NSES Content Standard A 
Science as Inquiry: Understanding about scientific inquiry 
Grades K-4, page 123 

Simple instruments, such as magnifiers, thermometers, and rulers, provide more information than scientists obtain using only their senses. 

Benchmark 1B The Nature of Science: Scientific Inquiry
Grades K-2, page 10
Tools such as thermometers, magnifiers, rulers, or balances often give more information about things than can be obtained by just observing things without their help.

Benchmark 5C The Living Environment: Cells
Grades K-2, page 111
Magnifiers help people see things they could not see without them.