Properties of light

Properties of Light

• light is all around us
• it reflects off surfaces, which allows us to see images in mirrors
• seep of light, which is the fastest anything has been observed to move
• when light enters a material, it slows down: amount depends on the material it enters and it’s destiny
• refraction means light bends when it passes from one medium to another
• when light enters a denser medium from one that is less dense, ti bends toward a line normal to the boundary between the two media
• another property that combines both refraction and reflection is total internal reflection
• dispersion: ability to break white light into its constituent colors. ex. rainbows are natural phenomena
• diffraction: light bends as it goes though an opening
• interference: occurs when tow beams of light meet

http://www.physicsplanet.com/articles/properties-of-light

Sources

Websites/ resources used:

http://hyperphysics.phy-astr.gsu.edu/hbase/ligcon.html#c1

http://www.physicsplanet.com/articles/properties-of-light
http://www.physicsclassroom.com/Class/light/U12l2f.cfm

physics: principle and problems by Zitzewitz Neff

Mathematics of Two Point source interference

MATHEMATICS OF TWO POINT SOURCE INTERFERENCE

Anatomy of a two point source interference pattern

• central antinodal line: antinodal line
• first antinodal line (1) / second antinodal line (2): etc. antinodal lines are present to the left and right of central antinodal line
• order number (m): each line in the pattern is assigned a number

The Path Difference

• from point A on the first antinodal line (m = 1) the path difference is equivalent to 1 wavelength
• two point source interference pattern with values:

Explaining the Path difference equation

• diagram shows two waves traveling along different paths from different sources to the same point in such a way that a crest is meeting a crest
• constructive interference will occur at this point
• the waves from source 1 (S1) travels a distance of 6 wavelengths 
• the wave from source 2 (S2) travels a distance of & wavelengths to reach the same point
• the difference in distance traveled by the two waves is one full wavelength
• when the path difference is one full wavelength, a crest meets a crest and constructive interference occurs

Young’s Equation

• circular waves from two sources can interfere in the surrounding space to produce a pattern of antinodes and nodes
• the nodal positions are present at locations where destructive interference always occurs and the path difference is a half-number of wavelengths
• the antinodal potions are present at locations where constructive interference always occurs and the path difference is equivalent to a whole number of wavelengths
• 

A light interference pattern

• two point source interference pattern, light waves from two coherent, monochromatic sources will interfere constructively and destructively to produce a pattern of antinodes and nodes
• light traveling in air is usually not seen because there is nothing of substantial size in the air to reflect the light to our eyes. Therefore the pattern formed by light interference can’t be seen it is somehow projected onto some form of a screen or a sheet of paper
• locations where light constructively interferes corresponds to an abnormally bright spot
• locations where light destructively interferes corresponds to an abnormally dark spot
• maxima: antinodes are locations where light from the two individual sources are reinforcing each other and correspond to points of brightness or maximum intensity
• minima: nodes are location where light from the two individual sources are destroying each other and correspond to points of darkness or minimum intensity 

Young’s experiment

= y • d / (m • L)

• in 1801, Young devised and performed an experiment to measure the wavelength of light

Color and Vision

COLOR AND VISION

The Electromagnetic and Visible Spectra

• electromagnetic waves are waves that are capable of traveling through a vacuum
• mechanical waves require a medium in order to transport energy, electromagnetic waves are capable of transporting energy through the vacuum of outer space
• electromagnetic waves are produced by a vibrating electric change and consist of both an electric and a magnetic component
• electromagnetic spectrum: continuous range of frequencies. The entire range of the spectrum is often broken into specific regions

Visible Light Spectrum

• our eyes are sensitive to only a very narrow band of electromagnetic waves, limit human eyes can see: visible light spectrum
• ROYGBIV: narrow band of visible light - each individual wavelength within the spectrum of visible light wavelengths is representative of a particular colour
• Isaac Newton discovered: light shining through a prism will be separated into its different wavelengths and will show various colors of visible light. Separation of colors: dispersion - each color is characteristic of a distinct wavelength and different wavelengths of light waves will bend varying amounts upon passage through a prism

Visible Light and the Eye’s Response

• our eyes are sensitive to a very narrow band of frequencies within the large range of frequencies of the electromagnetic spectrum: visible light spectrum
• specific wavelengths within the spectrum correspond to a specific color based upon how humans typically perceive light of that wavelength
• the long wavelength end of the spectrum corresponds to light that is perceived by humans to be red and the short wavelength end of the spectrum corresponds to light that is perceived to be violet. Other colors within the spectrum include orange, yellow, green and blue
• the picture depicts the approximate range of wavelengths that are associated with the various perceived colors within the spectrum

Color Cones

• color can be thought of as a psychological and physiological response to light waves of a specific frequency or set of frequencies impinging upon the eye
• light that enters the eye through the pupil ultimately strikes the inside surface of the eye known as the: retina - lined with a variety of light sensing cells known as rods and cones The rods on the retina are sensitive to the intensity of light, they can’t distinguish between lights of different wavelengths
• Three kinds of cones: red, green and blue. Since the red cone is sensitive to a range of wavelengths, it is not only activated by wavelengths of red light, but also by wavelengths of orange light, yellow light and even green light
• green cone is most sensitive to wavelengths of light associated with the color green but can be activated by wavelengths of light associated with the colors yellow and blue

