MP Board · Class 10 · Science · The Human Eye and Colourful WorldA person with a myopic eye cannot see objects beyond 1.2 m distinctly. What should be the type and focal length of the corrective lens used to restore proper vision?

Given:

• Far point of the myopic person, $v = -1.2\text{ m} = -120\text{ cm}$
• Object distance for the far point, $u = -\infty$
• Focal length of the lens, $f = ?$ \nUsing the lens formula: $$\frac{1}{f} = \frac{1}{v} - \frac{1}{u}$$ \nSubstitute the given values into the formula: $$\frac{1}{f} = \frac{1}{-1.2} - \frac{1}{-\infty}$|\nSince $\frac{1}{-\infty} = 0$: $$\frac{1}{f} = \frac{1}{-1.2}$$ $$f = -1.2\text{ m} = -120\text{ cm}$$ \nPower of the lens ($P$): $$P = \frac{1}{f\text{ (in meters)}} = \frac{1}{-1.2} = -0.83\text{ D}$| \nConclusion:\nThe person is suffering from myopia (near-sightedness) and requires a concave lens of focal length $-1.2\text{ m}$ (or $-120\text{ cm}$) and power $-0.83\text{ D}$ to correct this vision defect.