Electromagnetic RadiationTONYA PATTERSON

What is light and How does it behave?

Light acts like a wave

Has particle-like properties, as well

(Because of this it took many year to figure out.)

Waves have 3 primary characteristics: wavelength,

frequency, and speed.

Dual Nature of Light

Wavelength

Is the distance between two consecutive peaks or troughs in a wave.

Symbolized by the Greek letter lambda, λ.

Frequency

Is defined as the number of waves (cycles) per second that pass a given point in space.

All types of electromagnetic radiation travels at the speed of light, short-wavelength radiation must have a high frequency.

Symbolized by the lowercase Greek letter nu, ν.

The SI unit for frequency is 1/s or s-1 or commonly referred

to as the hertz (Hz)

Relationship

The wave with the shortest wavelength has the highest frequency and the wave with the longest wavelength has the lowest frequency.

This implies an inverse relationship between wavelength and frequency.

https://www.youtube.com/watch?v=cfXzwh3KadE

The wavelength times the frequency is equal to the speed of light.

Speed of light (c) = 2.9979 x 108 m/s or 3.0 x 108 m/s

We can use the formula c = λν to calculate the wavelength or frequency.

Electromagnetic Sprectrum

Frequency of Electromagnetic Spectrum

The brilliant read colors seen in fireworks are due to the emissions of light with wavelengths around 650 nm when strontium salts such as Sr(NO3)2 and SrCO3 are heated. (This can be easily demonstrated in the lab by dissolving one of these salts in methanol that contains a little water and igniting the mixture in an evaporating dish.) Calculate the frequency of red light of wavelength 6.50 x 102 nm.

View of particles & light during the 19th century

At the end of the 19th century the idea prevailed that matter and energy were distinct.

Matter was thought to consist of particles, whereas energy in the form of light (electromagnetic radiation) was described as a wave.

Particles were things that had mass, and whose position in space could be specified.

Waves were described as massless and delocalized; that is, their position in space could not be specified.

Also assumed that there was o intermingling of matter and light.

Everything known before 1900 seemed to fit neatly into this view.

Planck’s Constant

During the beginning of the 20th century, certain experimental results suggested that this picture was incorrect.

Max Planck (German physicist) was studying the radiation profiles emitted by solid bodies heated to incandescence, Planck found that the results could not be explained in terms of the physics of his day, which said that matter could absorb or emit any quantity of energy.

He was only able to account for these observations only by postulating that energy can be gained or lost only in whole-number multiples of the quantity hv.

h is a constant (Planck’s constant), and through experimentation, was determined to have a value of 6.626 x 10-34 J· s.

The change in energy for a system ∆E can be represented by the equation:

∆E = hv or

h = planck’s constant

v = frequency of the electromagnetic spectrum absorbed or emitted

Planck’s Results

Planck’s results was a surprise.

It had been assumed energy of matter was continuous, meaning that the transfer of any quantity of energy was possible.

It then seemed clear that energy is in fact quantized and can occur only in discrete units of size hv.

Each of these small “packets” of energy is called a quantum.

A system can transfer energy only in whole quanta. Thus energy seems to have particulate properties.

Energy of a Photon

The next big development in the knowledge of the atomic structure came with Albert Einstein.

The proposed that electromagnetic radiation is itself quantized.

Einstein suggested that electromagnetic radiation can be viewed as a stream of “particles” called photons.

The energy of each photon is given by the expression:

λ

hchνEphoton

Photon

Bundle of Light

Energy of a Photon

The blue color in fireworks is often achieved by heating copper (I) chloride (CuCl) to about 1200°C. then the compound emits blue light having a wavelength of 450 nm. What is the increment of energy (the quantum) that is emitted at 4.50 x 102 nm by CuCl?

Practice

Determine the energy of a photon that has a wavelength of 450nm.

2. Calculate the energy of a photon with a frequency of 1.85 x 1015Hz.

3. Calculate the energy of a photon with that has a wavelength of 1140nm.

The Photoelectric Effect

A phenomenon in which electrons are ejected from the surface of certain metals exposed to light of at least a certain minimum frequency, also called the threshold frequency.

The number of electrons ejected was proportional to the intensity (or brightness) of the light, but the energies of the ejected electrons were not.

Below the threshold not electrons were ejected no matter how intense the light.

The photoelectric effect could not be explained by the wave theory of light. Einstein, however, made an extraordinary assumption.

He suggested that a beam of light is really a stream of particles.

These particles of light are now called photons.