Comparison Table: Interference vs. Diffraction

Aspect	Interference (YDSE)	Diffraction (Single Slit)
Origin	Superposition of waves from two separate sources	Superposition of Huygens' secondary wavelets from a single slit
Number of sources	2 (or more)	1 (with finite width)
Central fringe	Bright; same width as others	Bright; TWICE as wide as others
Fringe width	Uniform: $\beta = \lambda D/d$	Non-uniform: central $= 2\lambda D/a$
Fringe intensity	All maxima equal: $I_\text{max} = 4I_0$	Rapidly decreasing: $I_1 \approx 4.5\%$ of $I_0$
Min condition	$\Delta = (2n-1)\lambda/2$	$a\sin\theta = n\lambda$
Max condition	$\Delta = n\lambda$	$a\sin\theta = (2n+1)\lambda/2$ (approx)
Key parameter	$d$ (slit separation)	$a$ (slit width)
Pattern type	Sharp, uniform bands	Wide center + rapidly fading wings