The Nature of Light

What is light has been a basic philosophical and scientific question since

time immemorial.

Isaac Newton• Known for his Law of

Universal Gravitation, English physicist Sir Isaac Newton (1643 to 1727) realized that light had frequency-like properties when he used a prism to split sunlight into its component colors, called dispersion.

• This was the first thought that it had wavelike characteristics.

Isaac Newton

• Nevertheless, he thought that light was a particle because the periphery of the shadows it created was extremely sharp and clear.

Francesco Grimaldi and Christian Huygens

• Light as a wave - The wave theory, which maintains that light is a wave, was proposed around the same time as Newton's theory. This means…that the explanation for the nature of light is that light travels as a transverse wave through different mediums. The site below has a helpful tutorial about light as a wave model. Check out science trek >under electromagnetic waves.(hi-lite link, right click >open hyperlink)

• http://www.colorado.edu/physics/2000/index.pl

Supporting observations

• In 1665, Italian physicist Francesco Maria Grimaldi (1618 to 1663) discovered the phenomenon of light diffraction and pointed out that it resembles the behavior of waves. Then, in 1678, Dutch physicist Christian Huygens (1629 to 1695) established the wave theory of light and announced the Huygens' principle.

Observations of diffraction using Light and Shadow

So, finally…

• In 1678, Dutch physicist Christian Huygens (1629 to 1695) established the wave theory of light and announced the Huygens' principle– when light goes through an aperture (an

opening within a barrier) every point of the light wave within the aperture can be viewed as creating a circular wave which propagates outward from the aperture

Diffraction of light

• Interactive simulation showing diffraction. The opening and frequency are adjustible.

• http://micro.magnet.fsu.edu/primer/java/scienceopticsu/diffraction/basicdiffraction/index.html

Augustin-Jean Fresnel

• French physicist Augustin-Jean Fresnel (1788 to 1827) asserted that light waves have an extremely short wavelength and mathematically proved light interference. In 1815, he devised physical laws for light reflection and refraction, as well. He also hypothesized that space is filled with a medium known as ether because waves need something that can transmit them.Ether & medium ideas didn’t hold up.

Thomas Young

• In 1817, English physicist Thomas Young (1773 to 1829) calculated light's wavelength from an interference pattern, thereby not only figuring out that the wavelength is 1 micron or less, but also having a handle on the truth that light is a transverse wave.

At that point, the particle theory of light fell out of favor and was replaced by the wave theory.

In Young’s Experiment…

• The waves from the two sources are in phase at the center. Bright areas are caused by constructive interference and the dark areas are caused by destructive interference.

• As the distance from the center increases, the path traveled by the light from one source is larger than that traveled by the light from the other source.

Young’s experiment continued…

• When the difference in path is equal to half a wavelength, destructive interference occurs. Instead, when the difference in path length is equal to a wavelength, constructive interference occurs.

• For destructive interference the waves are 180 degrees out of phase and for constructive interference they are exactly in phase.

Young’s Double Slot Diffraction Experiment

• Measurements from this type of experiment was used to determine the wavelength of light.

• http://vsg.quasihome.com/interfer.htm

Maxwell

• Maxwell's four equations have become the most fundamental law in electromagnetics. The equations actually predicted the existence of electromagnetic waves in 1861before we had devised technologies to detect details of electromagnetic radiation.

Light as a particle…A. Einstein

• The theory of light being a particle completely vanished until the end of the 19th century when Albert Einstein revived it.Now that the dual nature of light as "both a particle and a wave" has been proved, its essential theory was further evolved from electromagnetics into quantum mechanics..

Model Depicting the Duality of Light

• Model Depicting the Duality of Light

Einstein’s Quantum Theory

• Einstein believed light is a particle (photon) and the flow of photons is a wave.

• The main point of Einstein's light quantum theory is that light's energy is related to its oscillation frequency.

• Photon energy is the height of the oscillation frequency and the intensity of light is the quantity of photons.

