Refraction happens because light changes speed when it crosses into a different material. When it enters a slower medium, its speed decreases, and since the frequency stays the same, the wavelength becomes shorter. This change in speed at the boundary causes the ray to bend toward the normal. The relationship n1 sin θ1 = n2 sin θ2 (with n2 > n1) gives θ2 < θ1, so the refracted ray moves closer to the normal. In contrast, entering a faster medium would speed it up and bend away from the normal, and light does change speed in different media, so claims of no bending or no speed change aren’t correct. The wavelength adjusts with speed (it gets shorter in the slower medium) while the frequency remains constant.

Elastic potential energy is stored in a stretched or compressed spring. When you push or pull the spring, you do work on it and deform it, which stores energy in the spring's structure. The amount stored depends on how much you change its length (x) and how stiff the spring is (k), given by the form 1/2 k x^2. When the spring returns to its natural length, that stored energy can be released as motion. This is different from magnetic energy (stored in magnetic fields), gravitational potential energy (depends on height in a gravitational field), or chemical energy (stored in chemical bonds).

When a skydiver falls, gravity is doing work that turns gravitational potential energy into kinetic energy, so the speed increases. At the same time, air resistance acts opposite the motion and takes some of that energy away, warming the air and the diver. So the potential energy decreases, the kinetic energy increases, and some energy is transformed into thermal energy due to drag. Overall energy is conserved when you include the heat produced, but the mechanical energy (kinetic plus potential) drops as energy is dissipated as heat.

Magnetic field lines are a way to picture the magnetic field in space. Along each line, the tangent shows the direction a magnetic north pole would experience. The closer the lines are, the stronger the field in that region. They exist both outside and inside magnets, forming closed loops from the magnet’s north to its south outside, and returning inside. That’s why the best description is that a line shows the direction of the magnetic field, and line density indicates field strength. The other ideas don’t fit: magnetic monopoles aren’t what these lines mark, the lines don’t trace the path of an electric current, and the field is represented everywhere, not just inside magnets.