Chapter 26 Properties of Light
Electromagnetic Waves • Traveling, oscillating, electric and magnetic fields which are emitted by vibrating charges. • The electric and magnetic fields carry energy. • All electromagnetic waves travel at the speed of light in vacuum. (i.e.the wave speed = the speed of light)
Michael Faraday & James Clerk Maxwell • c 1831:Michael Faraday discovers electromagnetic Induction. Faraday also introduces the field-line concept for electricity and magnetism. • c 1865:James Clerk Maxwell develops mathematical relationships between electric and magnetic fields. Shows that all of electricity and magnetism are wave phenomena.
When Maxwell calculated the speed of these “Electro-magnetic” waves, he came up with thespeed of light. The speed of light = constant in vacuum c 3.00 x 108 m/s = 300,000,000m/s • electromagnetic wave in vacuum c = f
Note that everything you have learned about mechanical waves, with the exception of the need for a medium, also applies here to electromagnetic waves
Electromagnetic Spectrum -ray Radio Micro I.R. Visible U.V. X-ray 104 106 1010 1013 1023 Hz
Visible Part of EM Spectrum red =700nm fred =4.29 x 1014Hz violet=400nm f violet= 7.50 x 1014Hz
Transparent Materials • When light is incident on a particular material, the electrons and atoms of the material begin to vibrate while the light propagates through the material.
natural frequency of the electrons of the atoms in the material. • If the frequency of the light matches the: OR • the vibrational frequencies of the atoms and molecules as a whole. • then a resonant condition is established, and the light is absorbed.
For example: Most glass is made up of atoms (Si and other elements). They possess electrons that have natural frequencies of vibration that closely match the frequency of ultra-violet (UV)light. (1015- 1017) Hz
The larger atoms and molecules will vibrate at lower IR frequencies. So if IR is present, it will also be absorbed with the energy being dissipated as heat.
The visible radiation is sequentially absorbed, and then re-emitted from atom to atom. Eventually, the visible light passes through the entire piece of glass.
A state of resonance occurs for UV light. • We say that the glass is transparent to visible light, but not to UV or IR.
Glass IR radiation Visible light is transmitted Visible light UV radiation UV & IR get absorbed
Diffraction Any bending of light by means other than reflection and refraction is called diffraction.
Light Source Wide Window
Light Source Interference Pattern Narrow Slit
Two or more slits – Diffraction Grating Light Source Interference Pattern Two or more slits
Thin Film Phenomena – Constructive and destructive Interference Air OIL Water
Polarization The alignment of the transverse waves.
Time delay between absorption and re-emission of visible light Medium Average speed Vacuum c = 3 x 108 m/s Water v = 0.75c Glass v = 0.67c Diamond v = 0.41c
Opaque Materials • In Opaque materials, light is absorbed without re-emission. The energy of the light including visible, goes into the internal energy of the material – heating.
Metals are opaque but reflective. • These free electrons are responsible for why metals make such good electrical and thermal conductors. • Also, the light does not propagate from atom to atom as it does in a transparent material, but goes into reflection.
Light rays from the sun are essentially parallel Shadows Sharp shadow Large far away light source Sharp shadow Small nearby light source
Large nearby light source Penumbra (Partial shadow) Umbra (Total shadow)
Eclipses Sun Full Moon Not To Scale
Lunar Eclipse Earth Moon Sun Not To Scale
Solar Eclipse Earth Moon Sun SUN Drawing Not To Scale