Name _________________________
Date: ________
Properties of the Physical Universe, Part I - Light
Purpose: To familiarize students with the wave and particle properties of light.
Goals: Upon successful completion of this lab assignment, students will be able to:
1. Understand wave properties (wavelength, frequency, velocity, & amplitude)
as they pertain to light.
2. Understand how light energy is contained in discrete packets, called photons.
3. Compare different forms of radiation with regards to wavelength, frequency,
and individual photon energy.
4. Calculate the finite speed of light creates time delays in the transmission of
information across space.
The nature of light is quite different from anything we experience on an every day
basis. Some experiments have clearly shown that light behaves in a way similar to wave
phenomena, like sound. Light reflects off the surfaces of objects, refracts when passing
through different media, and its intensity can be described by the size of its amplitude.
Other experiments, most notably the photoelectric effect, reveal that light energy is
carried in discrete amounts, as if it were bundled inside of tiny little packages. As
contrary to common sense as it may seem, light is accepted to exhibit a dual nature; it has
properties of a wave and a particle.
Light as a Wave
When a rock is thrown into a pond, the pond is disturbed in a way that creates ripples
that spread across the water’s surface. These ripples are waves and they have particular
defining characteristics (see Figure 4.1).
Amplitude (A)
Wave velocity (v)
Wavelength ()
pond
Point where rock entered pond
Figure 4.1
The dashed horizontal line in Figure 4.1 represents the calm surface of the pond
before the rock created the disturbance. Once the rock enters the pond, the water is
disturbed in a way that forces the water to elevate to high points (crests) and depress to
low points (troughs). The amount that the water is disturbed is referred to as the
amplitude (A) of the wave and is measured from the point of equilibrium to the top of a
crest or the bottom of a trough. The units of measurement will depend on the wave
phenomena in question; for a water wave, units of distance are sufficient while the
amplitude of sound waves is measured in decibels.
The wave made when a rock is thrown into a pond will also travel with a particular
velocity (v). Every point on a wave moves with the same speed in a direction that takes it
away from the source. In this case, the wave moves away from the point where the rock
entered the water while sound waves move away from a set of stereo speakers and light
moves away from an illuminated light bulb.
The distance between crests (or troughs) is called the wavelength () and it is always
measured in units of distance. The wavelength and velocity are related to one another
through a quantity called the frequency (f). The frequency is a measurement of how
many waves pass a given point every second and is measured in units called Hertz (1 Hz
= 1 wave per second). For example, the note ‘middle C’ on a piano is a sound wave
with a frequency of 256 Hz. This means that 256 sound waves hit your ear drum every
second when the ‘middle C’ key is played.
The wavelength and frequency of any given wave are related to the wave’s velocity
by the following equation:
v =  f
The speed of sound is treated as a constant measurement (330 m/s), which means
that every sound wave has a unique wavelength and frequency (i.e. pitch). Similarly, the
speed of light is treated as a constant measurement (3 x 108 m/s) so every light wave will
also have a unique wavelength
 and frequency.
Exercises
1. Draw in a double-ended arrow (
) to indicate the wavelength () and
amplitude (A) of each wave in Figure 4.2. Measure the wavelength and
amplitude of each wave, in centimeters, and record your results in the table.
2. Calculate the frequency of each wave if they are both moving at the speed of
sound (v = 330 m/s). You will need to convert each of your measurements to
meters before substituting the values into the wave velocity equation.
Table 4.1
Amplitude
Wave A
Wave B
Wavelength
Frequency
Figure 4.2
Wave A
Wave B
3. Calculate the wavelength of a light wave that moves with a velocity of 3.0 x 108
m/s and has a frequency of 5.2 x 1014 Hz. Report your answer in nanometers
(nm).
4. What is the frequency of a light wave that has a wavelength of 125 m?
Light as a Particle
Waves also carry energy. It has been understood for quite some time that the energy
that is carried by a given wave is determined by the wave’s amplitude. The more intense
the wave, the more energy it carries (think of the damage done along the shoreline by
waves during a hurricane as opposed to the waves that normally crash on a beach). Light
waves, however, do not behave in this way at all. In order to correctly explain the
photoelectric effect, Albert Einstein changed our concept of the very nature of light. The
discovery he made, which won him the Nobel Prize, was that light energy is carried in
discrete amounts and not as a continual source of energy. When you flip on a light bulb,
tiny little packets of light (called photons, see Figure 4.3) come rushing out at the speed
of light instead of a single, continuous wave.
Figure 4.3: Conceptual drawing of a photon.
The energy of an individual photon is dependent upon its wavelength and not its
amplitude. The shorter the wavelength of a photon means that more waves can fit into
the package, thus the energy will be higher. The energy of a photon depends directly on
the photon’s frequency or inversely on the photon’s wavelength according to the
relationships below:
E photon = hf
- or -
E photon =
hc

c  speed of light = 3.00 x 10 m/s
8
h  Planck' s constant = 6.63 x 10 -34 J  s
5. Draw in a double-ended arrow (
) to indicate the wavelength of each photon

in Figure 4.4. Measure each photon’s wavelength and record your results in the
table in meters.
6. Calculate the frequency of each photon if they are both moving at the speed of
light (v = 3.0 x 108 m/s). You will need to convert each of your measurements to
meters before substituting the values into the wave velocity equation.
7. Calculate the energy of each photon using one of the photon energy equations
above. Record your answer in the table in units of Joules (J).
