1. If a lens has a power of +38 m−1, what is its focal length?
2. What is the power of a concave lens of focal length 25 cm?
3. What is the power of a convex lens of focal length 30 cm?
4. A concave lens of focal length 20 cm is placed in combination with a convex lens of focal length 30 cm. What is the resultant power and will it converge or diverge light rays?
5. Three lenses each of power +4 m−1 are placed in contact. What is the focal length of the combination?
6. A combination of two lenses has a focal length of 25 cm. If one of the lenses has a focal length of 50 cm, what is the focal length and power of the other lens?
7. If an eye has a fi xed lens system of +40 m–1 and a variable lens of maximum power +30 m–1, what is the minimum distance an object can be placed in front of the eye and still be in focus, assuming the retina is 1.75 cm from the lens system?
8. If a person has a cornea of power +35 m−1, what is the purpose of wearing goggles underwater to see clearly?
9. In order to see a distant object clearly, a person should have a lens power of +62 m−1. However, a particular person has a minimum power of +67 m−1. What is the focal length of the lens required to correct this vision defi ciency and what type of lens is it? Name the sight defi ciency this person has.
10. Explain how the eye focuses on objects at diff erent distances.
11. Draw a diagram to show how a specifi c type of lens is used to correct short-sightedness.
12. Explain how the cornea and lens work as an optical focusing system.
13. If the normal power of an eye is taken to be +65 m–1 and a woman has a sight problem resulting in her having a maximum power of +58 m–1, what would her sight defect be called and what is the focal length of the lens required to correct this problem?
14. A man goes to the optician and has test lenses placed in front of his eyes. In order to see correctly, he requires two lenses of +4 m–1 and +2.5 m–1 for his left eye and three lenses of +1.75 m–1 each for his right eye. What is the nature of his sight defect and what focal length will be prescribed for each eye to correct this defect?
