A periodic wave is a wave that follows a repeating pattern as it propagates through space. That is, the oscillating pattern repeats at regular intervals of time. This repeating motion is also known as periodic motion

Explanation:

What is simple harmonic motion?

"Simple harmonic motion is a type of motion directed along a straight line, whose acceleration is fixed towards a point  and is proportional to its distance from that point"

An object is said to be in simple harmonic motion if the following occurs:

• It moves in a uniform path.

• A variable force acts on it.

• The magnitude of the force is proportional to the displacement of the mass.

• The force is always opposite in direction to the displacement direction.

• The motion is repetitive and a round trip, back and

  forth, is always made in equal time periods

answer: the correct answer for the fill in the blank is rods.

rods are the specialized photoreceptor cells that are present in the retina of human eye. they are responsible for vision in the night time ( also called scotopic vision). they have very low visual acuity and color distinguishing ability.

as they are responsible for vision in night time, therefore they are found predominantly in the nocturnal animals.