

Index of Refraction Light travels at 186,000miles per second in a vacuum, but slower in everything else. In the process of slowing down as it enters water, glass, etc., the light beam bends inward. When it exits, it speeds up and bends away. The amount of bending depends on the Index of Refraction, the ratio of the speed of light in a vacuum to its speed in a medium. Can be used to make lenses, prisms, etc.

Lenses Flat (like a window) Concave (diverging). Spreads light waves. Corrects near-sightedness Convex (converging). Focuses rays to a point. Corrects far-sightedness.

Prisms Different wavelengths (colors) of light have different speeds in a medium. This causes them to bend different. White light is composed of all colors; a prism separates them.

Famous Album Cover

Total Internal Reflection When light tries to escape from a medium, if the angle is too great then it is completely reflected back down. (the explanation requires trig!!) “Circle of seeing” for fish and divers looking up. The physics behind fiber optics.

Physical Optics Diffraction Polarization

Diffraction The bending of light around corners. When any wave encounters an edge, it bends around it. Light will spread out through a thin slit in otherwise opaque material.

LASER Light Amplification by Stimulated Emission of Radiation. Useful for many things; especially for diffraction demonstrations. Exactly one color of light that doesn’t spread out (much).

Interference When a slit causes light to bend, the distance from one edge to a far-off wall could be ½ a wavelength different from the distance from the other edge. Therefore, one wave may be “up” while the other wave may be “down”. The interfere destructively (cancel out) If both waves are N’Sync (sorry) then they interfere constructively (add up)

Fringe Patterns The pattern of light and dark spots on the wall made by a LASER beamed through a slit. The reason? Diffraction and Interference! Gratings can also be used to separate colors, since the amount of bending depends on the light’s wavelength.

Polarization Light from most sources is disorganized. The plane of each wave is randomly oriented to each other. Think of many airplanes flying with different tilts to their wings: unpolarized. A polarizing filter will allow only the correctly oriented waves through, lining them all up: polarized. Two filters at 90 o will block light altogether! Good in sunglasses for blocking glare, which is polarized.