Diffraction In Cd

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AUTHOR:Adrian Birka
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MAIN FUNCTIONAL REQUIREMENT:Read data encoded in the features of the aluminum side of aCD and convert this data to output for speakers (or other uses).


DIFFRACTION FROM A COMPACT DISC (thanks to Kurt Seidman) Theory. The surface of a compact disc is covered with concentric grooves, and when a laser beam shines on it most of the light is reflected. However, the light that strikes the grooves is scattered in all directions, as illustrated in Figure 1. The light bounces off of the CD, it does not pass through it, the CD acts like a diffraction grating because the reflected light undergoes constructive and destructive interference due to the special geometry of the CD’s bottom surface. What made the diffraction grating.

It uses optical systems with a high precision laser to read thedata. It relies on the aluminum layer of the CD to reflect the laser beam, and usesthe reflected beam to determine the position of the features.

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A compact disc (known as a CD for short) is currently a de-facto standardmedium for high-volume data storage. It is used both for audio purposes (CD-AUDIO)and for computer data purposes (CD-ROM). Other formats (e.g. video) are alsoavailable. This page examines in brief the mechanismused to read the data off of a compact disc.

  • Bragg diffraction (also referred to as the Bragg formulation of X-ray diffraction) was first proposed by Lawrence Bragg and his father William Henry Bragg in 1913 in response to their discovery that crystalline solids produced surprising patterns of reflected X-rays (in contrast to that of, say, a liquid).
  • The diffraction of sunlight by clouds is a common phenomenon and referred to as a silver lining. Another example of diffraction is while observing the back of a compact disc (CD). Recorded data on CD is stored in tiny pits of different lengths, which carries the information.
  • A compact disc is a thin, circular disc of metal and plastic about 12cm (just over 4.5 inches) in diameter. It's actually made of three layers. Most of a CD is made from a tough, brittle plastic called polycarbonate.Sandwiched in the middle there is a thin layer of aluminum.Finally, on top of the aluminum, is a protective layer of plastic and lacquer.


A typical CD is a polymer disc 6 cm (2.36 in) in radius, with a 7.5 mm(0.30 in) radius hole in the center. It is about 1.2 mm (0.047 in) thick (seeDIAG_CD1). The CD consists mostly of clear polycarbonate plastic, with smallbump-like features on its top side to encode data. This is covered with a thinaluminum level (about 30 microns, or 0.003 in), then with an acrylic level (approximatelythe same thickness as aluminum level) and a label printed on top of that.

The features which store the data are arranged in a long,extremely thin, spiraling track, which is read by the CD reading mechanism starting fromthe center outwards (See DIAG_CD2). This track is 0.5 microns (0.00002 in) wide,with 1.6 microns (0.00006 in) separating the adjacent tracks. The features on thetrack are at 0.125 microns (0.000005 in) high (see DIAG_CD1).


In order to read the CD, light is first emitted by a laser(DIAG_CDPL1). It passes through a diffraction grating, which converts the light into acentral peak plus side peaks, but, as was stated before, only the three central beams arestrong enough to matter in the mechanism. The three beams go through a polarizingbeam filter, which reflects all but the horizontal polarization. The emerging light (nowhorizontally polarized) is then collimated by a lens, and is passed through a plate thatconverts it into circularly polarized light.
The circularly polarized light is then focused onto the disk by the objective lens.If the light strikes 'land' it is simply reflected back into the objective lens.If it strikes a bump-like feature, only the center beam (of the three beams) is reflectedby the feature, and the rest are still reflected by the land. The features are 1/4of wavelength high (DIAG_CDPL2), and therefore the reflected light rays have a 180 degreesphase difference; and hence interfere destructively completely. No light is receivedback in that case.
If any light is received back, it is passed through the wave plate again. Since itis going the reverse direction, it will be polarized vertically. When the verticallypolarized light hits the polarizing filter this time, it will be reflected (since thefilter reflects all non-horizontally polarized rays). Thus, it will reflect thoughthe focusing lens and be imaged on the photodetector array. The array treats this asa 1 bit, and if it receives no light due to destructive interference due to a hit of afeature, it reads a 0 bit. The digital information thus gathered is then sent to theDigital to Analog converter and is played through the speaker systems (or headphones).


A compact disc reader is an optical system, using light waves toread the CD’s data. Therefore, for better understanding of its operation, someknowledge of optics is required:

Reflection: light reflects accordingto the well known 'angle of incidence is equal to angle of reflection' law(DIAG_OP1).

Refraction: when light enters adifferent medium, its direction, speed and wavelength usuallychange. The direction change (DIAG_OP2) is given by Snell’s law (the ratio ofthe sines of angles of incidence and refraction is equal to the ratio of refractiveindices of media), while both the light’s speed and wavelength change proportionallyto the indices of refraction. Refraction is often used in lenses to collect ordisperse light rays (DIAG_OP3).

Diffraction: When a wave passesthrough an opening smaller than or of comparable size to its wavelength, it diffracts,producing a wider wave than the original (DIAG_OP4). If diffraction is combined withinterference (see paragraph below), as in a passage of a wave through two holessimultaneously, a distinctive diffraction pattern is seen. The diffraction gratingused in CD reading systems can be viewed as simply splitting a light ray into three parts(the other parts of the diffraction pattern are not intense enough to be noticed).

Interference: When two identicalwaves meet, they can superimpose and interfere with each other (DIAG_OP4). If theyare 180 degrees out of phase, they will cancel each otherout. If they are in phase, they will reinforce each other. In general, though, theinterference will be somewhere in between.

