Chapter 14 Properties of Light

Waves Wave - a moving disturbance Moving deformation of transport medium which propagates through the medium Example - a cork bobbing up and down …… ……...shows that a disturbance is passing rather than a flow of water (current) Waves transport energy from one point in space to another point

Types of Waves A) Mechanical waves - are those waves requiring a material medium to transport the disturbance A1) Transverse----- medium moves at right angles to the direction of transport A2) Longitudinal ------ medium moves in the direction of transport of the disturbance

Types of Waves EM waves are transverse waves Electromagnetic waves - are those waves requiring no material medium to transport the disturbance EM waves are transverse waves

The Electromagnetic Spectrum A range of light waves extending in wavelength from radio waves to gamma rays

The Electromagnetic Spectrum Gamma Rays - most energetic X-rays - penetrates tissue Ultraviolet - causes sunburns Visible Light - detected by your eyes Infrared - “heat waves” Microwaves - used to cook Radio Waves - communication

Electromagnetic Waves Electromagnetic Waves - energy-carrying waves emitted by vibrating electrons Also know as… Light Radiation Photons

Electromagnetic Wave Velocity The speed of light is the same for all seven forms of light. It is 300,000 kilometers per second or 186,000 miles per second.

Radio waves electrons moving up and down an antenna Visible Light electrons changing energy states in an atom

The Visible Spectrum A range of light waves extending in wavelength from about 400 to 700 namometers.

Let Their Be Light Light Emission

light

Excitation and de-excitation of electrons

Emission Lines for the Hydrogen Atom (Chaisson/McMillan) Astronomy

Excitation When an electron is raised to a higher energy level, the atom is said to be excited. When the electron returns to a lower energy level, energy is released in the form of photon (pellet of light).

Simple model of the atom Electron absorbs some energy which causes it to “jump” to a higher energy level. This is called excitation. E2 Energy E1 Nucleus

Excitation E2 UV light E1 Nucleus

de-excitation The electron, preferring to be in the lower energy, immediately drops back down to the lower energy level. Energy E2 E1

In order to conserve energy, a photon (discrete bundle of energy) is emitted. Photon Energy E1 Photon Energy = E2 – E1

. Because of the behavior of matter and energy at the atomic scale, which is governed by Quantum Mechanics, the energy of the photon is equal to the difference in the energies of the two energy levels and is also proportional to its frequency.

Emission Spectrum (Chaisson/McMillan) Astronomy

Emission Lines for the Hydrogen Atom (Chaisson/McMillan) Astronomy

Different transitions from high levels to low levels result in different colors of light.

The Visible Spectrum

Things that create a Spectrum Prism Raindrops CD’s Diffraction Grating

The Kirchhoff-Bunsen Experiment These two scientists found that burning chemicals over an open flame resulted in a spectrum with bright lines. They found that each chemical element produced its own characteristic pattern of bright spectral lines.

Emission Spectra Hot gas produces a bright line emission spectrum. Demo - hot hydrogen gas and diffraction gratings Emission Spectrum

Hydrogen Helium Oxygen Carbon Every element can be “fingerprinted” by it spectra.

Prism and colors

Prism and colors

Why Sky is Blue……...

Why Sky is Blue……...Rayleigh Effect

Why Sky is Blue……...

Incandescence Hot, dense solids produce a continuous spectrum. Demo - an incandescent light bulb and diffraction gratings Continuous Spectrum

The color of light emitted by a hot object changes with its temperature. Glowing object colors: Reddish  coolest glowing object Orange-ish Yellowish White Bluish  hottest glowing object

Absorption Spectra Cool gas in front of a continuous source of light produces an absorption line spectrum. Fraunhofer lines in our Sun's spectrum showed that cool helium gas surrounds the Sun. Absorption Spectrum

Fluorescence Some materials that are excited by ultraviolet light can emit visible light upon de-excitation. This is fluorescence.

Fluorescence Fluorescence occurs when UV light is absorbed by the electrons of an atom and visible light is emitted upon de-excitation.

Fluorescent Lamps Primary excitation - electron collisions with low pressure mercury vapor, and ultraviolet light is given off Secondary excitation - ultraviolet light is absorbed by phosphors and these emit visible light

Fluorescent Lamps e- Fluorescence High-speed electron Hg atom UV Photon Phosphor Coating Fluorescence

Phosphorescence Phosphorescence - a type of light emission that is the same as fluorescence except for a delay between excitation and de-excitation. Electrons get "stuck" in an excited state and de-excite gradually. Demos - glow-in-the-dark objects

Phosphorescence Atoms or molecules are excited by incident visible light and remain in an excited state for a long time before they de-excite. This allows phosphorescent materials to glow in the dark.

Phosphorescent Dove

Glow in the Dark Dove

Incoherent and Coherent light White light from an incandescent light bulb is called incoherent light because the light is composed of different colors and the waves are out of phase with each other.

If we pass the white light through a filter, we could get only one color (monochromatic) light which would still be out of phase. (incoherent red light)

Monochromatic, Coherent light By getting all the red light in phase, we would have monochromatic, coherent light. This is the kind of light emitted by a LASER.

Monochromatic, Coherent Red light

Lasers Light Amplification Stimulated by Emission Radiation of

Lasers Laser Light amplification by stimulated emission of radiation Lasers produce coherent light. Coherent light means that all the light waves have the frequency, phase and direction. Demo - Laser and chalk dust Demo - Laser and prism

Transparent Materials Transparent - the term applied to materials through which light can pass in straight lines

Opaque Materials Opaque - the term applied to materials that absorb light

Example Questions Are clouds transparent or opaque to visible light? Answer: opaque Are clouds transparent or opaque to ultraviolet light? Answer: transparent

Shadows Umbra - the darker part of a shadow where all the light is blocked Penumbra - a partial shadow These terms also apply to Solar Eclipses and Lunar Eclipses.

Solar Eclipse Umbra Sun Full Shadow Earth Moon Partial Shadow Penumbra A solar eclipse occurs when the Moon passes in front of the Sun.

Lunar Eclipse Sun Earth Moon A lunar eclipse occurs when the Moon passes into the Earth's shadow.

Questions Which type of eclipse is dangerous to view with the unprotected eye? Why are lunar eclipses more commonly seen than solar eclipses? *

SEEING LIGHT - THE EYE Cornea - does most of the focusing Iris - has the eye color and controls light intensity Pupil - the hole in the eye Lens - does remainder of focusing Retina - location of light sensors, has rods and cones Fovea - center of vision, predominantly cones Blind spot - optic nerve exit, no light sensors

Color Deficiency

Myopia (Near-Sightedness) People with near-sightedness cannot see clearly at distance.

Hyperopia (Farsightedness) People with far-sightedness cannot see clearly up close.

The speed of light... (a) has never been measured (b) is about the same as that of sound (c) is infinitely fast (d) is very fast, but not infinite (d) is very fast, but not infinite

In the dark at late evening, no color is seen because of lack of stimulation of a. rods. b. cones. c. cornea. d. crystalline lens. b. cones.