5th to 3rd Century
B.C. (500 – 300 BC)
Euclid explored and summarized the fundamental
knowledge of optics such as reflection, diffusion, and vision. He wrote a book
10th to 11th
Centuries (900 – 1100)
Ibn al-Haytham made many discoveries about the behaviour of light, and how it reflects and
refracts off of mirrors and through lenses.
17th Century (1601 – 1700)
Römer, a Danish astronomer noticed variations in the eclipse times of one of
Jupiter’s moons over the course of a year. Römer’s theory was that the light
took longer to reach Earth when it was further away from the moon. He was able
to roughly calculate the speed of the light to 200,000km/h
Newton believed that light was made of particles called corpuscles. He
published a book titled “Opticks” in which he talks about the “Light Particle
Huygens thought that light was a
vibrating wave travelling in a substance called ether.
18th to 19th Century
(1701 – 1900)
Isaac Newton was a great contributor to the science of optics using lenses,
prisms, mirrors, telescopes, and microscopes. Newton announced that “light is a mixture of various colours having different refractivity” and
demonstrated this using the famous prism experiment.
Thomas Young saw some flaws in Newton’s theory. Like
the explanation of why light reflects and refracts between air and water. Young
then came up with the basic idea for the double slit experiment that is so well
known today. When he did this experiment, Young was able to use the data that
he collected to calculate the wavelengths for the different colours of visible light, very close to the values that are accepted today.
This experiment was able to disprove Newton’s theory.
Hans Christian Orsted
discovered that electricity and magnetism are related. When he flicked a
battery on and off, he realized that the needle on a nearby compass would
suddenly jerk. This would later be described as a changing electrical field
creating a magnetic field.
Faraday had discovered the exact opposite of what Orsted said, to be true.
Clerk Maxwell predicted that electromagnetic waves exist, that they propagate
at the exact speed as light does, and as horizontal waves. Maxwell also
combined Orsted and Faraday’s findings that stated a changing electrical field
could create a changing magnetic field, which would then create a changing
electrical field, and so on. Using this he created the Maxwell Equations.
Maxwell was also able to calculate the speed of light at about 300,000,000m/s –
pretty close to the measurement used today.
Planck introduced Planck’s constant.
20th Century (1901 – 2000)
Albert Einstein’s research resulted in groundbreaking
findings. Einstein wrote about the photoelectric effect theory that said that
light is made up of small particles called photons. Another paper he published
was about the theory of Brownian motion that utilized the kinetic energy of
molecules. His third paper that was published in the early 1900’s was his
theory of special relativity. His work on the photoelectric effect and with
quantum physics won him the Nobel Prize in physics in 1921.
Electromagnetic radiation is a kind
of radiation including visible light, in which electric and magnetic fields
There are seven different types of
electromagnetic radiation: radio waves, microwaves, infrared light, visible
light, ultraviolet light, X-rays, and gamma-rays. They are arranged in what is known as the electromagnetic
spectrum. They are organized based on the wavelengths, the longest wavelength
being radio waves and the shortest being Gamma-Rays. As seen in the diagram
below, radio waves also have the least amount of energy whereas gamma rays have
the highest amount.
Radio waves are used by the radio
that you hear in your car but are also emitted by stars and gasses in space.
Microwaves are what is used in microwave ovens to heat food but are also used
by astronomers to learn about the structures of nearby galaxies. Next is
infrared light. It is used in night vision goggles to detect heat from a
person’s skin or other objects, but it is also used by scientists to map the
dust between stars. Ultraviolet light is emitted by hot objects in space, like
the sun. X-rays are used by doctors and dentists but are also emitted by hot
gasses in space. Gamma-rays are last on the spectrum. They are used by doctors
to see the inside of your body and are commonly emitted by stars and gasses in
Electromagnetic radiation can be
described using frequency and wavelength. The frequency of a wave, measured in
Hertz (Hz), counts the number of waves that pass by a point in one second. The
wavelength () of a wave, measured in meters
(m), measures the distance from one peak of one wave to the peak of the next.
These two measurements are inversely related. This means that the larger the
frequency, the smaller the wavelength. The smaller the frequency, the larger
The light properties equation shows
the relationship between the speed of light, the frequency of the wave of
light, and the wavelength. Because the speed of light is a constant figure, if
the frequency goes up then the wavelength has to go down, and vice versa.
c = the speed of light
= the frequency of the light wave
= the wavelength
Example: What is
the frequency of electromagnetic radiation that has a wavelength of 210.0nm?
