Diffraction & Interference of Light Kelly Conlin & Alexa Rufus

Light is a transverse electromagnetic wave. Diffraction and interference can be observed with all waves.

Diffraction Observed when light passes through a small opening in an otherwise opaque obstacle, the wavefront on the other side of the opening resembles the wavefront shown below.

The light spreads around the edges of the obstacle. This is Diffraction.

Diffraction involves a change in direction of waves as they pass through an opening or around a barrier in their path. Can be observed by placing barriers in a ripple tank. The waves will pass around the barrier. The amount of diffraction increases with increasing wavelength and decreases with decreasing wavelength.

Water waves in a ripple tank: Diffraction is a wave phenomenon and is also observed with water waves in a ripple tank. Water waves in a ripple tank: A single large slit: A single small slit:

In regions where they overlap we only observe a single disturbance. Interference Two or more waves traveling in the same medium travel independently and can pass through each other. In regions where they overlap we only observe a single disturbance.  This is Interference.

Constructive interference Occurs at any location along the medium where the two interfering waves have a displacement in the same direction. Constructive interference is observed at any location where the two interfering waves are displaced upward. Example:

When two or more waves interfere, the resulting displacement is equal to the vector sum of the individual displacements. Displacement is the shortest distance from the initial and final positions of a point. A displacement vector represents the length and direction of that imaginary straight path.

Destructive interference Destructive Interference occurs at any location along the medium where the two interfering waves have a displacement in the opposite direction. For instance, when a sine pulse with a maximum displacement of +1 unit meets a sine pulse with a maximum displacement of -1 unit, destructive interference occurs.

In the diagram above, the interfering pulses have the same maximum displacement but in opposite directions. The result is that the two pulses completely destroy each other when they are completely overlapped.

At the instant of complete overlap, there is no resulting displacement of the particles of the medium. When overlapped, the affect of one of the pulses on the displacement of a given particle of the medium is destroyed or canceled by the affect of the other pulse.

The two interfering waves do not need to have equal amplitudes in opposite directions for destructive interference to occur. For example, a pulse with a maximum displacement of +1 unit could meet a pulse with a maximum displacement of -2 units. The resulting displacement of the medium during complete overlap is -1 unit. This is still destructive interference since the two interfering pulses have opposite displacements. In this case, the destructive nature of the interference does not lead to complete cancellation.

The task of determining the shape of the resultant demands that the principle of superposition is applied.

Principle of superposition When two waves interfere, the resulting displacement of the medium at any location is the algebraic sum of the displacements of the individual waves at that same location.

Displacement of Pulse 1 Displacement of Pulse 2 = Resulting Displacement +1 +1 = +2 -1 -1 = -2 +1 -1 = 0 +1 -2 = -1

Quiz 1. What is diffraction? 2. How can diffraction be observed? 3. What is inference? 4. When two or more waves interfere, the resulting displacement is equal to the ___________ of the individual displacements. 5. When does destructive inference occur?

Diffraction & Interference of Light

Which is light from two or more sources that add together in superposition to produce smooth wave fronts A smooth wave front also can be created by multiple point sources when all point sources are synchronized, such as with a laser. Only the superposition of light waves from coherent light sources can produce the interference phenomena Interference of Light

English Physician: Thomas Young Proved that light has wave properties when he produced an interference pattern by shining light from a single coherent source through two slits Overlapping light from two slits fell on an observing screen, the overlap did not produce even illumination, but instead created a pattern of bright and dark bands called Interference fringes.

Double-slit Interference It produces a bright central band of the given color on the screen, as well as, other bright bands of near-equal spacing and near-equal width on either side. Intensity of bright bands decreases the farther the band is from the central band, between the bright bands are dark areas where destructive interference occurs.

When light passes through a slit with a size that is close to the light's wavelength, the light will diffract, or spread out in waves. When light passes through two slits, the waves from one slit will interfere with the waves from the other.

Constructive interference occurs when a wave front, or crest, from one wave coincides with a wave front from another, forming a wave with a larger crest. Destructive interference occurs when a wave front of one wave coincides with a trough of another, cancelling each other to produce a smaller wave or no wave at all.

Measuring the wavelength of light One common unit used to measure the length of light waves is not feet or inches but rather something called an angstrom.

Amplitude of a wave What exactly do we mean by a wave's "amplitude“? It is how do you measure the height or amplitude of a wave? Look at these diagrams and see if you can figure out a good definition for amplitude.

Wavelength tells you the type of light And The amplitude of a light's wave is important because it tells you about the intensity or brightness of the light relative to other light waves of the same wavelength. It's a measure of how much energy the wave carries. Remember ! Wavelength tells you the type of light And Amplitude tells you about the intensity of the light

Have you ever seen a spectrum of colors produced by a soap bubble or by the oily film on a water puddle in a parking lot? These colors WERE NOT the result of separation of white light by a prism. * The spectrum of colors was A RESULT of the constructive and destructive interference of light waves due to reflection in a thin film, a phenomenon called THIN-FLIM INTERFERENCE.

Diffraction of Light The cutting of coherent light on two edges spaced closely together produces a diffraction pattern Single- Slit Diffraction When coherent, blue light passes through a single, small opening that is larger than the wavelength of the light, the light is diffracted by both edges, and a series of bright and dark bands appears on a distant screen

The amount of bending depends on the relative size of the wavelength of light to the size of the opening. If the opening is much larger than the light's wavelength, the bending will be almost unnoticeable. However, if the two are closer in size or equal, the amount of bending is considerable, and easily seen with the naked eye.

Diffraction Grating A diffraction grating is a device made up of many single slits that diffract light and form a diffraction pattern that is an overlap of single slit diffraction patterns Diffraction gratings can have as many as 10,000 slits per centimeter or 10-6 or 1000 nm

Measuring wavelength In order to measure light wavelengths are used a diffraction grating called a grating spectroscope The diffraction pattern produced by a diffraction grating has narrow, equally spaced, bright lines. The larger the number of slits per unit of length, the narrower the lines in the diffraction pattern The distance between the bright lines can be measured much more precisely with a grating spectroscope than with a double slit

Quiz ! 1) ____________ is light from two or more sources that add together in superposition to produce smooth wave fronts. 2) ____________ proved that light has wave properties when he produced an interference pattern by shining light from a single coherent source through two slits. 3) What produces a bright central band of the given color on the screen as well as other bright bands of equal spacing and near equal width on either side? 4) __________ tells you the type of light and ___________ tells you about the intensity of light. 5)The cutting of coherent light on two edges spaced closely together produces a diffraction pattern called a ______________.

Answers! 1) Interference of Light 2) Thomas Young 3) Double- slit interference 4) Wavelength & Amplitude 5) Single- slit diffraction

Bibliography Physics Principles and Problems Textbook http://en.wikipedia.org/wiki/Diffraction http://electron9.phys.utk.edu/phys136d/modules/m9/diff.htm http://library.thinkquest.org/C001429/waves/diffraction_and_interference_of_.htm http://en.wikipedia.org/wiki/Interference_%28wave_propagation%29