What happens if there is no activation function in some layers of a neural network? What if I don't apply an activation function on some layers in a neural network. How will it affect the model?
Take for instance the following code snippet:
def model(x):
    a = Conv2D(64, (3, 3))(x)                         
    x = Conv2D(64, (3, 3), activation = 'relu')(x)
    b = Conv2D(128, (3, 3))(x)
    x = Conv2D(128, (3, 3), activation = 'relu')(b)
    return x, a, b
If you do not specify an activation for a layer you are effectively creating a linear transformation through that layer.  From the documentation:

activation: Activation function to use. If you don't specify anything, no activation is applied (see keras.activations).