Electromagnetic energy is produced
when electric charges change their potential energy. It can be identified as
electromagnetic energy if it is known as pure energy, which means that it does
not need matter to exist or move. Pure energy can then travel through the
vacuum of space, unlike sound.
James Clerk Maxwell took the known Ampere’s law and
Faraday’s law and combined them to create Maxwell’s equations. These equations
are complex and difficult to understand. These equations are laws (much like
the laws of gravity) and they describe the events of an electrical field
creating a magnetic field which creates an electrical field, and so on. They
were also used to calculate the speed of light.
Light can be used to push electrons
free from a solid surface. When experimenting, it was found that the intensity
of the light had no effect on the maximum kinetic energy of the dislodged
electrons. The light with the same frequency,
no matter the intensity, will have the same amount of energy of electrons.
Later, it was discovered that light had a threshold frequency that would not
eject electrons from a metal surface no matter how bright the source.
K = the
maximum amount of kinetic energy
E = the
energy of the absorbed electrons
= the work function of the surface
E = the
energy of a photon (Joules)
c = the
speed of light
= the wavelength (meters)
Example: What is
the energy of a photon of blue light (? = 450 nm)?
E = ?
Wave-Particle Duality refers to the idea that light behaves like a particle and a wave
interference experiment showed that light passing through the two slits add
together or cancel each other out to show interference fringes. This phenomenon could only be explained if the light was considered as waves. This experiment
was carried out, many, many years later, using technology that is able to
detect individual light particles. When the light source was weakened and
projected on a screen, the results showed light behaving as particles. When the
light source was extremely bright, the
results showed interference fringes which means that the light was behaving
like a wave.
The double slit experiment has been
carried out using different particles, not just light, with similar results.
When the experiment was conducted using electrons, they behaved the same way. Scientists
do not know why the particles behave like both a wave and a particle, so they
decided to try and measure it, but as they set up the equipment to do so, the
interference fringe disappeared. There is no possible way to measure what the
particle does when it goes through the slit.
Reflection, Refraction, Absorption, and
Reflection of Light
is reflected on a smooth surface, it bounces off of the surface at an angle
equal to the angle of the incoming light. Examples include mirrors and glass.
When light is reflected off of a rough surface the
light is reflected at all different angles. An example would be the Earth.
Refraction of Light
Light goes through an object and bends at an angle. Examples of
refractive objects would be diamond (a greater angle) and water (a lesser
Absorption of Light
Light stops and does not reflect or
refract. Objects that absorb light are
often dark and opaque. An example would be wood.
around an object, opening, or slit and spreads out. This occurs when the light
wave passes by a corner or through an
opening or slit that is approximately the same size as the light wavelength, or
smaller. Diffraction can also cause the light to separate into the visible
The refractive index is the measure
of the bending of a ray of light when passing from one medium to another.
Calculating the Refractive Index
Index of refraction for various
Index of Refraction
Carbon Dioxide Gas
When the light goes from a medium with a lower index of
refraction to a medium with a higher index of refraction, the light will bend
towards the normal. When the light goes
from a medium with a higher index of refraction to a medium with a lower index
of refraction, the light will bend away from the normal.
This link is an interactive lab
that lets one explore the relationship between the incident ray and the
refracted ray of light between two mediums. It also calculates Snell’s Law for
The equation allows you to define
the relationship between the angles of
the incident ray and the refracted ray of light.
n = the
refractive index of each medium
= the angles of the light travelling through
the mediums with respect to the normal
Example: Light travelling through an optical fibre (n=1.44) reaches the end of the fibre and exits into the air. If the angle of incidence on the end of
the fibre is 30°, what is the angle of
refraction outside the fibre?
constant was introduced in 1900. It can be defined as the distribution of
energy emitted by a blackbody. In this case, radiation, such as light, is
emitted, transmitted, and absorbed in quanta determined by the frequency of the
radiation and the value of Planck’s constant. The energy of each quantum or photon equals Planck’s constant
times the radiation frequency.
equations are for calculating the energy of a photon. In the first equation, if
the energy goes up, so does the frequency. In the second equation, if the
energy goes up, the wavelength goes down.
This formula is used in quantum mechanics and to describe the
particle aspect of light.
E = energy
of each quantum or photon (in Joules)
= the frequency (sometimes will be replaced by an f, but they both
Example: What is
the energy of green light with a frequency of ?