• the cone sensitivity curve shown above helps us to better understand our response to the light that is incident upon the retina

• light is simply a wave with a specific wavelength or a mixture of wavelengths; it has no color in or of itself
• an object that is emitting or reflecting light to our eye appears to have a specific color as the result of the eye brain response to the wavelength. Technically there is no such thing as yellow light - light with a wavelength of about 590 nm together appears yellow

Light Absorption, Reflection and Transmission

• visible light waves consist of a continuous range of wavelengths or frequencies
• when a light wave with a single frequency strikes an object, several thing can happen
• light wave could be absorbed by the object: energy is converted to heat
• light wave could be reflected by the object 
• light wave can be transmitted by the object
• atoms and molecules contain electrons: the electrons and their attached strings have a tendency to vibrate at specific frequencies
• electrons of atoms have a natural frequency which they tend to vibrate
• if a light wave of a given frequency strikes a material with electrons having the same vibrational frequencies, then those electrons will absorb the energy of the light wave and transform it into vibrational motion. During vibration, the electrons interact with neighboring atoms in such a manner as to convert its vibrational energy into thermal energy. As a result the light wave with that given frequency is absorbed by the object, never again to be released in the form of light
• reflection and transmission of light wave happen because the frequencies of the light do not match the natural frequencies of vibration of the object]
• if the object is transparent - transmitted: if the object is transparent, then the vibrations of the electrons are passed on to neighboring atoms through the bulk of the material and reemitted on the opposite side of the object
• if the object is opaque - reflected: vibrations of the electrons are not passed from atom to atom through the bulk of the material. The electrons of atoms on the material’s surface vibrate for short periods of time and then reemit the energy as a reflected wave
• the color of the objects that we see are largely due to the wave those objects interact with light and ultimately reflect or transmit it to our eyes
• the color of the object is the light that shines upon it and reflected/transmitted to our eyes

Color and Vision

• when you look at an object and perceive a distinct color, you are not necessarily seeing a single frequency of light

example: looking into a purple shirt - there are several frequencies of light striking your eye but the brain interprets the eye and decodes as being the color purple

• white is not a color at all, but rather the presence of all the frequencies of visible light
• primary colors of light: three colors of light that produce white light when combined with specific intensity. ex. red, green and blue
• secondary colors of light: they are produced by the addition of equal intensities of two primary colors of light .ex. yellow, magenta and cyan

• complementary colors of light: any two colors of light that when mixed together in equal intensities produce white are said to be complementary colors of each other. ex. complementary color of light is cyan light 

Color Subtraction

• materials that have been permeated by specific pigments will selectively absorb specific frequencies of light in order to produce a desired appearance
• process of color subtraction: if white light is shining on a shirt, then red, green and blue light is shining on the shirt. If the shirt absorbs blue light, then only red and green light will be reflected from the shirt. So while red, green and blue light shine upon the shirt, only red and green light will reflect from it. Red and green light striking your eyes always gives the appearance of yellow
• the ultimate color appearance of an object is determined by beginning with a single color or mixture of colors and identifying which color or colors of light are subtracted from the original set

Complementary colors and color subtraction

• a pigment that absorbs a single frequency is known as a pure pigment
• rule: pigments absorb light. Pure pigments absorb a single frequency or color of light. The color of light absorbed by a pigment is merely the complementary color of that pigment

Filters and color subtraction

• opaque materials selectively absorb one or more frequencies of light and reflect what is not absorbed
• transparent materials selectively absorb one or more frequencies of light and transmit what is not absorbed
• both materials are permeated by pigments that contain atoms that are capable of absorbing light with a single frequency or even a range of frequencies

Primary Colors of Paint

• thee primary colors of paint used by artists/ color printer/ film developer are cyan, magenta and yellow
• each primary color of paint absorbs one primary color of light
• the color absorbed by a primary color of paint is the complementary color of that paint

Waves and Behaviour of Light

WAVES AND BEHAVIORS OF LIGHT

• Light reflects in the same manner that any wave would reflect
• Light behaves similar to the concept of waves. Ex. undergoes interference and exhibits the Doppler effect

Reflection of Light waves

• All waves/light waves undergo reflection (bouncing off an obstacle)
• Reflection off of a mirrored surface results in the formation of an image
• One characteristic of wave reflection is that the angle at which the wave approaches a flat reflecting surface is equal to the angle at which the wave leaves the surface

Refraction of Light Waves

• All waves are known to undergo refraction when they pass from one medium to another medium
• the direction that the wavefront is moving undergoes a sudden change; the path is “bent”
• “bending” is dependent upon the relative speed of the two media. A wave will bend one way when it passes from a medium n which it travels slowly into a medium in which it travels fast; and if moving from a fast medium to a slow medium, the wavefront will bend in the opposite direction
• the amount of bending is dependent upon the actual speeds of the two medium on each side of the boundary
• refractive behavior of light provides evidence for the wavelike nature of light

