  The ground-breaking image of a black hole's event horizon, which captured the
public's attention and imagination in April 2019, was captured using the power
of interferometry: many separate telescopes working together to observe the
cosmos in incredible detail. Many recent astrophysical discoveries that have
revolutionized the scientific community's understanding of the cosmos were made
by interferometers such as LIGO, ALMA, and the Event Horizon Telescope. Astro
101 instructors who want their students to learn the science behind these
discoveries must teach about interferometry. Decades of research show that
using active learning strategies can significantly increase students' learning
and reduces achievement gaps between different demographic groups over what is
achieved from traditional lecture-based instruction. As part of an effort to
create active learning materials on interferometry, we developed and tested a
new Lecture-Tutorial to help Astro 101 students learn about key properties of
astronomical interferometers. This paper describes this new Lecture-Tutorial
and presents evidence for its effectiveness from a study conducted with 266
Astro 101 students at the University of North Carolina at Chapel Hill.
