Objectives
I can calculate
wavelength, frequency
or energy of light.
I can explain the
emission spectrum of an
element.

equation #1
y  4 x
warmup
equation #2
1.
2.
3.
4.
4
y
x
In equation #1, if the value of x increases, then y will ____________
In equation #2, if the value of x increases, then y will _____________
Which equation is a direct proportion? ____________
Which equation is an inverse proportion? ____________
5. In equation #3, y is ______________ proportional to x.
(directly, inversely)
6. In equation #3, y is ______________ proportional to z.
(directly, inversely)
equation #3
x
y
z
How do Neon lights work?




First we need to know what light is and
how to quantify properties of light:
frequency
wavelength
energy
Electromagnetic Waves

Light is an electromagnetic wave.
1.
2.
3.
4.
label
label
label
label
wavelength
crest,
trough
amplitude
Frequency of a wave (f ) is the number of wave cycles
that pass a given point in one second: cycles per second
or Hertz (Hz)
Wave simulation
Electromagnetic Waves

Electromagnetic radiation includes radio
waves, radar, microwaves, visible light,
infrared waves, ultraviolet light, x-rays, and
gamma rays.
Electromagnetic Waves

Key Point! For every type of electromagnetic wave,
the speed is the same.
 For every electromagnetic wave, if you multiply
the frequency by the wavelength you always get
the speed of light!
Speed of Light = (frequency)(wavelength)
c  f
Frequency and Wavelength
formula
/sf = frequency
formula:
frequency:
wavelength:
λ (lamda) = wavelength
c  f
Frequency and Wavelength
formula
velocity of a light wave: c  f
velocity of a light wave: c = 3.0 x 108 m/s
f = frequency
λ (lamda) = wavelength
speed of light:
frequency:
wavelength:
c  f
c
f 

c

f
c
f

If the wavelength gets decreases,
then frequency will ________.
A.
B.
C.
decrease
increase
remain constant
c
f 

Therefore wavelength and frequency
are ______________.
A.
B.
C.
directly proportional
inversely proportional
not related
c
f 

Example 1: Calculate
wavelength
c  f

Bob FM broadcasts music at a
frequency of 103.5 MHz
= 103.5 x 106 Hz. What is the
wavelength of a Bob FM wave?
Example 1: Calculate
wavelength
c  f

c
f
Bob FM broadcasts music at a
frequency of 103.5 MHz
= 103.5 x 106 Hz. What is the
wavelength of a Bob FM wave?
c 3.00108 m / sec
 
 2.90 m
6
f
103.5 10 Hz

Example 2: Calculate
frequency
c  f

A DVD player uses a red laser with a
wavelength of 6.5 x 10-7 m. What is the
frequency of this light?
Example 2: Calculate
frequency
c  f

c
f

A DVD player uses a red laser with a
wavelength of 6.5 x 10-7 m. What is the
frequency of this light?
c 3.0010 m / sec
14
f  

4
.
6

10
Hz
7

6.5 10 m
8
“Color” of light depends on the wavelength, the
shorter the wavelength the higher the frequency.
The shorter the wavelength, the higher
the energy.
sketch an electromagnetic wave getting shorter and shorter
Which wave has the higher
energy per photon?
A.
B.
[Default]
[MC Any]
[MC All]
Wave A
Wave B
Light is particles of energy!!!


When light reacts with matter it acts
as particles called a photons.
A photon is a quanta of energy that
is carried by the light wave.
Energy depends on the ‘color’


The energy carried by a photon can be
calculated if we know the frequency or
the wavelength of the photon
Energy = h x f



h = Plank’s constant (6.63 x 10-34)
f = frequency
since f= c /  … Energy = h x c

How do Neon lights work?
Energy = h x f
The higher the frequency, the higher the
energy!
= Directly Proportional
h = Plank’s constant (6.63 x 10-34)
How do Neon lights work?
Energy = h x c = h x f
λ
The shorter the wavelength, the higher
the energy! = inversely proportional
To calculate energy of a
photon given frequency

What is the energy of a photon of
microwave radiation with a frequency of
3.20x1011 Hz?
ans: 2.21E-22 J
To calculate energy of a
photon given its wavelength

What is the energy of an x-ray photon that
has a wavelength of 0.05 nanometer.

note: 1nanometer (nm) = 1x10-9 m
1.99 x10
-16
J
Electromagnetic Spectrum
exit quiz
How do Neon lights work?



Einstein proposed that light could be described as a
quanta of energy and behave like particles.
Einstein proved this phenomenon by explaining the
photoelectric effect.
This effect occurs when light hits the surface of a metal,
and electrons are ejected.
http://phet.colorado.edu/en/simulation/photoelectric
How do Neon lights work?


When elements in the gaseous state are
electrically stimulated, they emit a light of a
specific wavelength, or color.
Every element emits a different colored light.
Neon = orange red
Xenon = blue
Argon = lavender
How do Neon lights work?


So where exactly does the neon light
come from?
As electrons gain energy, they become
excited and move into high energy
levels around the atom.
How do Neon lights work?

When those electrons drop back down
to the original energy level, the energy
is released as a photon of light.
http://phet.colorado.edu/en/simulation/discharge-lamps
How do Neon lights work?


The specific color that each element
emits is called its atomic emission
spectrum.
This spectrum can go outside of the
visible light range.
Concepts