Could you clarify what type of spectra is this (Absorption? reflection? Raman?, UV-Visible?), ?   What are the coordinates units?  It is hard to answer your questions without sufficient information. 

I am so sorry. Its UV-Vis absorbance spectra of iron oxide nanoparticles dispersed in water. 

Thanks Abbas for your input. Some paper refer maximum wavelength has to be taken for calculation and some paper refer to take cut off wavelength for calculation. So I am confused..

Here you can find good explanation and band gap calculator,

http://www.instanano.com/characterization/theoretical/uv-vis-spectroscopy-band-gap-calculation/

you can use Kubelka Munk method,

there is some details on my paper:

Article Hydrothermal-assisted sol–gel synthesis of Cd-doped TiO2 nan...

In my opinion the best way is just follow the following link:

https://www.youtube.com/watch?v=KIfaVTfHMZc

Aadhinath W

Tauc used an equation in 1968 to calculate the absorption edges (bandgaps) of amorphous Ge and Si from their absorption data.

(αhν)γ=A(hν-Eg)

In this equation, α is the absorption coefficient, h is Planck’s constant, υ is the frequency of the incident photon, A is a proportionality constant (which is determined by the index of refraction, electron, and hole effective masses; however, it is usually taken as 1 for amorphous materials), and Eg is the bandgap energy. The important term is the exponent γ, which denotes the nature of the electronic transition, that is, when γ=2 it is a direct allowed transition, and when it is equal to 1/2, it is an indirect allowed transition. For γ=2/3 it is a direct forbidden transition, and for γ=1/3 an indirect forbidden transition. Typically, the allowed transitions dominate the basic absorption processes, giving either direct or indirect transitions.

Thus, the basic procedure for a Tauc analysis is to acquire optical absorbance data for a sample in question that spans a range of energies from below the bandgap transition to above it.

Plotting the (αhν)γ versus (hν) is a matter of testing γ=2 or γ=1/2 to compare which provides the better fit and thus identifies the correct transition type.

I have provided the Template file (Origin file) in the video description. Thanks

https://youtu.be/LLESVSsZmlg