The thinnest film in which the reflected light will be a max

1. The problem statement, all variables and given/known data
Monochromatic light, at normal incidence, strikes a thin film in air. If λ denotes the wavelength in the film, what is the thinnest film in which the reflected light will be a maximum?

• 
  
  A
  
  λ/4
  
  
• 
  
  B
  3λ/4
  
  
• 
  
  C
  λ
  
  
• 
  D
  
  much less than λ
  
  
• 
  
  E
  
  λ/2

2. Relevant equations
n air ∠ n glass
so at the first reflection it will cuase a 180° ray out of phase while the second ray refected from the inside it will stay inphase because n air ∠ n glass
so for constructive interference: 2L=odd number*λ/2
3. The attempt at a solution
least odd number is one , so L=λ/4 ?? why it is incorrect