![]() Step 2: Recording data to determine the width of the tracks in the CD. After marking the pattern use a ruler to measure the distance between maxima and record the values as your ∆x value. Focus the laser through the diffraction grating and while it is on, mark and draw the diffraction pattern onto the paper.Measure the distance from the diffraction grating to the textbook, this will be your L value.Before you begin, record the number of slits in the diffraction grating, this will be your d value.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.Step 1: Recording data for determining wavelength of the laser.įigure 1: This is the set up for gathering information to determine the wavelength of the Red Laser source -Diffraction grating(s) of known spacingįor 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 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. 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 wavelength of the laser is within the range of the wavelength of its colour, which is 630nm to 650nm. We will demonstrate the diffraction patterns formed when the laser passes through the gratings. The purpose of this lab is to experimentally determine the wavelength of a laser, and the widths of tracks on a CD & 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. Using this information we will then determine the track spacing for a CD and DVD. D 1 or D 99).Using the formula λ = ∆x L / d, we will determine the wavelength of a red laser to be 641nm. The beginning and end of the distribution are commonly defined by D 10 and D 90, although other D values can be used to define the cumulative distribution as well (e.g. D 50 defines the point where 50 % of the particles are smaller and 50 % bigger than that certain diameter. In either direction, the cumulative curve always ranges from 0 % to 100 %, with the middle point D 50 being the most commonly reported result of particle sizing by laser diffraction. ![]() ![]() This is done either from the smallest to the biggest diameter (called the "undersize curve") or in the opposite direction (called the "oversize curve"). To get this distribution, values for all previous classes are added to the next. ![]() For this reason, usually the cumulative distribution is analyzed. spikey, flat, etc.), so peak values are rather unreliable. However, there might be more peaks or the peak might be weakly defined (e.g. The D mode value defines the position of the highest peak. The basic particle size distribution might have one or more peaks for size classes, which indicate the most common particle sizes. The sample de-agglomerates (breaks down into smaller sized particles) as particles collide with each other or with the wall of the dispersion unit.Ī typical result of a laser diffraction measurement is shown in Figure 11. In dry mode the powder is put into motion either by compressed air or by gravity, creating a dry flow which is positioned in front of the laser beam. The liquid dispersion unit is usually equipped with a mechanical stirrer with adjustable speed and with a sonicator with adjustable duration and power. The sample keeps circulating until the measurement is done. In liquid mode the particles are dispersed in a liquid and pumped into a glass measurement cell which is placed in front of the laser. it should be measured in liquid mode if the final product is a liquid dispersion and in dry mode if the final product is a powder. Usually a sample should be analyzed in a state relevant to its application, i.e. This means that each particle should be visible as a single particle in front of the laser, moving through either liquid medium or air. In order to get a clear diffraction, it is necessary to have a proper dispersion of the sample.
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