1) http://physics.bu.edu/~duffy/PY106/Instruments.html

2) http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

3) http://www.nasa.gov/audience/foreducators/topnav/materials/listbytype/Optics.Guide.html

4) http://college.hmco.com/cgi-bin/SaCGI.cgi/ace1app.cgi?FNC=AcePresent__Apresent_html___physicalscience_shipman_physsci_10e_students_ace_quiz2_07