title: Make rigid body bounce off the screen edge
body: <p>I'm currently working on an object that involves bubble-like movement. This object has a rigidbody and a sphere collider. </p>
  
  <p>I use the <code>AddForce</code> method every 1-3 seconds to make it move continuously and slowly. </p>
  
  <p>Now I'd like to know how to make the rigidbody move in the opposite direction (a.k.a bounce off) when they reach the screen edge already. I have already computed the screen edges using <code>ViewportToWorldPoint</code> method. </p>
  
  <p>One ineffective solution that I thought of is to put empty game objects with collider at the edges but that won't work in my case since I'm building for mobile devices which have different screen resolutions/sizes.</p>
  
  <p>Anyone know a good solution for this?</p>