Properties of Light

Light Photons ElectromagneticRadiation

These three are the same… Light *pure energy Electromagnetic Waves *energy-carrying waves emitted by vibrating electrons Photons *particles of light

Electromagnetic Waves Demo –Tesla Coil, Radio, Fluorescent light

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

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

THE ELECTROMAGNETIC SPECTRUM EM Waves Sources Radio Waves Microwaves Infrared Visible Ultraviolet X-rays Gamma rays Vibrating charges Molecular vibrations Atomic vibrations Nuclear vibrations

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

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

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.

Penumbra Umbra Partial Shadow Full Shadow Solar Eclipse Sun Moon Earth 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 - Pupil - has the eye color and controls light intensity Lens - the hole in the eye (red eye demo) does remainder of focusing Retina -location of light sensors, has rods and cones Blind spot - Fovea - center of vision, predominantly cones 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.

You will observe a total eclipse of the sun when... (a) you stand in the penumbra of the moon’s shadow (b) you stand in the umbra of the moon’s shadow (c) sunlight diffracts around the moon (d) sunlight reflects from the moon to the earth

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

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.

Light Emission The following slides will illustrate facts about Light and electrons as they relate to atomic structure and release of energy

Structure of the Atom Proton Neutron Electron Energy Levels

Excitation When an electron is raised to a higher energy level, the atom is said to be excited.

Emission When the electron returns to a lower energy level, energy is released in the form of light.

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

Robert BunsenGustav Kirchhoff “The Taste Test” versus “The Flame Test”

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 of Hydrogen Prism Photographic Film Film Slit Low Density Glowing Hydrogen Gas Discrete Emission Spectrum

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

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

Emission Spectra Continuous Emission Spectrum Prism Photographic Film Slit White Light Source

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

The brightness and 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

Discrete Absorption Spectrum Absorption Spectra of Hydrogen Prism Photographic Film Film Slit White Light Source Discrete Emission Spectrum Hydrogen Gas

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

Matching Questions Type of SpectrumAppearance 1. Emission Spectraa. All Colors 2. Continuous Spectrab. Dark Lines 3. Absorption Spectrac. Bright Lines

Matching Questions 1. Emission Spectraa. Hot Solids 2. Continuous Spectrab. Hot Stars 3. Absorption Spectrac. Hot Gases

Fluorescence Some materials that are excited by ultraviolet light can emit visible light upon de-excitation. This is fluorescence. Demo - Black light and chalk See Figures & 30.11

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

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

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 or diffraction grating

Matching Questions 1. Incandescencea. mercury vapor light tubes 2. Fluorescenceb. glow-in-the-dark paints and plastics 3. Phosphorescencec. light bulbs with filaments

Matching Questions 1.Fluorescent Lampa. color changes with temperature 2.Incandescent Bulbb. only one color of light 3. Laserc. converts ultraviolet light to visible light

End of Chapter 30

Brightness versus color curve for different temperatures (measured in Kelvins)

Emission Spectra Therefore, each element emits is own characteristic pattern of light frequencies.