The phase difference between two waves from successive half period zones or strips is:
(A) $\dfrac{\pi }{4}$
(B) $\dfrac{\pi }{2}$
(C) $\pi$
(D) zero

Hint: We can say that frequency is the number of occurrences of a repeating event per unit of time. It is also referred to as temporal frequency, which emphasizes the contrast to spatial frequency and angular frequency. Frequency is measured in units of hertz (Hz) which is equal to one occurrence of a repeating event per second. Frequency measures the number of times something occurs in a specific amount of time. While frequency can be used to measure the rate of any action, in technical applications it is typically used to measure wave rates or processing speed.

Complete step by step answer
We know that phase difference is used to describe the difference in degrees or radians when two or more alternating quantities reach their maximum or zero values. Previously we saw that a Sinusoidal Waveform is an alternating quantity that can be presented graphically in the time domain along a horizontal zero axis.
We can say that the phase difference between the two electrical quantities is defined as the angular phase difference between the maximum possible value of the two alternating quantities having the same frequency. The angle between zero points of two alternating quantities is called angle of phase differences.
The half period zone is provided by an optical device known as zone plate. It is simply a plane parallel gloss plate having concentric circles of radii accurately proportional to the square roots of the consecutive natural numbers 1,2,3 ... etc. The area is given by $\pi \gamma^{2} .$
 Hence the areas are $\pi, 2 \pi, 3 \pi, 4 \pi,-$ -
The phase difference is thus $\pi$ between each successive half period zones of strips.

So, the correct answer is option C.

Note: We know that wave frequency is the number of waves that pass a fixed point in a given amount of time. The SI unit for wave frequency is the hertz (Hz), where 1 hertz equals 1 wave passing a fixed point in 1 second. T is the time it takes for one complete oscillation; it is measured in seconds. All waves, including sound waves and electromagnetic waves, follow this equation. For example, a wave with a time period of 2 seconds has a frequency of 1/ 2 = 0.5 Hz. A sound wave has a time period of 0.0001 seconds.