What's the difference between a Haidinger fringe and Newton's Ring? I know visually, the central fringe of a Newton ring is dark, whereas the central Haidinger fringe is bright, but what's the difference in physical set-up that would cause these different types of interference?
[Physics] Difference between haidager fringes and Newton rings
interference
Related Solutions
Why is it the opposite in both experiment? (i.e. maxima occurs for multiple of λ/2 in single slit diffraction but for YDSE multiples of λ/2 are where dark fringes occur).
When two point sources separated by some distance $d$ produce waves in phase, they will produce constructive interference at any point $P$ for which the path difference from the sources is $n \lambda$. I believe this is more or less clear, do you see it? Since two very narrow slits may be considered, for this purpose, as point sources, this results also applies for Young's (very narrow)double-slit experiment.
When you have 1 slit with finite width, the usual approach is to consider the slit to be divided into several tiny strips (point sources) each of which produce waves. The following image is taken from Young and Freedman's book, in which the topic is nicely explained), considering 2 little strips in the slit, but the same reasoning applies considering 4, 6, 8,... strips yielding that you get dark bands at $y_m = x \frac{m \lambda}{a}$. In the book you may find a more detailed explanation.
Why is the central maximum in diffraction is twice as wide as the other secondary maxima?
You may look at this using the formula. Basically it is because the central maxima goes from $m = -1$ to $m = 1$ (it jumps two values of $m$) while the secondary maxima jumps only 1 value in $m$ (e.g. from $m=1$ to $m=2$.)
Why is diffraction measured in radian while fringe width in mm?
I don't know what you mean by this. You may measure in radians the angular position of some maxima/minima from the normal line coming from the slits. You can do something similar to measure the width of the lobes in the diffraction pattern. Equivalently, you may measure the locations or the widths of the lobes in mm as distances over the screen.
A detector, such as your eye, does not respond at the rate of oscillation of the optical or RF frequency, instead it responds to the variation of its peaks or to variation of its average power (energy per cycle). These variations are many orders of magnitude slower than the oscillation rate. For example your eyes can tell the difference of brightness but only at the rate of few hundred hertz. In an ideal double-slit experiment the nulls are completely zero non-oscillating in intensity, there really is no energy; at the intensity peaks those do fluctuate at the rate of the oscillation (optical or RF) frequency but the detector ignores that and responds to the peaks or rms energy.
Best Answer
For Haidinger fringes, the thin film has a constant thickness and the path difference is associated with a variable inclination of rays. With an extended source, localization is at infinity.
For Newton fringes, the thickness is variables and the inclination is constant. The path difference is associated with a variable thickness. With an extended source, localization is near the lens.
Sorry for my poor english.