Energy. Radiant Energy Radiant: think light…. How does light carry energy through space???

EnergyEnergy

Radiant EnergyRadiant Energy

• Radiant: think light….

• How does light carry energy through space???

WavesWaves• Light waves

– electromagnetic – consist of an electric & magnetic fields

oscillating at right angles to each other in direction of motion

WAVE WAVE CHARACTERISTICSCHARACTERISTICS• WavelengthWavelength –

distance between crests

• AmplitudeAmplitude – height of wave from origin to crest or peak– Brightness

• FrequencyFrequency – how fast a wave oscillates – The number of

times a wave completes an up and down cycle

– Measured in cycles per second

– s-1 = 1/s = hertz = Hz

Speed of LightSpeed of Light

• constant speed in space

c = 3.00 x 108 m/s

For all waves,• The shorter the wavelength () of a

wave, higher the frequency ()• c = x = c/

Practice ProblemPractice Problem

• Calculate the wavelength of a radio wave with a frequency of 93.1 x 106 s-1

= c/ = 3.00 x 108 m/s / 93.1 x 106 s-1

= 3.22 m

Electromagnetic SpectrumElectromagnetic Spectrum

• Beyond red is the infrared portion. This is also called radiant heat– Used in microwaves,

radio waves, tv waves

Electromagnetic SpectrumElectromagnetic Spectrum

• Beyond violet is the ultraviolet radiation (UV)– Responsible for

sunburns and skin cancer.

– X-rays are even shorter in wavelength

Electromagnetic SpectrumElectromagnetic Spectrum

Quantum TheoryQuantum Theory

Planck’s TheoryPlanck’s Theory

• Max Planck predicted accurately how the spectrum of radiation emitted by an object changes with temperature

• Proposed that there is a fundamental restriction on amounts of energy that an object emits or absorbs, called quantum.– Means FIXED AMOUNTS

• Related the Frequency of the radiation to the amount of energy.

• E = h

• Energy = Planck’s Constant x Frequency

Know Planck’s Constant Know Planck’s Constant

(h) = 6.6262 x 10(h) = 6.6262 x 10-34-34 J-s (Joule x second) J-s (Joule x second)

• Energies absorbed or emitted by atoms are quantized, which means that their values are restricted to certain quantities.

• Energy is not continuous!

• Quantized Continuous

Planck’s Theory AppliedPlanck’s Theory Applied

• Determining temperatures of distant planets and stars by measuring the wavelengths of the EM radiation that they emit

• Why can’t we see quantized energy– Planck’s Constant is very small.– Quanta are too small for every day notice,

but to a very small atom they make a big difference!

The Photoelectric EffectThe Photoelectric Effect(Einstein thanks Planck…)(Einstein thanks Planck…)

• electrons ejected from surface of a metal when light shines on it

• Einstein realized that Planck’s idea of energy quanta could explain this

• Light consists of quanta of energy that behave like tiny particles, called PHOTONS– Each photon carries an amount of energy

given by Planck’s equation.