**Question:**
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> 
> The angular position at which first minima is observed is $\theta=\pi/6$ rad for a slit of width $e$ and light of wavelength $\lambda$. The angular position at which secondary maximum is observed is = ?
> 
> 
> 

**My attempt:**
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For first minima, $e\sin\theta=\lambda...(i)$ Unfortunately, formula for maxima is not given in my book so I derived it on my own:

**For deriving second maxima**  

Consider $2N$ wavelets all throughout the slit.  

Path difference between the $i$-th wavelet and $(N+i)$-th wavelet ($i\in [1,N]$) should be $\lambda$ for constructive interference.  

So, if maxima occurs at $\theta'$ angular position, $\frac{e}{2}\sin\theta'=\lambda$ or $e\sin\theta'=2\lambda...(ii)$

Dividing (i) by (ii), we get:

$$\frac{\sin\theta}{\sin\theta'}=\frac{1}{2}$$
$$\Rightarrow \sin\theta'=2\sin\theta=1$$
$$\Rightarrow \theta'=\pi/2$$

which is the wrong answer -\_-

**My question:**
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Please explain my mistake and suggest correct approach.