A roller coaster car sits on a peak. Just before it moves down the track, the car's potential energy is 10,000 joules. Halfway down the hill, its potential
energy decreases to 4,000 joules. What is the work done by the car to reach
this point? Assume that the track has negligible friction.

1.   what is light?

light is an electromagnetic wave visible to the human eye. 

2. what is a reflection?

the rebounding of a light ray when it hits a mirror.

3. what is the incident ray?

contacts the surface of an object.

4. what is the reflected ray?

bounces off a reflective surface.

5. what is the normal ray?

perpendicular to the surface of a mirror.

6. what is the angle of incidence?

an angle formed by the incident ray and the normal.

7. what is the angle of reflection?

an angle formed by the reflected ray and the normal.

8. what is refraction?

(bending.)

9. what is a converging lens?

bring together light rays that pass through.

10. what is a diverging lens?

spreads out light passing through.

11. focal point?

the real point where the refracted rays meet when the incident rays run parallel.

i copy pasted btw from an website..