Power of accommodation is the ability of the eye lens to adjust its focal length so that objects at different distances are focused clearly on the retina.
A concave, or diverging, lens should be used. It diverges light from distant objects so that the image is formed on the retina of the myopic eye.
For a normal human eye, the far point is infinity and the near point is about 25 cm from the eye.
The student is likely suffering from myopia, or near-sightedness, in which distant objects are not seen clearly. It can be corrected using a concave lens of suitable power.
- a. presbyopia.
- b. accommodation.
- c. near-sightedness.
- d. far-sightedness.
The ability to adjust focal length is called accommodation.
(b) accommodation.
- a. cornea.
- b. iris.
- c. pupil.
- d. retina.
The eye lens focuses the image on the retina.
(d) retina.
- a. 25 m.
- b. 2.5 cm.
- c. 25 cm.
- d. 2.5 m.
The normal near point for a young adult is about 25 cm.
(c) 25 cm.
- a. pupil.
- b. retina.
- c. ciliary muscles.
- d. iris.
Ciliary muscles change the curvature and focal length of the eye lens.
(c) ciliary muscles.
Use f = 1/P, where f is in metres. For P = -5.5 D, f = 1/(-5.5) = -0.182 m. For P = +1.5 D, f = 1/1.5 = +0.667 m.
(i) For distant vision, f = -0.182 m = -18.2 cm.
(ii) For near vision, f = +0.667 m = +66.7 cm.
For a distant object, the correcting lens should form a virtual image at the far point, 80 cm in front of the eye. Thus f = -80 cm = -0.80 m. Power P = 1/f = 1/(-0.80) = -1.25 D.
A concave lens of power -1.25 D is required.
For the correcting lens, u = -25 cm and v = -100 cm. Lens formula: 1/f = 1/v - 1/u = -1/100 - (-1/25) = 3/100 cm⁻¹. Therefore f = 100/3 cm = 33.3 cm = 0.333 m. Power P = 1/f = +3.0 D.
A convex lens of power +3.0 D is required. In the diagram, the convex lens forms a virtual image of an object at 25 cm at the person's near point of 1 m, so the eye can focus it.
For objects closer than 25 cm, the ciliary muscles cannot make the eye lens sufficiently thick to reduce its focal length enough. The image is not focused sharply on the retina, so the object appears blurred.
The image distance remains nearly constant because the image must form on the retina. The eye changes the focal length of its lens to focus objects at different distances on the retina.
Stars twinkle because their light is refracted by layers of the earth's atmosphere with changing density. This atmospheric refraction causes the apparent position and intensity of starlight to fluctuate.
Planets are much closer than stars and appear as extended sources of light. The fluctuations from different points of the planet average out, so their brightness appears steady and they do not twinkle noticeably.
The sky appears blue on earth because air molecules scatter sunlight. In space there is almost no atmosphere to scatter sunlight, so the sky appears dark to an astronaut.