P ( p h o t o n r e a c h e s x g o i n g t h r o u g h e i t h e r s l i t ) = P ( p h o t o n r e a c h e s x g o i n g t h r o u g h s l i t 1 ) + P ( p h o t o n r e a c h e s x g o i n g t h r o u g h s l i t 2 ) − − P ( p h o t o n r e a c h e s x g o i n g t h r o u g h b o t h s l i t s ) = P ( p h o t o n r e a c h e s x | w e n t t h r o u g h s l i t 1 ) P ( g o i n g t h r o u g h s l i t 1 ) + P ( p h o t o n r e a c h e s x | w e n t t h r o u g h s l i t 2 ) P ( g o i n g t h r o u g h s l i t 2 ) − − P ( p h o t o n r e a c h e s x g o i n g t h r o u g h b o t h s l i t s ) = P ( p h o t o n r e a c h e s x | w e n t t h r o u g h s l i t 1 ) 1 2 + P ( p h o t o n r e a c h e s x | w e n t t h r o u g h s l i t 2 ) 1 2 − − P ( p h o t o n r e a c h e s x g o i n g t h r o u g h b o t h s l i t s )