NOTE: As you read through this lesson take notes. At the
end of the lesson you will find questions to complete from
the book. I will check the notes and questions the day after
it is assigned.
Lesson 7.1 Introduction to Electromagnetic Radiation
Suggested Reading

Zumdahl Chapter 7 Sections 7.1 & 7.2
Essential Questions

What is the basic nature of light?
Learning Objectives


Describe the electromagnetic spectrum.
Solve problems relating to energy, wavelength, and frequency.
Introduction
Very soon we will begin talking about the formation of chemical bonds.
However, in order to understand chemical bonding, you need to know
something about the electronic structure of atoms. The present theory of
the electronic structure of atoms started with an explanation of the colored
light produced during flame tests such as the ones you carried out last year
in Honors Chemistry. Before we can discuss this, we need to describe the
nature of light. This is a complex topic, but you are only expected to have a
cursory understanding of it. My plan is to give you only what is needed to
understand atomic structure.
The Wave Nature of Light
If you drop a stone into one end of a quiet pond, the impact of the stone
with the water starts a wave. A wave is a continuously repeating change or
oscillation in matter or in a physical field. Light is also a wave. It consists of
oscillations in electric and magnetic fields that can travel through space.
Visible light, x-rays, and radio waves are all forms of electromagnetic
radiation (EMR).
Wavelength and Frequency
Waves are characterized by their wavelengths and frequencies. The
wavelength, denoted by the Greek letter λ (lambda), is the distance
between any two adjacent identical points of the wave. Thus, the
wavelength is the distance between two adjacent peaks or troughs.
The frequency of a wave is the number of wavelengths of that wave that
pass a fixed point per unit of time (usually 1 second). Frequency is denoted
by the greek letter ν (nu, pronounced "new").The unit of frequency is /s or s1
, which is also called the hertz (Hz).
The wavelength and frequency are related to each other. For two waves
traveling with a given speed, wavelength and frequency are inversely
related: the greater the wavelength, the lower the frequency and vice
versa.
The product νλ, is the total length of the wave that has passed the fixed
point in 1 second. This length of wave per second is the speed of the wave.
For light of speed c
c = νλ
This is commonly referred to as "light speed" or the speed of light, which, in
a vacuum, is equal to 3.00x108 m/s. The speed in air is only slightly less,
so this value is treated as a constant.For calculations involving the speed of
light, we assume the light is traveling in a vacuum.
This is a very simple equation, so the problems relating to this equation are
fairly easy. Watch the tutorial to see some examples:
Watch this YouTube Video
https://www.youtube.com/watch?v=goJl54Y1hio
The Electromagnetic Spectrum
The range of frequencies or wavelengths of electromagnetic radiation is
called the electromagnetic spectrum. Note that visible light extends from
the violet end of the spectrum, which has a wavelength of about 400 nm, to
the red end, with a wavelength of less then 800 nm. Beyond these
extremes, electromagnetic radiation is not visible to the human eye.
Infrared radiation (IR) has wavelengths greater than 800 nm (greater then
the wavelength of red light), and ultraviolet radiation has wavelengths less
than 400 nm (less than the wavelength of violet light).
The following mnemonic device is often used to help remember the visible
light spectrum:
Quantum Effects and Photons
In the early 1800s scientists debated about the nature of light. Isaac
Newton suggested that light consisted of a beam of particles while others
thought it had wave properties. By the early part of the twentieth century,
the wave theory of light was well entrenched. But in 1905, German
physicist Albert Einstein (emigrated to the U.S. in 1933) discovered that he
could explain a phenomena known as the photoelectric effect by
postulating tht light had both wave and particle properties. Einstein based
his work on the work of another German physicist named Max Planck.
Planck's Quantization of Energy
Planck carried out experiments with solids and discovered that the atoms of
solids oscillate, or vibrate, with a definite frequency, ν, depending on the
solid. This led him to conclude that the vibrational energies of the atoms
were quantized, which means they were limited to certain values. The
equation Planck used to calculate this energy was E = nhν, where n = 1,2,3
... The symbol h is a constant now called Planck's constant, which is
equal to 6.63 x 10 -34 J ∙ s. The numbers symbolized by n are called
quantum numbers, where the value of n must be 1 or 2 or some other
whole number.
This probably sounds like a bunch of gobly goo, but Planck's work provides
a basis for the quantum model of the atom; our present theory of atomic
structure. Therefore, you must be familiar with Planck and his work.
Photoelectric Effect
Einstein used Planck's work in his explanation of the structure of light.
Einstein suggested that lights consists of quanta (now called photons), or
particles of electromagnetic energy, with energy E proportional to the
observed frequency of light:
E = hν
The photoelectric effect is the ejection of electron from the surface of a
metal when light shines on it. Electrons are only ejected when the
frequency of light exceeds a certain threshold value characteristic of the
particular metal. To explain this, Einstein assumed that the photons in the
light must have a least enough energy to remove the electron from the
attractive forces of the metal. Thus, Einstein showed that the photoelectric
effect depended on both the frequency of light (waves) and the photons
within the light (particles), which means that light has properties of both
particles and waves. This is called the wave-particle duality of light, which
is displayed in the equation E = hν. E is the energy of a light particle or
photon, and v is the frequency of the associated wave. Neither the wave
nor the particle view alone is a complete description of light.
Example: Calculating the Energy of a Photon (This is an important
problem type)
The red spectral line of lithium occurs at 671 nm (6.71 x 10-7 m). Calculate
the energy of one photon of this light.
Solution:
The frequency of light can be found using c = νλ by rearranging to give ν
= c/λ = (3.00 x 108 m/s) ÷ (6.71 x 10-7 m) = 4.47 x 1014/s.
The energy of one photon is E = hv = (6.63 x 10 -34 J ∙ s) x (4.47 x 1014/s)
= 2.96 x 10-19 J
Good dimensional analysis skill will come in handy here. Analyze the units
to see your way to the solution!!!
**This lesson is courtesy of Kimberly Richardson
Homework problems:
Book questions pg. 321 questions 31, 33, 35,37