Physics 142
Lecture Quiz 3
Name
September 8, 2000
Reci. Section time:
Please do your own work. Open book and notes. Please do not talk to your neighbors.
You may earn up to 10 points based on showing your work and explaining it.
Pretend you are a successful research scientist who has invented a thin plastic filter that can protect
human skin from ultraviolet radiation. In fact, the optical density of a thin sheet of this plastic, onetenth of a millimeter in thickness, has an optical density that is exactly the same, to three significant
figures, as three digits of your student ID number. To determine what value of optical density this is
put a decimal in front of the first non-zero digit in the last four digits of your student ID number and
then round this decimal to three significant figures, e.g. if the last four digits of your student ID
number are 5678 your film would have an OD of 0.568, or if the last four digits of your student ID
number are 0234 your film would have an OD of 0.234.
1. (2 points) Determine the optical density of your filter to three significant figures:
Optical Density (OD) = 0.__ __ __
2. (4 points) What is the fraction of ultraviolet radiation that is transmitted by one sheet of your filter?
3. (4 points) To provide a Sun Protection Factor of 100 (SPF 100, as used for sunscreen ratings), the
UV intensity must be reduced to 1.0% of the incident radiation, i.e. you must build a filter with an
optical density of 2.00. How thick must a filter made of your material be, in millimeters, to provide
this level of protection?
(If you need more space for writing please write on the back of this page)