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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.

Summary

  • 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

what is bohrs model for hydrogen atom
Swagatika Reply
hi
Tr
Hello
Youte
Hi
Nwangwu-ike
hi
Siddiquee
what is the value of speed of light
Propessor Reply
1.79×10_¹⁹ km per hour
Swagatika
what r dwarf planet
Sivalakshmi Reply
what is energy
Isiguzo Reply
কাজের একক কী
Jasim
কাজের একক কী
Jasim
friction ka direction Kaise pata karte hai
Rahul Reply
friction is always in the opposite of the direction of moving object
Punia
A twin paradox in the special theory of relativity arises due to.....? a) asymmetric of time only b) symmetric of time only c) only time
Varia Reply
b) symmetric of time only
Swagatika
fundamental note of a vibrating string
fasoyin Reply
every matter made up of particles and particles are also subdivided which are themselves subdivided and so on ,and the basic and smallest smallest smallest division is energy which vibrates to become particles and thats why particles have wave nature
Alvin
what are matter waves? Give some examples
mallam Reply
according to de Broglie any matter particles by attaining the higher velocity as compared to light'ill show the wave nature and equation of wave will applicable on it but in practical life people see it is impossible however it is practicaly true and possible while looking at the earth matter at far
Manikant
a centeral part of theory of quantum mechanics example:just like a beam of light or a water wave
Swagatika
Mathematical expression of principle of relativity
Nasir Reply
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
Zahid
at subatomic level wave and matter are associated. this refering to mass energy equivalence
Zahid
it is key of quantum
Manikant
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)
Jerry
I see my message got garbled , anyway use the chain rule with u= wt , etc...
Jerry
de broglie wave equation
LoNE Reply
vy beautiful equation
chandrasekhar
what is electro statics
fitsum Reply
when you consider systems consisting of fixed charges
Sherly
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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