I am unable to preview ARW photo files in finder

ARW is a file suffix for raw files from many Sony cameras. However, as they are raw files, the designation ARW merely tells you these are from a particular brand. These are not files like .jpg or .txt which all operate in standard ways. The support for the raw from each model of camera has to be added individually. Not all cameras are supported, though Apple do add more from times to time. If you want more you'll need to tell us what model of camera you are using.

I have RAW Right Away installed (it is not free and requires a purchase from the AppStore). It helps a lot to make browsing RAW files in the Finder easier, including the histograms, focus points, and metadata:

http://www.rawrightaway.com/