E =Energy of a photon (Joules)
h = Planck’s constant
c = the speed of light
= wavelength (meters)
Wien’s Law is a law used for black
bodies, perfect radiation (light) emitters and absorbers and indicates at what
wavelength they tend to give off their light. This law is used to explain why
the colours of objects change as you
change their temperatures. The wavelength and the temperature are related. So it the wavelength
goes up so does the temperature. If the wavelength goes down the temperature
= wavelength that most
of the light is emitted at
T = the temperature of the object (K)
Example: An object has a temperature of
10,000 K. what is its max (lambda max) value?
Sources of Light
When something is burning it goes
through a chemical reaction called combustion. Combustion occurs when fuel
reacts with oxygen to create carbon dioxide, water, and a lot of energy. The
energy is in the form of heat, light, and even sound. The light is produced
when the atoms of the hot item gain energy and become unstable. In order to
become stable again, the atoms must give off the energy in the form of photons.
Electromagnetic energy from the sun
comes to the earth in the form of radiation through space, at the speed of
light. The sun radiates energy in every direction. The earth intercepts some of
that light. The side of the planet that is facing the sun will receive, at the top of the atmosphere. The sun provides the largest
amount of natural light. Other natural sources include fire, the moon,
volcanos, lightning, and bioluminescence, even though they are not productive
for humans to use.
Before the invention of the
lightbulb, humans relied on natural sources of light after the sun went down.
Candles and oil lamps were the main sources
of lighting, especially within homes, but were
not very useful or made a large mess.
examples of artificial light would be incandescent light bulbs and
Light-Emitting Diodes (LEDs). Incandescent light bulbs make light using heat.
The filament within the bulb is both thin and short (good conductor qualities) so
that electricity while still having to work hard. The filament heats up when
connected to a source of electricity so that it will glow red or even white
hot. These light bulbs are known to waste electricity because about 95% of the
energy that it produces is in the form of excess heat. LED lights are more
energy efficient. The diagram below illustrates how the LED light works.
N-type silicon has extra electrons
P-type has extra holes
Battery connected across
the p-n junction makes the diode forward biased, pushing holes in the opposite
Electrons and holes cross the junction and combine
Photons are given off as the electrons and holes
(Steps from explainthatstuff.com)
invention of the light bulb is commonly credited to the famous Thomas Edison,
but that is not entirely true. The first edition of the light bulb was created
in 1950 by Joseph Swan. His light bulb had a carbonized paper filament in a
vacuum tube that minimized the exposure to oxygen. The light bulb was very
impractical because of the large amount
of energy needed and the short lifespan. After Swan presented his version of
the light bulb in 1979, Thomas Edison realized the fault in his design. Edison
replaced the filament with a thin filament made of a metal with a high
electrical resistance. Edison and Swan joined forces after a lengthy legal
battle to create the Edison-Swan United light bulb company.
The Speed of Light (c)
In a vacuum but is often rounded to for easier calculations.
This speed is known to be constant
throughout space but is also highly debated. The Quantum Field Theory states
that a vacuum is not empty but is filled with elementary particles that are
rapidly popping in and out of existence. These particles are the cause of
electromagnetic ripples that, theoretically, could alter the speed of light.
This theory was never proven correct and we still believe that the speed is
constant in the vacuum of space.
The speed of light does vary on
earth, however, it is slowed when in a
different clear medium such as water, air, or glass. The speed will depend on
what is called the refractive index of the substance and it is usually greater
“When we look at
the stars it’s like looking back in time.”
look up into the sky, the light, from the stars and other objects in space that
is hitting the Earth now will have started from the object a long time ago. An
example would be Proxima Centauri, the closest star to the Earth besides the
star. The Proxima Centauri is about four light-years away from the Earth
meaning that it takes about four years for the light from the star to reach the
Earth. The light that we see from the star would have left four years prior to
us seeing it. If something catastrophic happened to the star we wouldn’t know
until four years later.
you could travel almost the speed of light?”
If you were able to travel about 90%
of the speed of light, you would experience some interesting effects. Time
dilation would occur causing time to run slower for the objects travelling rapidly. The passage of time during
the trip would be halved. The trip that took 20 minutes on earth would be 10
minutes for the person on the trip. Another problem with travelling at 90% of the speed of light would
be a visual aberration and a visual
Doppler effect. Light waves from stars in front of you would crowd together and
appear blue. Light waves from the stars behind you would spread apart and
appear red. The faster that you would travel the more extreme this phenomena
becomes until all visible light from the stars in front and behind the
spacecraft become completely shifted out of the known visible spectrum.
Redshift occurs when the light goes from the right to the
left on the electromagnetic spectrum, like the second diagram in the picture.
Blueshift occurs when the light
goes from the left to the right, like the last diagram on the picture