Q.38 If an optician prescribes a corrective lens of power -2.0 D, the required lens
(A) is a concave lens.
(B) is a convex lens.
(C) has a focal length of +50 cm.
(D) has a focal length of -50 cm.
Lens Power Basics
Lens power P measures the ability to converge or diverge light, calculated as P = 1/f where f is focal length in meters. The unit diopter (D) applies when f is in meters; negative power indicates a diverging (concave) lens for distant objects.
Option Analysis
- (A) Concave lens: Correct. Negative power (-2.0 D) signifies a diverging concave lens, used for myopia where distant images focus in front of the retina.
- (B) Convex lens: Incorrect. Convex lenses have positive power and converge light for hyperopia, not myopia.
- (C) +50 cm focal length: Incorrect. Positive f implies converging lens; calculation yields negative value.
- (D) -50 cm focal length: Correct. f = 1/P = 1/-2.0 = -0.5 m = -50 cm, confirming diverging nature.
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Introduction to Corrective Lens Power -2.0 D
When an optician prescribes a corrective lens power -2.0 D, it addresses myopia (nearsightedness) using specific lens properties. This power indicates a diverging lens that shifts focus for clear distant vision. Understanding the formula P = 1/f reveals the lens type and focal length essential for exams and eye care.
Why Concave Lens for -2.0 D Power
A negative power like -2.0 D corresponds to a concave lens, which diverges parallel rays from infinity. This corrects myopia by forming virtual images at the eye’s far point. Convex lenses (+ power) suit farsightedness instead.
Focal Length Calculation
Convert power to focal length: f = 1/-2.0 = -0.5 m or -50 cm. The negative sign confirms divergence; +50 cm would indicate convergence. Always use meters for diopters.
| Option | Description | Correct? | Reason |
|---|---|---|---|
| A | Concave lens | Yes | Negative power = diverging |
| B | Convex lens | No | Requires positive power |
| C | +50 cm focal length | No | Positive f = converging |
| D | -50 cm focal length | Yes | Matches f = 1/P |
