Which of the following equation is used for the constructive interference in optical grating?

• A. a tan‚âà√≠¬¨¬±=n‚âà√≠¬¨‚Ñ¢
• B. a cot‚âà√≠¬¨¬±=n‚âà√≠¬¨‚Ñ¢
• C. a sin‚âà√≠¬¨¬±=n‚âà√≠¬¨‚Ñ¢
• D. a sec‚âà√≠¬¨¬±=n‚âà√≠¬¨‚Ñ¢