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An illustration of the details of a compact disc. A laser beam hits the disc from below at right angles. The disc consists of three layers. The lower layer is a polycarbonate plastic layer with alternating pits and bumps. A thin layer of Aluminum is deposited on top of the plastic layer. A layer of laquer covers the disc, filling in the bumps and pits and forming a smooth upper surface. The entire disc, including all three layers, is 1.2 m m thick.
A compact disc is a plastic disc that uses bumps near its surface to encode digital information. The surface of the disc contains multiple layers, including a layer of aluminum and one of polycarbonate plastic.

A CD player uses a laser to read this digital information. Laser light is suited to this purpose, because coherent light can be focused onto an incredibly small spot and therefore distinguish between bumps and pits in the CD. After processing by player components (including a diffraction grating, polarizer, and collimator), laser light is focused by a lens onto the CD surface. Light that strikes a bump (“land”) is merely reflected, but light that strikes a “pit” destructively interferes, so no light returns (the details of this process are not important to this discussion). Reflected light is interpreted as a “1” and unreflected light is interpreted as a “0.” The resulting digital signal is converted into an analog signal, and the analog signal is fed into an amplifier that powers a device such as a pair of headphones. The laser system of a CD player is shown in [link] .

A photograph of the inner working of a CD player
A CD player and its laser component.

Blu-ray player

Like a CD player, a Blu-Ray player reads digital information (video or audio) stored on a disc, and a laser is used to record this information. The pits on a Blu-Ray disc are much smaller and more closely packed together than for a CD, so much more information can be stored. As a result, the resolving power of the laser must be greater. This is achieved using short wavelength ( λ = 405 nm ) blue laser light—hence, the name “Blu-” Ray. (CDs and DVDs use red laser light.) The different pit sizes and player-hardware configurations of a CD, DVD, and Blu-Ray player are shown in [link] . The pit sizes of a Blu-Ray disk are more than twice as small as the pits on a DVD or CD. Unlike a CD, a Blu-Ray disc store data on a polycarbonate layer, which places the data closer to the lens and avoids readability problems. A hard coating is used to protect the data since it is so close to the surface.

The different pit sizes and player-hardware configurations of a CD, DVD, and Blu-Ray player are illustrated. In each case, the pits are smaller than the size of the spot made by the laser beam on the surface of storage medium. On the left, the CD player, with 0.7 GB storage capacity, is shown. The CD laser has a wavelength of lambda equal to 780 nanometers, corresponding to a red color. It is focused by a lens, penetrating the CD material to a depth of 1.2 m m and forming a relatively large spot on the surface of the CD. In the middle, the DVD player, with 4.7 GB storage capacity, is shown. The DVD laser has a wavelength of lambda equal to 650 nanometers, corresponding to a reddish-orange color. It is focused by a lens, penetrating the DVD material to a depth of 0.6 m m and forming a smaller spot on the surface of the DVD than we saw on the CD. On the right, the Blue-Ray player, with 25 GB storage capacity, is shown. The blue-Ray laser has a wavelength of lambda equal to 405 nanometers, corresponding to a blue color. It is focused by a lens, penetrating the blue-ray disc material to a depth of 0.1 m m and forming a small spot on the surface of the disc.
Comparison of laser resolution in a CD, DVD, and Blu-Ray Player.


  • Laser light is coherent (monochromatic and “phase linked”) light.
  • Laser light is produced by population inversion and subsequent de-excitation of electrons in a material (solid, liquid, or gas).
  • CD and Blu-Ray players uses lasers to read digital information stored on discs.

Key equations

Orbital angular momentum L = l ( l + 1 )
z -component of orbital angular momentum L z = m
Radial probability density function P ( r ) d r = | ψ n 00 | 2 4 π r 2 d r
Spin angular momentum S = s ( s + 1 )
z -component of spin angular momentum S z = m s
Electron spin magnetic moment μ s = ( e m e ) S
Electron orbital magnetic dipole moment μ = ( e 2 m e ) L
Potential energy associated with the magnetic
interaction between the orbital magnetic dipole
moment and an external magnetic field B
U ( θ ) = μ z B = m μ B B
Maximum number of electrons in a subshell of
a hydrogen atom
N = 4 l + 2
Selection rule for atomic transitions in a
hydrogen-like atom
Δ l = ± 1
Moseley’s law for X-ray production ( Z 1 ) = constant f

Conceptual questions

Distinguish between coherent and monochromatic light.

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Questions & Answers

given that the velocity v of wave depends on the tension f in the spring, it's length 'I' and it's mass 'm'. derive using dimension the equation of the wave
obia Reply
What is the importance of de-broglie's wavelength?
Mukulika Reply
he related wave to matter
at subatomic level wave and matter are associated. this refering to mass energy equivalence
how those weight effect a stable motion at equilibrium
Nonso Reply
how do I differentiate this equation- A sinwt with respect to t
Evans Reply
just use the chain rule : let u =wt , the dy/dt = dy/du × du/dt : wA × cos(wt)
I see my message got garbled , anyway use the chain rule with u= wt , etc...
de broglie wave equation
LoNE Reply
vy beautiful equation
what is electro statics
fitsum Reply
when you consider systems consisting of fixed charges
Diagram of the derive rotational analog equation of v= u+at
Nnamnso Reply
what is carat
Arnulfo Reply
a unit of weight for precious stones and pearls, now equivalent to 200 milligrams.
a science that deals with the composition, structure, and properties of substances and with the transformations that they undergo.
what is chemistry
Mrs Reply
what chemistry ?
where are the mcq
Fred Reply
acids and bases
How does unpolarized light have electric vector randomly oriented in all directions.
Tanishq Reply
unpolarized light refers to a wave collection which has an equal distribution of electric field orientations for all directions
In a grating, the angle of diffraction for second order maximum is 30°.When light of wavelength 5*10^-10cm is used. Calculate the number of lines per cm of the grating.
Micheal Reply
OK I can solve that for you using Bragg's equation 2dsin0over lander
state the law of gravity 6
cletus Reply
what is cathodic protection
Ebe Reply
its just a technique used for the protection of a metal from corrosion by making it cathode of an electrochemical cell.
what is interferometer
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what is interferometer
Practice Key Terms 6

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Source:  OpenStax, University physics volume 3. OpenStax CNX. Nov 04, 2016 Download for free at http://cnx.org/content/col12067/1.4
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