My chemistry teacher today told that only two mono chloro substituted products are possible for naphthalene. He explained this by saying since the naphthalene molecule is symmetrical about the axis passing through the C-C bond where the benzene rings are fused, the corresponding positions should be identical & hence only two such products are possible. I personally don't find this convincing & hope to look for a genuine answer. I will be extremely thankful to anyone who helps.
If someone is unable to understand my teacher's explanation, just ignore it. I myself wasn't able to understand it properly so could have mistaken it for something else.