5.3 Notes5.3 NotesLight & SpectrometryLight & Spectrometry

Pg. 136-149Pg. 136-149

ObjectivesObjectives Appreciate the phenomenon of how an

atom absorbs and releases energy in the form of light

Understand the differences between wave and particle theories of light

Describe the electromagnetic spectrum Understand and explain the dispersion of

light through a prism

Theory of LightTheory of Light Color is a visual indication of the fact that Color is a visual indication of the fact that

objects absorb certain portions of visible objects absorb certain portions of visible light and transmit or reflect others. light and transmit or reflect others.

Different chemical substances absorb Different chemical substances absorb certain types and certain amounts of light.certain types and certain amounts of light.

Two Models for Two Models for Explaining LightExplaining Light

1.1. Light described as a Light described as a continuous wavecontinuous wave

2.2. Light described as a Light described as a stream of discrete stream of discrete energy particlesenergy particles

Light as a continuous waveLight as a continuous wavec = c = λ λ x f x f

Wavelength (Wavelength (λλ - - lambdalambda))Unit of measure = nanometerUnit of measure = nanometer

Frequency (Frequency (ff) refers to the speed at ) refers to the speed at which wavelengths pass a given point in which wavelengths pass a given point in a unit of time a unit of time Cycles per secondCycles per second

Speed of light is 300 million meters per Speed of light is 300 million meters per second (second (cc))

LightLight Visible light is identified by the Visible light is identified by the

electromagnetic spectrumelectromagnetic spectrum Distinguishable by wavelength and Distinguishable by wavelength and

frequencies.frequencies. Can be described as continuous as long Can be described as continuous as long

as it travels spaceas it travels space

LASERLASER

LLight ight AAmplification by the mplification by the SSimulated imulated EEmission of mission of RRadiationadiation

Coherent light Coherent light Light that has all its waves pulsating in Light that has all its waves pulsating in

unisonunison

Light as a stream Light as a stream of discrete particlesof discrete particles

Occurs when radiation is absorbed by a Occurs when radiation is absorbed by a substancesubstance

Discrete particles are known as photons Discrete particles are known as photons each with a definite amount of energyeach with a definite amount of energy

E = hfE = hf ( (EE energy, energy, f f is frequency) is frequency) h = Planckh = Planck’’s constants constant 6.6262 x 106.6262 x 10-34-34 J/s J/s

SpectrophotometrySpectrophotometry

Measure the quantity of radiation that a Measure the quantity of radiation that a particular material absorbs as a function of particular material absorbs as a function of wavelength or frequencywavelength or frequency The invisible radiations of the electromagnetic The invisible radiations of the electromagnetic

spectrum.spectrum. This absorption by chemical substances is This absorption by chemical substances is

selective because different materials have selective because different materials have different energy requirements and therefore different energy requirements and therefore absorb at different frequenciesabsorb at different frequencies

SpectrophotometerSpectrophotometer

An instrument that measure absorption An instrument that measure absorption spectrum of a chemical substancespectrum of a chemical substance

Produces a graph that shows the Produces a graph that shows the absorption of UV, visible, and IR radiations absorption of UV, visible, and IR radiations

Some radiation reflected so not all is Some radiation reflected so not all is absorbedabsorbed

Just how much is absorbed depends on Just how much is absorbed depends on the concentration of the absorbing the concentration of the absorbing substancesubstance

Used to obtain qualitative data.Used to obtain qualitative data.

In-Class Assignment/HomeworkIn-Class Assignment/Homework