Interference vs Diffraction: Differences for CBSE Class 12 Physics

Interference

• Superposition of light waves from TWO coherent sources
• Produces equally spaced bright and dark fringes
• All bright fringes have equal intensity
• Fringe width β = λD/d (depends on slit separation d)
• Condition: Two narrow, coherent, monochromatic sources needed
• Young's Double Slit Experiment is the classic example

Diffraction

• Bending of light waves around edges of a SINGLE slit/obstacle
• Central maximum is widest and brightest; secondary maxima get progressively dimmer
• Intensity of secondary maxima decreases rapidly
• Central fringe width = 2λD/a (depends on slit width a)
• Occurs with a single slit — no need for two sources
• Single slit diffraction pattern: wide central bright band with fainter bands on sides

Key Differences

• Sources: Interference = 2 sources, Diffraction = 1 slit
• Fringes: Interference = equal width, Diffraction = central maximum much wider
• Intensity: Interference = all fringes equal, Diffraction = central brightest, others fade
• Spacing: Interference fringes are narrow and uniform, Diffraction fringes are wider and non-uniform