🔍 Key Concepts

• Notice the two labeled P.E.C. components and the central lamp with mirrors between a reference tube and a measuring tube. • Think about how light passing through two different paths can be compared to detect changes in a gas or vapor stream. • Ask yourself which choice best matches a system that compares light signals from a reference path and a sample (measuring) path.

💭 Think About

• What does P.E.C. likely stand for in this context, and how does it normally create an electrical signal? • Why would the device need both a reference tube and a measuring tube instead of just one passage? • Which answer choice directly connects to light intensity changes being converted into an electrical output?

✅ Before You Answer

• Identify what P.E.C. means and how it operates in optical instruments. • Confirm which choice explicitly involves a change in electrical output due to a sensing element, rather than fluid dynamics or mechanical force. • Check that your selected principle can use a lamp, mirrors, and two tubes to compare a sample against a reference.