The UV-Vis spectrum can be used to calculate the band gaps of material ( direct, indirect, forbidden direct, and forbidden indirect band gap), by plotting the graph between (ahv)^(n) on y-axis versus photon energy (hv)on x-axis.

where a: is the optical absorption coefficient, which can be calculated from absorbance(A), and thickness of the sample(t) using: (a=2.303A/t); and (hv) can be calculated form wavelength using: (hv = 1240/wavelength); The power factor (n) takes the values of (1/2, 2, 1.5, and 3 for direct, indirect, forbidden direct and forbidden indirect band gaps respectively .

Then you will extraplot the vertical sagments of the plot to intersect on x-axis where y-axis is zero. That value is your band gap. It might be direct or indirect or forbidden direct or forbidden indirect band gap depending upon the formula

Hope this paper can help you in Section 3.4. We have some details in SD

Article A high-capacity and photo-regenerable composite material for...

Muhammad Zahid

Actually i am confuse about thikness because in my case its powder not a thin film. So with powder form how can i do?