Diffraction: waves spread out after they pass through an aperture
(small opening)
Refraction: waves will `bend' when going through certain materials
What is light?
An electric `field' surrounds all charged objects
An oscillating charge will produce as oscillating electro-magnetic field
The oscillating electro-magnetic field propagates as transverse
wave.
Electro-magnetic waves of a certain frequency (wavelength) are
perceived as `light'.
We perceive different frequencies of light as different 'colors'
All hot objects emit light.
A black-body is a body that absorbs all radiation that falls on it.
When a black body is heated, it starts to glow and emits a
characteristic black-body spectrum.
More energy is emitted from hot objects than from cool ones. The
peak emission occurs at higher frequencies for hot objects, at lower
frequencies for cooler objects.
An electron, such as those in the atoms in your eye, will be
pulled along with the oscillating electric field. This is how we
`see' light
The electrons in an atom or molecule can absorb or emit photons
(light quanta of a certain energy) and thus produce a discrete
spectrum.
Emission spectrum: Atoms or molecules emit photons (light quanta)
of a certain discrete energy when the electrons jump from a higher level
to a lower level. These photons have an energy characteristic for the
atom or molecules, thus show distinct colors in the visible spectrum.
Absorption spectrum: Atoms or molecules absorb photons of a
certain energy, thus there are black lines (missing frequencies) in the
light spectrum.
Light wavelengths
The wavelength is related to the frequency be
The speed of light in the vacuum isc = 3 * 108m/s.
Very long wavelengths of light are called radio waves
Long wavelengths of light are called infrared.
Visible light has wavelengths of 400 to 700 nanometers (a nanometer
is one billionth of a meter, or 10-9meter).
Short wavelengths of light are called ultraviolet
Light Sources
Incandescent light: produce light through being heated, e.g.
light bulbs, candles.
Fluorescence: The process of absorbing ultraviolet light and
emitting visible light. Excited atoms in fluorescent materials drop to
their normal level in about 10-8s.
Phosphorescent materials continue to glow long after the
illumination has been removed. The atoms remain in an excited metastable
state for periods ranging from a few seconds to several hours.
Triboluminescence: The process of producing light by mechanical
forces exerted on an object (e.g. crushing a wintergreen candy).
Photometry
Intensity is power per unit area and is measured in cd
[candles].
Luminance: intensity per unit area [candles/m2].
The intensity falls of inversely proportional to the square of the
distance from the light source.