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Name: Connie
Status: Student
Grade: 9-12
Location: IL
Country: United States
Date: October 2006

I am conducting a experiment to decide whether I should use plastic or glass mirrors to form a design using lasers. Part of that experiment requires finding out how much energy is lost after each time the laser hits a mirror. So, what I need to know is, what can my teachers and I use to complete these measurements of the strength of the laser that can be done in a high school laboratory?

Hi Connie,

It makes no difference whether the mirror substrate is plastic or glass, if you use the correct type of mirror for this application. The type of mirror recommended for use in accurate optical work is called a "First Surface Mirror". This type of mirror has its reflective coating on the front side of the mirror, so the light being reflected never "sees" the plastic or glass. These types of mirrors are available from many scientific sources, including Edmund Scientific.

With regards to plastic mirrors, there are two general types of plastic used: acrylic and polycarbonate. My recollection is that optical grade polycarbonate (this is what CDs are made from) has slightly better optical characteristics than acrylic. But the differences small; either type is comparable with glass regarding its light absorbent properties. But as mentioned above, the use of a First Surface mirror is really the way to go. You can then determine the amount of light being lost as a result of reflective losses, without the uncertainty of absorption losses in the mirror's substrate. Note that you must never touch the reflecting surface of a First Surface mirror, because it is very delicate and cannot be cleaned of fingerprints.

Another variable is what the reflecting surface is made of. Glass mirrors have silver deposited on them as the reflecting medium, or they may have vacuum-deposited aluminum. Nearly all plastic vacuum-deposited layer of aluminum instead of silver. Silver and aluminum have slightly different reflectivities, so you may want to experiment to see what the differences are.


Bob Wilson.

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