Least converging power of eye lens, \(P_{e}=+20 \mathrm{D}\) 

Power of the eye-lens, \(P=P_{c}+P_{e}\)

\(=40 \mathrm{D}+20 \mathrm{D}=60 \mathrm{D}\)

Power of the eye lens

\(P = \frac{1}{{{\text{ Focal length of the eye lens }}(f)}}\)

\(f = \frac{1}{P} = \) \(\frac{1}{{60{\text{D}}}} = \frac{1}{{60}}{\text{m}}\) \( = \frac{{100}}{{60}}{\text{cm}} = \frac{5}{3}{\text{cm}}\)

Distance between the retina and cornea-eye lens

\(=\) Focal length of the eye lens

\(=\frac{5}{3}\, \mathrm{cm}=1.67 \,\mathrm{cm}\)