Figure 4.4
Photon A
Photon B
Table 4.2
Wavelength
Frequency
Energy
Photon A
Photon B
Types of Photons: the EM Spectrum
All photons move through empty vacuum at the speed of light. This being the case,
it is useful to categorize photons into groups according to their wavelength, frequency,
and energy. Photons are then often arranged by type according to their wavelengths,
from short to long. The boundaries are not strictly defined so a photon may be
considered to be “high energy UV” or “low energy X-Ray” depending on the scientist’s
interpretation. Figure 4.5 shows how the EM spectrum arranged is from short
wavelength to long wavelength. Table 4.3by a table that list one set of accepted
wavelength boundaries.
Figure 4.5
Gamma
Short 
High f
High E
Blue
X-Rays
Green
Ultraviolet
(UV)
Visible
Violet
Yellow
Orange
Infrare
(IR)
Microwave
Red
Radio Waves
Long 
Low f
Low E
Table 4.3
Radiation Type
Gamma Ray
X-Rays
Ultraviolet (UV)
Visible Light
Infrared (IR)
Microwaves
Radiowaves
Wavelength Range
Frequency Range
Energy Range
< 1 pm
1 pm – 1 nm
1 nm – 400 nm
400 – 750 nm
750 nm – 25 m
25 m – 1 mm
> 1 mm
> 1020 Hz
1020 - 1017 Hz
1017 - 1014 Hz
~ 1014 Hz
1014 - 1013 Hz
1013 - 1011 Hz
< 1011 Hz
> 10-13 J
10-13 - 10-16 J
10-16 - 10-19 J
~10-19 J
10-19 - 10-21 J
10-21 - 10-22 J
< 10-22 J
8. Table 4.4 lists a set of photons of different significance. Complete the table by
calculating the wavelength, frequency, and/or energy where necessary. Note that
all wavelengths need to be converted to meters before they can be used to
calculate any of the other quantities.
Table 4.4
Photon
Sun Light
Human Radiation
Microwave Oven
Medical X-Rays
UV-B
Satellite TV Signal
Wavelength
Frequency
Energy
517 nm
3.10 x 1013 HZ
2.45 x 109 Hz
1.00 nm
6.63 x 10-19 J
5.00 x 109 Hz
Speed of Light
It may seem instantaneous for light to travel across the room when we flip on a light
switch, but light travels at such a high speed that it takes an unperceivable amount of time
to travel across a tiny room that our eyes cannot perceive the time delay. Though the
speed of light is the fastest that anything in the universe can travel, it is possible to
measure it to high accuracy using sophisticated lab equipment. Currently, the speed of
light is measured to be 299,792,458 m/s, though in this course it will be sufficient to use
the rounded version of the number: c = 3.00 x 108 m/s. There are a couple of things to
keep in mind about the speed of light.
- Only light can travel at the speed of light. Anything with a mass, no matter how
small, CANNOT achieve the speed of light (though some tiny particles can get really
close).
- When talking about the speed of light we can use either the wave model or the
photon model. In the wave model, we can think of the entire wave moving at the
speed of light whereas in the photon model we think of the packet of energy moving
at that speed. The reason that a photon packet can move at the speed of light is
because the packet is massless being that it is pure energy.
- The speed of light is constant regardless of the frame of reference. This empirical
fact of nature has some strange, and possibly useful, consequences that we will
explore in a later lab.
- All forms of light will move at the same speed in vacuum. In other words, visible
light from the Sun, x-rays from a distant stellar explosion, and radio waves being
broadcast by a possible alien civilization all travel through empty space at the same
3.00 x 108 m/s. However, the speed of light will slow down to different degrees
once it travels through any substance (air, water, glass).
Since so much of our information is transmitted through various forms of light, it is
important to understand that there is a time delay between when information is given out
vs. when the information is received. The fact that the speed of light is constant as it
travels through space is critical when computing these delay times. Below are several
examples where it is useful to understand how long delay times are. Determine the delay
times in each of the cases below using the relation:
c =
d
t
9. A scientist operating the Hubble Space Telescope wants to look at a new object.
In order to reposition the telescope, the controller needs to send a radio signal to
the Hubble, which is orbiting the Earth at an elevation of 560 km. Calculate how
long it takes a for the
radio signal to get from the command center to the
telescope.
10. If the Sun were to blow up, we would not know it instantly because it takes time
for the light from the explosion to reach Earth. Calculate how long after the
explosion occurs we would receive its light given the distance to the Sun is 150
million km.
11. When trying to communicate with any of the spacecraft or rovers on Mars, the
operators have to consider the time delay in the transmission when sending
commands. Calculate the time delay that would be expected when a command is
sent to a rover on Mars when Mars is at a distance of 76.5 x 106 km.
Reflection
Light will interact with matter in ways that are explained on a macroscopic and
microscopic level. In this section we will focus on one macroscopic interaction
(reflection) while saving the microscopic ones for when we discuss the quantum nature of
matter.
When light encounters an object, the object will absorb some of the light while the
rest of the light will reflect, or bounce, off of the surface. Different amounts of incident
light will reflect depending on the properties of the object, namely density and
composition. As a simple example, think of how a mirror made of glass produces a better
reflection than a wooden tabletop. Scientists can measure the fractional amount of light
that reflects off of an object and interpret that amount as an indication of the object’s
composition. Such a measurement is referred to as the object’s albedo and is reported as
a decimal value between 0 and 1.00 (corresponding to 0% to 100%).
12. Below is a table of measured albedos for different objects/substances:
Object/Substance
Albedo
Grass
Forest
Granite
Sand
Ice
Soil
Snow
0.05 – 0.30
0.05 – 0.10
0.30 – 0.35
0.20 – 0.40
0.60
0.05 - 0.30
0.80 – 0.90
Based on the information in the above table, would scientists be better served to look
for a world with a low albedo or a high albedo if they were interested in detecting
possible life on that world? Explain your reasoning.