In the context of Gestalt psychology, which principle explains why we tend to group similar objects together in our perception?

In a psychology class, students are shown an image of several circles — some circles are red, and others are blue. The students immediately perceive the red circles as one group and the blue circles as another. This perception is best explained by the Gestalt principle of:

During a psychology lecture on perception, a professor shows students an image with various lines: some are thick and vertical, others are thin and vertical, and a few are thick and horizontal. According to the Gestalt principle of similarity, which group of lines is students most likely to perceive as being grouped?

What is the primary focus of Gestalt psychology?

How does Gestalt psychology differ from structuralism in terms of perception?

How do Gestalt principles explain the perception of fluid motion in films with higher frame rates?

What are the Principles of Perceptual Organization in Gestalt psychology?