Polarization: Light can be polarizedin a few different ways. The ones used in the CD mechanism are verticalpolarization, horizontal polarization and circular polarization. For the purposes ofunderstanding CD reader’s function it is enough to know that polarizing filters existwhich let only light of certain polarization through, reflecting all the rest. Polarization converters also exist, which convert between different polarizations.


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A CD, due to the small dimensions of its features, can hold a lot of data (about 650MB or 72 minutes of music). However DVD technology, which uses a similar principlewith much smaller features, can hold 4.7 GB per side, and as such can be used to holdentire movies (133 minutes of motion video at 40:1 compression (MPEG-2)). Thus, theoptical disctechnology might have an even brighter future invideo industry than it has had in audio.


Portable music players, computers, car stereos, data storagedevices


Brain, Marshall. 'How Compact Disks (CD’s)Work.' How Stuff Works. http://www.howstuffworks.com/cd.htm.

Kuhn, Kelin J. 'Audio Compact Disk - Anintroduction'. http://www.ee.washington.edu/conselec/CE/kuhn/cdaudio/95x6.htm.

Nakajima, Heitaro and Ogawa, Hiroshi. Compact DiscTechnology. Translation by Charles Aschmann. Washington: IOS Press,Inc., 1992.

Learning Objective

How is a cd an example of diffraction
  • Recognize the difference between constructive and destructive interference, and between interference and diffraction

Key Points

  • In physics, interference is a phenomenon in which two waves superimpose to form a resultant wave of greater or lower amplitude.
  • Constructive interference occurs when the phase difference between the waves is a multiple of 2π, whereas destructive interference occurs when the difference is π, 3π, 5π, etc.
  • Diffraction refers to various phenomena that occur when a wave encounters an obstacle. In classical physics, the diffraction phenomenon is described as the apparent bending of waves around small obstacles and the spreading out of waves past small openings.


  • interferenceAn effect caused by the superposition of two systems of waves, such as a distortion on a broadcast signal due to atmospheric or other effects. In physics, interference is a phenomenon in which two waves superimpose to form a resultant wave of greater or lower amplitude.
  • amplitudeThe maximum absolute value of some quantity that varies, especially a wave.
  • diffractionThe breaking up of an electromagnetic wave as it passes a geometric structure (e.g., a slit), followed by reconstruction of the wave by interference.

In physics, interference is a phenomenon in which two waves superimpose to form a resultant wave of greater or lower amplitude. Interference usually refers to the interaction of waves that are correlated or coherent with each other, either because they come from the same source or because they have the same (or nearly the same) frequency. Interference effects can be observed with all types of waves, including light, radio, acoustic, and surface water waves. In chemistry, the applications of interference to light are the most relevant to the study of matter.

Mechanism of Interference

The principle of superposition of waves states that when two or more waves are incident on the same point, the total displacement at that point is equal to the vector sum of the displacements of the individual waves. If a crest of a wave meets a crest of another wave of the same frequency at the same point, then the magnitude of the displacement is the sum of the individual magnitudes; this is known as constructive interference. If a crest of one wave meets a trough of another wave, then the magnitude of the displacements is equal to the difference in the individual magnitudes; this is known as destructive interference.

Constructive interference occurs when the phase difference between the waves is a multiple of 2π, whereas destructive interference occurs when the difference is π, 3π, 5π, etc. If the difference between the phases is intermediate between these two extremes, then the magnitude of the displacement of the summed waves lies between the minimum and maximum values.

Consider, for example, what happens when two identical stones are dropped into a still pool of water at different locations. Each stone generates a circular wave propagating outwards from the point where the stone was dropped. When the two waves overlap, the net displacement at a particular point is the sum of the displacements of the individual waves. At some points, these will be in phase and will produce a maximum displacement. In other places, the waves will be in anti-phase and there will be no net displacement at these points. Thus, parts of the surface will be stationary.


Diffraction refers to various phenomena that occur when a wave encounters an obstacle. In classical physics, the diffraction phenomenon is described as the apparent bending of waves around small obstacles and the spreading out of waves past small openings. Similar effects occur when light waves travel through a medium with a varying refractive index or a sound wave through one with varying acoustic impedance. Diffraction occurs with all waves, including sound waves, water waves, and electromagnetic waves such as visible light, X-rays, and radio waves. As physical objects have wave-like properties (at the atomic level), diffraction also occurs with matter and can be studied according to the principles of quantum mechanics. Italian scientist Francesco Maria Grimaldi coined the word diffraction and was the first to record accurate observations of the phenomenon in 1665.

The effects of diffraction are often seen in everyday life. The most striking examples of diffraction are those involving light; for example, the closely spaced tracks on a CD or DVD act as a diffraction grating to form the familiar rainbow pattern seen when looking at a disk. This principle can be extended to engineer a grating with a structure such that it will produce any diffraction pattern desired; the hologram on a credit card is an example. Diffraction in the atmosphere by small particles can cause a bright ring to be visible around a bright light source like the sun or the moon. A shadow of a solid object, using light from a compact source, shows small fringes near its edges. All these effects occur because light propagates as a wave.

Richard Feynman said, “No one has ever been able to define the difference between interference and diffraction satisfactorily. It is just a question of usage, and there is no specific, important physical difference between them.”

He suggested that when there are only a few sources, say two, we call it interference (as in Young’s slits), but with a large number of sources, the process can be labelled diffraction.

While diffraction occurs whenever propagating waves encounter such changes, its effects are generally most pronounced for waves where the wavelength is roughly similar to the dimensions of the diffracting objects. If the obstructing object provides multiple, closely spaced openings, a complex pattern of varying intensity can result. This is due to the superposition, or interference, of different parts of a wave that traveled to the observer by different paths (see diffraction grating).

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