an em wave of intensity I falls on a surface kept in vaccum what is the radiation pressure if wave is totally reflected

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$R a d i a t i o n p r e s s u r e (p) i s t h e f o r c e e x e r t e d b y e m w a v e o n u n i t a r e a o f t h e s u r f a c e, i . e . r a t e o f c h a n g e o f m o m e n t u m p e r u n i t a r e a o f t h e s u r f a c e . M o m e n t u m p e r u n i t a r e a p e r u n i t t i m e = \frac{I n t e n s i t y}{S p e e d o f w a v e} = \frac{I}{c} C h a n g e i n m o m e n t u m p e r u n i t a r e a p e r u n i t t i m e = \frac{∆ I}{c} N o w w h e n t h e w a v e i s t o t a l l y r e f l e c t e d t h e n, m o m e n t u m o f t h e r e f l e c t e d w a v e p e r u n i t a r e a p e r u n i t t i m e = - \frac{I}{c} R a d i a t i o n p r e s s u r e = p = \frac{I}{c} - (- \frac{I}{c}) = \frac{2 I}{c}$
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