## Abstract

Using the formula λ = ∆x L / d, we will determine the wavelength of a red laser to be 641nm. Using this information we will then determine the track spacing for a CD and DVD. We calculate the track spacing for the CD to be 1.5 μm and the track spacing for the DVD to be 0.7 μm and compare our results for the spacing.

## Purpose

The purpose of this lab is to experimentally determine the wavelength of a laser, and the widths of tracks on a CD & DVD. We will demonstrate the diffraction patterns formed when the laser passes through the gratings.

## Hypothesis

The wavelength of the laser is within the range of the wavelength of its colour, which is 630nm to 650nm. In addition to this, the width of tracks for a CD is approximately 1.6 μm and for the DVD it is 0.74 μm. The difference in widths of tracks is due to the fact that both the CD and DVD have different storage capabilities therefore the spacings are different.

## Materials

-Red Laser source                   -Diffraction grating(s) of known spacing

-Grating holder                       -1 CD

-1 DVD                                   -Ruler

-Tape

For this lab, we will undergo three main steps to gather the information we will need to determine the wavelength of the laser and the groove spacing for the CD and DVD

Step 1: Recording data for determining wavelength of the laser.

Figure 1: This is the set up for gathering information to determine the wavelength of the
laser.

• The first thing to do is to set up the lab according to the diagram in Figure 1, make sure the papers are taped onto the textbook and the table.
• Before you begin, record the number of slits in the diffraction grating, this will be your d value.
• Measure the distance from the diffraction grating to the textbook, this will be your L value.
• Focus the laser through the diffraction grating and while it is on, mark and draw the diffraction pattern onto the paper. After marking the pattern use a ruler to measure the distance between maxima and record the values as your ∆x value.

Step 2: Recording data to determine the width of the tracks in the CD.

Figure 2: The setup for the CD

• Set up the lab according to Figure 2, making sure that the CD and the paper are taped onto the corresponding surfaces.
• When focusing the laser onto the CD it is very important that the laser is parallel to the table or that while it is focused on the CD its reflection is directly on the source of the laser. This helps ensure that the laser is hitting the CD at a 90-degree angle.
• For a CD the diffraction pattern would appear on the paper, this is why the lab is set up this way. While the laser is focused on the CD measure the height at which the laser’s light hits the CD and record it.
• Next, draw/mark the diffraction pattern on the paper and measure the distance between the two maxima.

Step 3: Recording data to determine the width of tracks in a DVD.

Figure 3: The set up for the DVD

• The setup for a DVD is similar to that of the CD, the difference is the disk. Again while setting up be sure to tape the paper and the DVD onto the textbook and the table.
• Just like the CD, when focusing the laser on the DVD it is important to make sure that the light hits the DVD at a 90-degree angle.
• For a DVD only one dot will appear in the diffraction pattern, draw the pattern on the paper and measure the distance from the DVD to the dot.
• Next, measure the height from where the laser hits the DVD and record it.

## Analysis

How do CDs and DVDs work?

Ans: CDs and DVDs work by having lasers focused on the shiny part of the disc. The shiny part of the disk contains bumps that reflect the focused laser beam. The bumps act as a diffraction grating so that when the laser beam is reflected the pattern contains a binary code that is read by pickups and transmitted or amplified.

How do CDs and DVDs impact our society?

Ans: CD’s and DVD’s impact our society by changing the way we store information. Compared to other means of storage CDs and DVDs are used by focusing laser beams onto the disc, since light is the only thing that touched the disc in theory it never wears out. This is better in comparison to cassettes for example, that contains physical contact between the player and the tape. This reduces the chances of damage and is why CDs and DVDs are popular choices for our society.

Describe the concepts of diffraction and interference in CDs or DVDs.

Ans: In CDs or DVDs, the discs themselves act as the diffraction gradient for the laser beam. As the laser beam is focused directly on the CD or DVD the laser beam is reflected. When it is reflected interference occurs, the waves overlap and diffraction patterns become visible. For CDs, the diffraction pattern shows two fringes while the DVD has only one. This is because the spacing of the CD is bigger than that of the DVD.

Use the concepts of diffraction and interference to describe the separation of white light into colors through a diffraction grating.

Ans: As white light goes through a diffraction grating it separates into colors. This occurs when the diffraction grating contains very closely spaced slits, or in CDs and DVDs closely spaced tracks. The closely spaced tracks cause the white light to separate because it scatters the different wavelengths within the white light, which causes interference that results in colors being visible.

## Validity

During the experiment, to ensure that our results were approximate we taped the papers to the textbook and to the table. So that when we measure and mark our drawings of the differentiation patterns the length does not change. In addition to that while, we conducted our lab we also made sure that the laser itself was as close to 90 degrees when hitting the CD and DVD. This was the only way we could make sure that the reflected pattern was not distorted.

## Conclusion

After we have conducted our lab, we have determined that the wavelength of the laser is 641nm. We know that our answer is valid because the range for a red laser is between 630nm and 650nm and our result is within the range. Furthermore, according to our lab, the track spacing of the CD is 1.5μm, the CD track spacing is said to be 1.6 μm. The experimental value we obtained is 6.25% off compared to the 1.6 μm. And for the DVD the spacing we determined is 0.7μm, while the theoretical value is 0.74μm.

The experimental value for the DVD is only 5.7% off compared to the spacing that was given. Since both our experimental spacing values for the CD and DVD are within 10% of error, we have obtained a pretty approximate value. From the values given for the spacing of the CD and DVD comparatively, the spacing for the DVD is smaller than the CD.

This also means that the diffraction pattern for the DVD is sharper than the CD. The difference in diffraction pattern and spacing contributes to the fact that the DVD has more storage than the CD.

Sources:

diffraction.” Encyclopædia Britannica. 2010. Encyclopædia Britannica Online. 22 Mar. 2010 <http://www.britannica.com/EBchecked/topic/163008/diffraction>.

How CD’s work.” How Stuff Works. 2010. How Stuff Works. Inc. 22 Mar. 2010

< http://electronics.howstuffworks.com/cd5.htm>

CD’s and DVD’s.” 2010. Explain That Stuff. 22 Mar. 2010
< http://www.explainthatstuff.com/cdplayers.html>

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