What are the cases in which dopler effect is observed

The Doppler effect shows up whenever the distance between a

source of waves and the observer is increasing or decreasing.

I said it that way because it doesn't show up every time the source

and observer are moving. There has to be some 'radial component'

of motion ... motion in the direction that's bringing them farther apart

or closer together.

If a source of waves was buzzing around in a perfect circle, and

you were at the center of the circle, then the distance from you to

the source wouldn't be changing, and you wouldn't observe any

Doppler effect.

The Doppler Effect is observed when the source of waves is moving with respect to an observer, the doppler effect can be observed for any type of waves amoung these are sound waves, water waves, light waves or other type of waves.