Lenses
Chapter 7: Lenses
Chapter tests
See all →Top questions
See all →- The SI unit of the power of a lens is Answer: B. dioptre Power P = 1/f with f in metres; its unit is the dioptre (D). 1
- Myopia (near-sightedness) is corrected by using a Answer: B. concave lens In myopia the image forms in front of the retina; a concave lens of suitable power moves it back onto the retina. 1
- The image formed on the retina of the human eye is Answer: A. real and inverted The eye lens is convex and forms a real, inverted image on the retina; the brain interprets it as erect. 1
- The power of a convex lens of focal length 50 cm is Answer: D. +2 D P = 1/f = 1/0.5 m = +2 D. 1
- A lens has power −2.5 D. It is a Answer: C. concave lens of focal length 40 cm f = 1/P = 1/-2.5 = -0.4 m = -40 cm. A negative power means a concave lens. 1
- The impression of an image persists on the retina for about Answer: D. 1/16 second This persistence of vision lasts about 1/16 s. 1