I would not advise to try actual ray tracing in OpenGL because you need a lot hacks and tricks for that and, if you ask me, there is not a point in doing this anymore at all. If you want to do ray tracing on GPU, you should go with any GPGPU language, such as CUDA or OpenCL because it makes things a lot easier (but still, far from trivial).
To illustrate the problem a bit further: For raytracing, you need to trace the secondary rays and test for intersection with the geometry. Therefore, you need access to the geometry in some clever way inside your shader, however inside a fragment shader, you cannot access the geometry, if you do not store it “coded” into some texture. The vertex shader also does not provide you with this geometry information natively, and geometry shaders only know the neighbors so here the trouble already starts. Next, you need acceleration data-structures to get any reasonable frame-rates. However, traversing e.g. a Kd-Tree inside a shader is quite difficult and if I recall correctly, there are several papers solely on this problem. If you really want to go this route, though, there are a lot papers on this topic, it should not be too hard to find them.
A ray tracer requires extremely well designed access patterns and caching to reach a good performance. However, you have only little control over these inside GLSL and optimizing the performance can get really tough.
Another point to note is that, at least to my knowledge, real time ray tracing on GPUs is mostly limited to static scenes because e.g. kd-trees only work (well) for static scenes. If you want to have dynamic scenes, you need other data-structures (e.g. BVHs, iirc?) but you constantly need to maintain those. If I haven’t missed anything, there is still a lot of research currently going on just on this issue.