2.2 CD and DVD

Question 1

The nearer the slit size is to the wavelength…

Answer 1

…the more the wave will diffract.

Question 2

The smaller the gap…

Answer 2

…the greater the diffraction.

Question 3

The longer the wavelength…

Answer 3

…the greater the diffraction.

Question 4

What are coherent waves?

Answer 4

Coherent waves are waves with a constant phase difference.

They will have the same frequency and wavelength.

Question 5

What do two waves do when they meet?

Answer 5

They interfere and superpose.
After they have passed they return to their original forms.
At the point they meet, the two waves will combine to give a resultant wave whose amplitude (or intensity) may be greater or less than the original two waves.

Question 6

What is destructive interference?

Answer 6

If two waves of the same type and the same frequency combine so that the crest of one coincides with the trough of the other, they will completely cancel each other out.

Question 7

What is constructive interference?

Answer 7

Two waves could combine when their crests coincide; then there would be constructive interference and the resultant amplitude would be equal to the sum of the separate amplitudes.

Question 8

Define superposition.

Answer 8

The effect of two waves adding together when they meet.

Question 9

What are the bright and dark fringes formed in the Young’s double slit experiment a result of?

Answer 9

The superposition of the waves coming from each of the slits.

Question 10

What was the young double slit experiment?

Answer 10

In Young’s Double Slit experiment, a double slit is used to diffract one light beam into two.
This causes constructive and destructive interference in the area where the two diffracted beams overlap (superposition) to create fringes.
Where there is a bright line, the waves from each slit have arrived in phase and reinforce each other giving twice the amplitude of a single wave.
A dark fringe is created when the waves from each slit arrive 180 degrees out of phase (a crest meets a trough etc) and completely cancel each other out .

Question 11

Why has a transmission diffraction grating got a shiny surface between the lines?

Answer 11

So that light gets reflected off it.

Question 12

Whats the difference between diffraction grating and Young’s double slit experiment?

Answer 12

Diffraction grating has a maxima more intense, more sharply defined and the angles the beam are diffracted through are larger so that they can be measured with greater precision.

Question 13

Explain the basic principles of a compact disc.

Answer 13

The surface of the CD contains one long spiral track of data.
Along the track, there are flat reflective areas and non-reflective bumps.
A flat reflective area represents a binary 1, while a non-reflective bump represents a binary 0.
The CD drive shines a laser at the surface of the CD and can detect the reflective areas and the bumps by the amount of laser light they reflect.
The drive converts the reflections into 1s and 0s to read digital data from the disc.

Question 14

What are the height of the bumps of a CD?

Answer 14

A quarter of a wavelength high.
This means that light striking the flat part travels a quarter of a wavelength in one direction and a quarter of a wavelength in the other direction (making a total distance of a half wavelength) further than light striking to top of the bump.

Question 15

How much is light deflected from the land in a CD delayed?

Answer 15

By half a wavelength and is us exactly out of phase with the light reflected from the bump.
These two waves will interfere destructively meaning that effectively no light has been reflected.
This means that the detecting optical sensor senses bumps as binary zeros whereas the land does not produce interference and is sensed as ones.

Question 16

Why is a motor rotating the disc at between 200-500 revolutions a minute?

Answer 16

So that the laser can pick up data at the same rate whether the beam is scanning near the centre where the disc is moving quickly or scanning at the outer parts where it is moving less quickly.

Question 17

How do CD-R discs work?

Answer 17

A laser creates a non-reflective area on the disc by darkening an area in the layer of dye positioned underneath the reflective aluminium surface (the laser bean shining upwards from below the disc).
A less powerful laser is then used to read the pattern with the darkened area not reflecting the light (as with bumps) while the remainder of the dye remains translucent and allows the aluminium surface to reflect the light.

Question 18

How do CD-RW (rewritable) discs work?

Answer 18

To create a writeable CD (CD-R), you need to modify the surface of a CD so you can burn data onto it, turning it into a CD-R.
There are no bumps on a CD-R. A clear dye layer covers the CD’s mirror.
A write laser heats up the dye layer enough to make it opaque.
The read laser in a CD player senses the difference between clear dye and opaque dye the same way it senses bumps – it picks up on the difference in reflectivity.

Question 19

What can stores more music, CD’s or DVDs?

Answer 19

Single sided DVDs have about seven times the capacity for storing music.

Question 20

Why can DVDs store so much more music than CDs?

Answer 20

Tigher spiral
Shorter bumps
More efficient tracking system

Question 21

How are DVD and HD-DVD made compatible?

Answer 21

By being accessed from the top and bottom of the same disc by the read write lasers.

Question 22

What is a dual layer DVD?

Answer 22

Dual-Layer DVD have two layers of Inks which, when the lasers are focus to the check layer they can write and read the data.
This makes it so you have double the amount of data on one disc.

Question 23

Why is Blu-ray higher definition?

Answer 23

Read by a blue laser, which has a shorter wavelength than the red laser that a DVD uses. The allows a more precise focus point for the laser and so pits and bumps can be smaller.
Blu rays diffract less.

Question 24

State two changes that were made to the CD format.

Answer 24

wavelength of light decreased/frequency increased
tighter spiral
bumps not as high

Question 25

Reading a CD relies on the production of an observable interference pattern. State two properties that the laser light must have in order to read the CD.

Answer 25

constant phase difference

wavelength is 4 × bump height

Question 26

Explain why the pits must be a quarter of a wavelength high.

Answer 26

Reflections from pit and land interfere destructively-waves from pit and land are antiphase.
Allowing detection of change from pit to land or vice versa.
Producing binary 0.

Question 27

Explain how the reflected first-order beams ensure that the tracking is correct

Answer 27

Equal intensity/amplitude beams mean reflection occurs from same surface.
Reflected beams should have equal intensity/amplitude
If one beam overlaps with pit/goes off track there will be different intensities/amplitude (so error/re-tracking initiated).

Question 28

Explain how the diffraction grating produces the bright spectral lines for a particular wavelength.

Answer 28

Superposition
Light from slits is coherent 
Path difference from slits is a multiple of one wavelength
Waves arrive in phase
Interference is constructive
Waves add to produce larger amplitude