This site presents the photon theory for light is a particle.

http://www.colorado.edu/physics/2000/index.pl click on…>science trek, >quantum atom, scroll to bottom, >next, >

click on energy levels and watch photons.

Einstein’s Photoelectric Effect• Albert Einstein (1879 to 1955), famous for his theories of relativity,

conducted research on the photoelectric effect, in which electrons fly out of a metal surface exposed to light.

The strange thing about the photoelectric effect is the energy of the electrons (photoelectrons) that fly out of the metal does not change whether the light is weak or strong. (If light were a wave, strong light should cause photoelectrons to fly out with great power.)

Another puzzling matter is how photoelectrons multiply when strong light is applied. Einstein explained the photoelectric effect by saying that "light itself is a particle," and for this he received the Nobel Prize in Physics.

• http://www.canon.com/technology/s_labo/light/001/11/014.html

Einstein’s Oscillation Frequency

• The main point of his light quantum theory is the idea that light's energy is related to its oscillation frequency). In short, Einstein was saying that light is a flow of photons, the energy of these photons is the frequency of their oscillation frequency, and the intensity of the light is the quantity of its photons.

• http://www.canon.com/technology/s_labo/light/001/11/015.html

Photoelectric EffectClick on the icon below for an

interactive simulation.

photoelectric.jnlp

Einstein’s Quantum Theory

• Einstein proved his theory by proving that the Planck's constant which he developed, exactly matched the Planck’s constant of 6.6260755 x 10-34 that Planck independently discovered from his experiments.

Einstein’s Quantum Theory

• This showed light as a wave and the properties of light as both a particle and a wave.

• http://www.canon.com/technology/s_labo/light/001/11/016.html

Electromagnetic Spectrum

• All electromagnetic energy is a continuum, gradually changing in wavelength from less than a billionth of a meter to many miles long. All forms of energy within this group travel as electrical energy in one plane and magnetic energy oriented 90 degrees and in phase with the electric field.

Electromagnetic Spectrum

Electromagnetic Spectrum

• Makes up all of the radiation in the universe, some visible and most invisible.

• All forms of radiation within the spectrum , 186,000 miles per second, travel at the speed of light in a vacuum.

• Radiation travels through some mediums and without a medium as well.

Light Intensity

• Light Intensity is inversely proportional to the square of the distance I = 1/d2 .

• According to Einstein, the intensity of the light is determined by the quantity of the photons hitting the surface.

Electromagnetic spectrum continued…

• We see only a very narrow portion of the radiation of the spectrum, the rest simply lies outside the threshold of our eyes to detect it and is therefore invisible to our eyes.

Parts of the electromagnetic spectrum

• Gamma Radiation – invisible, shortest wavelength, highest frequency, highest energy radiation, can penetrate up to 3 m of concrete. Source radioactive elements. Used to treat cancer.

• X-ray radiation – invisible, next highest energy, passes through skin and soft tissue, but absorbed by bone.

Electromagnetic spectrum continued…

• Ultra-violet radiation, penetrates skin cells, causes sunburn, cellular mutations (DNA damage) and skin cancer (melanoma)

• Visible Light: The only part of the EMS you can see, broken down to different wavelengths (400 nm violet – 700 nm red) and frequencies and perceived as different color ROYGBIV

Electromagnetic Spectrum continued…

• Infrared Light- or heat radiation - although we can not see it directly, we can detect it with heat sensors embedded in our skin.

It’s wavelength is longer than red light

• Microwave radiation – longer wavelength than IR, it is used in microwaves to heat water in food and for cellular communication.

Electromagnetic spectrum final

• Radio waves – composed of waves lengths which are assigned to AM radio, FM radio, TV, and radar. They have the longest wavelength and the lowest frequency and the lowest energy.

Why does this happen? Let’s see..

Candles viewed in white light Same candles viewed in red light

Color Filters

• Filters are placed between the object and the observer. They work by letting a specific frequency pass through ONLY and the rest of the frequencies are blocked or absorbed.

