$\left({h}=6.63 \times 10^{-34}\, {J} \cdots\right.$, $\left.{c}=3 \times 10^{8} \,{ms}^{-1}\right)$
\(\Rightarrow {eV}_{{S}}=\frac{1240}{280}-2.5=1.93\, {eV}\)
\(\rightarrow {V}_{{S}_{1}}=1.93\, {V} \ldots\) (i)
\(\rightarrow {eV}_{{S}_{2}}=\frac{1240}{400}-2.5=0.6 \,{eV}\)
\(\Rightarrow {V}_{{S}_{2}}=0.6 \,{V} \ldots\) (ii)
\(\Delta {V}={V}_{{S}_{1}}-{V}_{{S}_{2}}=1.93-0.6=1.33\, {V}\)