Abstract: An optical interference apparatus with a regular triangle structure，which can

generate double - beam interference，is introduced according to the optical wave theory. The

formulae of optical path difference and phase difference have been derived; the

interference conditions and distribution laws of spots or fringes are also quantitatively

analyzed in detail. Its possible application and supplement function in teaching and researching

activ- ities are also discussed.

Key words: optical interference, interference condition, interference fringes, regular-triangle optical cavity, double beam