Line of Effect and Line of Sight are almost identical. Both require only a single line from you to the target.
Line of Effect: You can target a creature or a square if there’s an unblocked path between it and you—that is, if you have line of effect to it. If every imaginary line you trace to a target passes through or touches a solid obstacle, you don’t have line of effect to the target.
Line of Sight: To determine whether you can see a target, pick a corner of your space and trace an imaginary line from that corner to any part of the target’s space. You can see the target if at least one line doesn’t pass through or touch an object or an effect—such as a wall, a thick curtain, or a cloud of fog—that blocks your vision.
The bold lines are functionally identical, except that line of sight cares about vision and line of effect cares about how solid the obstruction is.