Ask to students to set up their laser and their target on opposite sides of the wall. Tell them they can use a mirror to try and get around the wall and hit their target. Most students will do this with no difficulty at all. But further ask them to measure the angles that the laser makes with the mirror. What angles do they get? What's the relationship? Briefly discuss the Law of Reflection, and the importance of measuring from the Normal Line.
Next, ask them of the exact same scenario, but replace that mirror with a lens. They may take a moment to realise that lens will reflection too. And another moment to realize that they should take advantage of the curved side to act perpendicular to the laser, but the flat side to reflect. Through measuring with protractors, and a discussion, note that the Law of Reflection still holds. Also note that some light escapes.
Now we enter to "Level 3", where we place the target on the opposite side of the lens. Ask the students to try and hit the target. At first, they'll be adjusting it in either direction, before hitting that sweet spot. Ask them to measure the angles, and see if they can find a connection between the angles. These slides will help assist 'which angles' you're talking about. They might come up with some correlation, however very unlikely the correct one.
At this point, you can now introduce Snell's Law, as seen in the PowerPoint slide above. Show that the connection between the angles is a little more complicated than the Law of Reflection. Snell's Law, you should explain, depends on the two materials that the light is going through. They have a constant, known as the index of refraction, and these determine the angles on either side of the boundary. If you provide them with the index of refraction for air, and have them measure the angles, you can now ask them to calculate the index of refraction of their lens. Depending on the education level of the students, you could further discuss error.
The last, final, "level 4" of this demonstration is to suppose a big, scary, monster, with thick skin. Suppose the students need 100% of their laser to reach the monster to kill it. However, they're still stuck with the problem that they're behind a wall, and with only a lens. Ask to see if they can somehow aim the laser at the lens to ensure that all the light reaches the monster. Some students may be able to find this angle, while others may try to get clever and walk around the lens (attempting to use refraction). Once some of the students do it correctly, ask them to then find the smallest angle they can (relative to the Normal Line) that they can still achieve this phenomenon.
Now explain to them that this phenomenon is known as Total Internal Reflection. Furthermore, that there's a critical angle that allows for Total Internal Reflection to occur (and for all values after that point). Using the index of refraction they found from earlier, you can ask the students to calculate the critical angle. They can cross-check this theoretical value with the value that they measured. Ask them to discuss why the number might be slightly different (due to error). Furthermore, you can discuss applications of this Total Internal Reflection, the most popular of which is Fibre Optic Cables.
The plan below is aimed at high school students. My PowerPoint slides are presented, with a description of the content that would be discussed on each slide. Before the demonstration begins, split the students up into small groups (smaller the better), and provide them each with some whiteboard space, a laser, a wall, a mirror, a lens and a target. All of these (as per the construction page) will be magnetized. Also provide them each with a protractor.
The lesson plan above requires the students have already mastered basic trigonometry. However, the lesson plan can be altered for a younger audience. In fact, one of my demonstrations was done for a group of elementary school students. I adapted the plan appropriately, and the video for this can be seen below.