• You see the object only because of the frequency that reaches your eye, the color you perceive is assembled by your brain as it receives signals from the stimulation of the cone receptors inside your eye.

Color Filter Interactive Demo Site

• http://micro.magnet.fsu.edu/primer/java/primarycolors/colorfilters/index.html

Additive Color Light

Light is perceived as white by humans when all three cone cell types of the eye are simultaneously stimulated by equal amounts of red, green, and

blue light. These are called Primary colors, because when they are added together, white light is formed.

Primary Additive Color Interactive Tutorial Site

• Primary Additive Color Interactive Tutorial Site

Subtractive Color

• The complementary colors (cyan, yellow, and magenta) are also commonly referred to as the primary subtractive colors because each can be formed by subtracting one of the primary additives (red, green, and blue) from white light.

• The color observed by subtracting a primary color from white light results because the brain adds together the colors that are left to produce the respective complementary or subtractive color.

Check this out!

• Subtractive Color Interactive Site

Color perception and color separation.

• Pigments and dyes within objects are responsible for most of the color that we perceive.

• When any two of the wave frequencies of the primary subtractive colors are added, they produce a primary additive color. For example, adding magenta and cyan together produces the color blue, while adding yellow and magenta together produces red and adding yellow and cyan is interpreted as green.

Color separation continued…

• When all three primary subtractive colors are added, the three primary additive colors are removed from white light leaving black (the absence of any color).

• White cannot be produced by any combination of the primary subtractive colors, which is the main reason that no mixture of colored paints or inks can be used to print white.

Color Separation Interactive tutorial

• http://micro.magnet.fsu.edu/primer/java/primarycolors/colorseparation/index.html

Paint theory…

• Paint is produced from Base pigments which contain the subtractive primaries that are mixed together. Depending what pigments that are mixed together initially, a specific color combination is created in final paint. The final color that is perceived is determined by the amount of signals the brain receives from the different frequencies in the light that strikes the eye and are detected by the cones and sent to the brain where the combination of signals are processed.

White light illuminates objects A and B

Pigments

Pigments in various light

Reflection • Occurs when radiation transitions from one density to another

different density.

• http://www.physicsclassroom.com/mmedia/waves/ltm.gif

Reflection from Plane Mirror

• Reflection is the bouncing off of radiation, or visible light, from a surface.

The angle of incidence is equal to the angle of reflection.  

Virtual Image for Plane Mirror

Plane Mirror, virtual image and ray path

• http://www.4physics.com/phy_demo/flat_mirror/mirror.html

Concave Mirror ray paths

• http://www.4physics.com/phy_demo/mirage/focal-length.html

Refraction

• The bending of light ray caused by transitioning from one medium to another of medium of a different density.

Refraction ray path

• http://www.ps.missouri.edu/rickspage/refract/refraction.html scroll down to the refraction simulator, Figure 3. It is interactive.

Convex Lens Refraction

Convex Lens Refraction Rules

• Any incident ray traveling parallel to the principal axis of a converging lens will refract through the lens and travel through the focal point on the opposite side of the lens.

• Any incident ray traveling through the focal point on the way to the lens will refract through the lens and travel parallel to the principal axis.

• An incident ray which passes through the center of the lens will in effect continue in the same direction that it had when it entered the lens.

Bi-Convex lens Real and Inverted Image

Bi-Convex Virtual and Upright Image

Bi-Convex Image location and Size

Bi-Concave Lens Refraction

Refraction Rules for Divergent (Bi-concave) Lens

• Any incident ray traveling parallel to the principal axis of a diverging lens will refract through the lens and travel in line with the focal point (i.e., in a direction such that its extension will pass through the focal point).

• Any incident ray traveling towards the focal point on the way to the lens will refract through the lens and travel parallel to the principal axis.

• An incident ray which passes through the center of the lens will in effect continue in the same direction that it had when it entered the lens.

Rainbows

Rainbow Refraction ray path