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The far point of a myopic person is 80 cm in front of the eye. Assuming the far point for normal vision is infinity, what is the required power (P) of the corrective lens?
+0.80 D
+1.25 D
0.80 D
1.25 D
25 D
To correct myopia, a concave lens is required to diverge incoming light so that distant objects appear to be at the person's far point. Since the far point is 80 cm (0.8 m), the lens must form a virtual image of an object at infinity at this distance, resulting in a power of -1.25 D.
To correct myopia, a concave lens is required to diverge incoming light so that distant objects appear to be at the person's far point. Since the far point is 80 cm (0.8 m), the lens must form a virtual image of an object at infinity at this distance, resulting in a power of -1.25 D.
Think of a concave lens like a wide-angle lens for the eye; it spreads out the 'crowded' light rays coming from a distance so the myopic eye can focus them properly on the retina.
Mnemonic: 'Myopic Needs Concave' (MNC) - Myopia requires a Negative Concave lens.
f1тАЛ=v1тАЛтИТu1тАЛ тАФ The Lens Formula relating focal length, object distance, and image distance.
P=f(m)1тАЛ тАФ Definition of optical power in Diopters (D).
The lens formula is given by f1тАЛ=v1тАЛтИТu1тАЛ. For a myopic person, an object at infinity (u=тИЮ) must be imaged at the far point v=тИТ80 cm = -0.8 m. Using the sign convention, the focal length is f=тИТ0.8 m, and power is calculated as P=f1тАЛ.
Myopia (nearsightedness) occurs when the eyeball is too long or the lens is too curved.
A negative power indicates a diverging (concave) lens.
Power is always calculated with focal length in meters.
Restores clear distance vision.
Non-invasive solution compared to surgery.
Requires constant use of eyewear.
Peripheral vision may be slightly distorted by thick lens edges.
Spectacles for nearsighted individuals.
Contact lenses for myopia correction.
The focal length f=тИТ0.8 m gives P=тИТ0.81тАЛ=тИТ1.25 D.
Option A is positive, indicating a converging lens (used for hypermetropia). Option B is positive, and Option C is the wrong magnitude.
D is correct тАФ The required power is calculated using the lens formula P=f1тАЛ, yielding a value of -1.25 D.
Always convert cm to meters before calculating power, as the unit 'Diopter' is defined per meter (mтИТ1).
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