First off, opacity is an object-level attribute, so you can't be changing the opacity of a layer, so let's presume you meant you are lowering the opacity of a single object that is the only thing on a layer on that page.
Adding transparency to a page can, indeed affect the appearance of every other object on the page. Does switching the transparency blend space (Edit > Transparency Blend Space...) make a difference?
As rar as the text, best practice is to keep it above all transparency to minimize interaction that can cause it to be outlined or rasterized. Normally we here that it gets darker, rather than lighter, though. What do you have for Appearance of Black in your Prefs?
i went to Edit > Transparency Blend Space,and when i changed the colour mode to rgb it worked out as i wanted to.in cmyk mode it all had a different opacity appearance.