The following is a basic diagram of a spectrometer. 
What is component 3?
Definitions:
Graded School Classes
A method of organizing school classes by age and academic ability level, allowing for a more standardized education system.
Adolescence
The transitional stage of physical and psychological development that generally occurs during the period from puberty to legal adulthood.
Peer Group
A social group whose members have interests, social positions, and age in common, playing a significant role in the socialization of individuals.
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