Q3. The far point of a myopic eye is 50 cm in front of the eye. A correcting lens is placed 2 cm in front of the eye. The power needed for clear distant vision is approximately:
Options

Question

Q3. The far point of a myopic eye is 50 cm in front of the eye. A correcting lens is placed 2 cm in front of the eye. The power needed for clear distant vision is approximately:

G1 Physics · Accepted Answer

GIVEN: Eye's far point = 50 cm in front of eye; spectacles 2 cm in front of eye. Step 1: Distance from spectacle lens to eye's far point = 50 − 2 = 48 cm. Note that the 2 cm gap matters. Step 2: Distant parallel light must be made to appear to originate from this far point. Step 3: For an object at infinity, v = f; the required virtual image has v = −48 cm = −0.48 m. Step 4: Power P = 1/f (metres) = 1/(−0.48) = −2.08333… D. FINAL ANSWER: Approximately −2.08 D (concave lens). Option B.