Do question 70 only. Typed preferred. Thanks.
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For perpendicular incidences of light:
For mth dark fringe,
2*n*t = (m-1)*lembda .....(1)
At the separated edge (separation = t) of ...
This solution is provided in 125 words. It discusses incidences of light and dark fringes, as well as provides calculations for dark fringe and how thickness affects this.