Interference is amplitude addition with phase.

Two coherent sources create a stable interference pattern on a screen. The geometry connects path difference to positions of bright and dark fringes.

• Two-slit setup and what is measured
• Path difference as the key quantity
• Bright vs dark condition (conceptual)
• Small-angle geometry idea (intro)

Fringe spacing describes how far apart adjacent bright fringes appear on the screen. It depends on wavelength, slit separation, and screen distance.

• What determines “spacing”
• Dependence on wavelength
• Dependence on slit separation
• How screen distance enters

Reflections from the top and bottom of a thin film can interfere. The key is path difference inside the film plus possible phase shifts on reflection.

• Two reflected rays picture
• Optical path length idea (intro)
• Phase flips on reflection (conceptual)
• Why soap bubbles show colors

A stable interference pattern requires a coherent source: phase relationships must persist over the time and distances involved in the setup.

• Why ordinary light often washes out fringes
• Temporal coherence and path differences
• Spatial coherence and slit illumination
• Practical ways coherence is improved

Practice using path difference rules, computing fringe spacing, and applying thin-film reasoning with clear assumptions.

• Bright/dark condition drills
• Fringe spacing computation sets
• Thin-film “what color?” concept checks
• Coherence reasoning questions
• Exam-style interference problems