MouseEnterLeave

Public Class Form1

    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load

        AddHandler Me.MouseEnter, AddressOf Form_MouseEnter
        AddHandler Me.MouseLeave, AddressOf Form_MouseLeave

    End Sub

    Private Sub Form_MouseEnter(sender As Object, e As EventArgs)
        Me.Opacity = 0.5
    End Sub

    Private Sub Form_MouseLeave(sender As Object, e As EventArgs)
        Me.Opacity = 1
    End Sub

End Class