Using lens formula, \(\frac{1}{v}-\frac{1}{u}=\frac{1}{f}\)
or \(\frac{1}{\infty}-\frac{1}{4}=\frac{1}{f}\) or \(f=-4\, \mathrm{m}\)
Lens should be concave.
Power of lens \(=\frac{1}{f}=\frac{1}{-4}=-0.25 \,\mathrm{D}\)
(શૂન્યવકાશમા પ્રકાશનો વેગ $=3 \times 10^{8} \,{m} / {s}$ અને $\left.\cos 30^{\circ}=\frac{\sqrt{3}}{2}\right)$