One complete wave passes a putrefying pile of poached pickerel in 0.50 seconds. the piled pickerel protrude 20 cm along the beach. calculate its wavelength

If the wave passes the putrefying pile of a poached pickerel in 0.5 seconds, then the wave has a period of 0.5s. 

But we say that if F=1/T,
where F⇒frequency,
          T⇒period.

F=1/0.5
  = 2Hz

velocity of the wave = distance/time taken
                                = (20÷100)/0.5
                                = 0.2÷0.5
                                = 2/5

Now form the wave equation,
V=λF 
Where V ⇒ velocity of the wave.
             λ⇒wavelength of the wave.
       
So, λ=V/F

    λ = 2/5 ÷ 2
       =2/5 ×1/2
       = 2/10
       =0.2 m

The wavelength in cm is, 0.2×100 = 20 cm. 

Aform of energy that is reflected or emitted from objects in the form of electrical and magnetic waves that can travel through space. there are many forms of  electromagnetic  energy including gamma rays, x rays, ultraviolet radiation, visible light, infrared radiation, microwaves and radio waves.

The answer is as follows in the photo