\ 
??????Equation 1
by
Dr. Didar Islam,
Central Michigan Uuniversity Professor of
Physics
INTRODUCTION
Even though the laser is the main topic of discussion, most of the activities are concerned with the Conservation of Energy and Thermodynamics. Most high school physics curricula and most textbooks include these topics. Using the laser to teach or reinforce these concepts is the key. When researchers are constructing these laser cutters, the basic principles of physics are used to determine the same quantities as we have included here.
Laser surgery, especially of the eye, is a growing field. The new laser surgery to correct nearsightedness has been in the news recently and it seems they are developing new methods of laser surgery every day. With all of these items making the news, it is hoped that these lessons will continue to stimulate interest and bring the real physics into the real world of the high school student.
ACTIVITY ONE
Although the instructor will determine the placement and use of this activity, it probably is best placed when studying Thermodynamics. Most of the activities are applications and calculations from that unit. The equations for light energy, mass of melted material, latent heat of fusion, and specific heat capacity will undoubtedly have been covered but could be given without derivation if necessary. These equations are given here for reference. They were left out of the student section purposely so the classroom teacher could decide how and when to introduce them to the students.
\ ??????Equation 1
Where:
P = Power
W = Work or Energy
t = time
\ 
??????Equation 2
Where:
r = density
m = mass
V = volume
???????Equation 3
Where:
Q = heat absorbed
m = mass
Hf = latent heat of fusion
???????Equation 4
Where:
Q = heat absorbed
m = mass
c = specific heat capacity of the material
D T = change in temperature
It should be emphasized that the equations used to calculate the time for the sun to melt 150 g of ice could also be used to calculate the time to melt a certain amount of steel using the laser. The only difference is the source of thermal energy.
The additional problems are an extension of the original activity and can be removed from the activity if the teacher so desired. They are intended to give additional practice to the students in using graphs to solve problems. They also give additional practice in solving thermal energy problems.
The additional activity can be removed also if the teacher so desired. It basically is an extension of graphical analysis. The merits of the activity are that the student needs the concepts and skills it reinforces. The teacher on a "need to know" basis can give out the hints or they can be given wholesale to the students. The teacher could also ignore the hints and generate some of their own if the need arises.
ACTIVITY TWO
Laser surgery is one of the most interesting uses of lasers today. The eye surgery that is being done today is truly miraculous. It is hoped the student would make the connection that the same equations used in their physics class is used in laser surgery. The laser produces heat and the tissue is vaporized. Laser cutting speeds can be calculated in the same manner as for cutting steel. The only difference is in the power of the laser.
The equations are given to the student without derivation. Most students will accept this. Sometimes it is better to do without derivation if that process only takes away from what is being taught.
Notice that the problems to be solved are similar in nature to the problems for cutting steel. Again, this reinforces the use of the thermodynamic equations for heat capacity and latent heats in actual every day situations. The teacher should reinforce the research done to determine the cutting speed of lasers for specific surgeries when the students study thermodynamics. Years of research and testing are required before this marvel of technology is released to the public.
Listed below are some web sites that may be of interest. These sites were consulted during the development of these activities.
WEB SITES
http://www.gs.uk.com/laser-summary.html???UK Laser Site Summary
http://met121.bham.ac.uk/app-D.htm???Particle Size Analysis by Laser
http://www.shimadze.com?????Home Page/Measuring Instruments
http://www.achilles.net/~jtalbot/news/EtaCarinae.html?Violet Laser Star
http://www.achilles.net/~jtalbot/history/index.html??Index of Laser Stars
http://www.achilles.net/~jtalbot/glossary/diffract/plots?Diffraction Patterns
http://www.iptsg.epfl.ch/aps/BAPSAPR97/abs/S30002.html?Femtosecond Laser Breakdown of Water
http://www.templex.com/slide_show/slide25.html??Data Storage
http://basil.computer.org/next50/213E.html???Holographic Storage
http://www.lbl.gov?????Lawrence/Berkley Lab
http://www.laurin.com/Content/Oct97/techFemto.html?Femtosecond Laser
Questions? Comments??
James Gormley