EMR / Lenses

Electromagnetic Radiation

Electromagnetic waves consist of fields of electric and magnetic energy that are oscillating perpendicularly to the direction of propagation of the wave. The electric and magnetic fields are in phase with each other and are also perpendicular to each other. Since the fluctuations are perpendicular to the direction of propagation, electromagnetic waves are transverse waves.

Electromagnetic Spectrum

Type of Radiation (in order of decreasing frequency)	Comments	Approximate frequency
grays	Produced by radioactive decay; nuclear reactions	³1020 Hz
X-rays	Produced by deceleration of high-speed electrons, e.g. X-ray tube.
Ultraviolet	Produced by electrons going from higher energy orbitals to lower higher energy orbitals
Visible light	Produced by electrons going from higher energy orbitals to lower higher energy orbitals, e.g. electric light bulb	1014 Hz
Infrared (heat waves)	Produced by and causes movement of atoms and molecules, vibration of bonds. Used in IR spectroscopy.
Microwaves (higher-frequency radio waves)
Lower-frequency radio waves	Used in NMR spectroscopy	106 Hz

Polarization

Since electromagnetic waves are transverse, they can be polarized. This means that the electric field fluctuates along a single plane as opposed to fluctuating in every plane (for simplicity, the magnetic field component is ignored).

A polarizing material placed in the path of a beam of unpolarized light will absorb the electric field components in a certain plane. The transmitted light will thus be polarized in the plane perpendicular to plane in which absorption is occurring. The intensity of the light transmitted is half the intensity of the light incident on the polarizing material.

Disorders of Vision

Scientific name of disorder	Common name of disorder	Basic problem	Treatment
Myopia	Nearsightedness	Cannot focus on distant objects	Diverging lenses
Hyperopia	Farsightedness	Cannot focus on near objects	Converging lenses

Lens Aberrations

Aberration	Explanation	Measures to reduce it
Spherical aberration	The rays from a single point on the object are not focused to a single point on the image.	Use an aperture so that only rays close to the principal axis can pass through the lens. Or use 2 lenses instead of 1.
Chromatic aberration	Dispersion in the lens causes different colors to to be focused at different points along the principal axis.	Use an achromatic lens (a converging and diverging lens in tandem). This combination rejoins separated colors.