Diffraction of Light Waves

• diffraction involves a change in direction of waves as they pass through an opening or around an obstacle in their path
• Light diffracts around obstacles
• interference effects occur due to the diffraction of light around different sides of the object

TWO POINT SOURCE INTERFERENCE

• wave interference happens when two waves meet while traveling along the same medium: constructive/ deconstructive interference
• constructive interference: occurs at any location along the medium where the two interfering waves have a displacement in the same direction
• destructive interference: occurs at any location along the medium where the two interfering waves have a displacement in the opposite direction

Example: Interference Patterns

• the diagram above depicts an interference pattern produced by two periodic disturbances
• crests: thick lines
• troughs: thin lines
• constructive interference happens when a thick line meets a thick line or a thin line meets a thin line: antinode (red dote - located on antinodal lines)
• deconstructive interference happens wherever a thick line meets a thin line: node (blue dot: located on nodal lines)
• central antinodal line: line of points where the waves from each source always reinforce each other by means of constructive interference

Two Point Source Light Interference Patterns

• any type of wave should produce a two point source interference pattern if the two sources periodically disturb the medium at the same frequency
• when light constructively interferences, the two waves act to reinforce one another and to produce a “super light wave”
• when light destructively interferes, the two waves act to destroy each other and produce no light waves
• result: two point source interference pattern would still consist of an alternating pattern of antinodal lines and dark lines

application example: if such an interference pattern could be created by two light sources and projected onto a screen, then there ought to be an alternating pattern of dark and bright bands on the screen. Since central line in such a pattern is an antinodal line, the central band on the screen ought to be a bright band

• 1801, Thomas Young proved light produces a two point source interference pattern, using monochromatic light (light of a single color; by use of such light, the two sources will vibrate with the same frequency)
• it is important that the two light waves be vibrating in phase with each other: coherent light (the crest of one wave must be produced at the same precise time as the crest of the second wave)

THIN FILM INTERFERENCE

• reflection, refraction and diffraction is one strand of evidence
• interference of light waves is another strand of evidence

examples:

• streaks of color on a car windshield shortly after it has been swiped by a windshield wiper/ squeegee at a gas station
• explanation: interference of light by a very thing film of water or soap that remains on the windshield
• streaks of color in a thin film of oil resting upon a water puddle/ concrete driveway
• explanation: result of interference of light by a very thin film of oil that is spread over the water surface

• light wave interference results when two waves are traveling through a medium and meet up at the same location
• when a wave reaches the boundary between the two media, a portion of the wave reflects off the boundary and a portion is transmitted across the boundary. The reflected portion of the wave remains in the original medium. The transmitted portion of the wave enters the new medium and continues traveling through it until it reaches a subsequent boundary. If the new medium is a thin film, then the transmitted wave does not travel far before it reaches a new boundary and undergoes the usual reflection and transmission behavior. Result: two waves that emerge from the film - one wave that is reflected off the top of the film and the other wave that reflects off the bottom of the film 

POLARIZATION

• light wave is an electromagnetic wave that travels through the vacuum of outer space
• light waves are produced by vibrating electric charges
• electromagnetic wave is a transverse wave that has both an electric and a magnetic component
• a light wave that is vibrating in more than one place is referred to as unpolarized light

example: light emitted by the sun, by a lamp in a classroom or by a candle flame is unpolarized light. Such light waves are created by electric charges that vibrate in a variety of directions, creating an electromagnetic wave that vibrates in a variety of directions

Methods of Polarization

Polarization by use of a Polaroid filter

• made with special material that is capable of blocking one of the two planes of vibration of an electromagnetic wave
• when unpolarized light is transmitted through a polaroid filter, it emerges with one half the intensity and with vibrations in a single plane; it emerges as polarized light
• alignment of molecules gives the filter a polarization axis

Polarization by Reflection

• unpolarized light can undergo polarization by reflection off of nonmetallic surfaces
• extent polarization occurs depends upon the angle at which the light approaches the surface and upon the material that the surface is made of
• metallic surfaces reflect light with a variety of vibrational directions; such light is unpolarized

Polarization by Refraction

• Polarization can occur by the refraction of light
• occurs when a beam of light passes from one material into another material
• at the surface of two materials, the path of the beam changes its direction
• if an object is viewed through crystal, two images are seen because of double refraction of light. Both refracted light beams are polarized - one in a direction parallel and the other perpendicular to the surface

Polarization by Scattering

• occurs when light is scattered while traveling through a medium
• when light strikes the atoms of a material, it will often set the electrons of those atoms into vibration
• the vibrating electrons then produce their own electromagnetic wave that is radiated outward in all directions
• this newly generated wave strikes neighboring atoms, forcing their electrons into vibrations at the same original frequency. These vibrating electrons produce another electromagnetic wave that is once more radiated outward in all directions
• polarization by scattering is observed as light passes through our atmosphere
• the scattered light often produces a glare in the skies