/etc/ld.so.cache is the cache containing ordered list of libraries found in /etc/ld.so.conf. Our applications search in this cache for any library.
Before searching in the above cache you can make the application to search in LD_LIBRARY_PATH to figure out the library.
This is exactly similar to your PATH environmental variable, wherein executables are searched. Here you are searching for a shared object i.e., .so file.
Here /opt/lib is being appended to your old LD_LIBRARY_PATH. If the above way of appending is confusing you can follow this way
Both are one and the same..