5. In order to see a distant object clearly, a person should have a lens power of +60 m−1. However, a particular person has a maximum power of +52 m−1. What is the focal length of the lens required to correct this vision deficiency and what type of lens is it?
You may now complete Exercise 15C (page 165). Solution
+60 m−1 − 52 m−1 = 8 m−1 = additional power required for normal vision P = 1
__ f ⇒ f = 1
__ P ⇒ f = 1
__ 8 = +0.125 m
The + sign indicates a convex lens is required, focal length 0.125 m.
CHAPTER 15 EXERCISE 15A
1. A magnifying glass is required to look at a spider from a distance of 15 cm. If a magnifi cation of 3 is desired, what type of lens and what focal length should be used?
2. Draw a ray diagram to show how an image is formed in a convex lens.
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3. An object is placed in front of a 25 cm focal length convex lens. Find the two positions at which the image is twice the size of the object.
4. An image appears three times the size of the object in a convex lens of focal length 40 cm. Find its position if it is (i) real, (ii) virtual.
5. What type of lense and what focal length are required to produce a real image with magnifi cation of 10, when the object is 2.5 m from the lens?
6. A convex lens is placed 15 cm from an object and forms a real image 22.5 cm from a screen. The lens can be moved to form another real image. Determine the object and image distance for this situation and the focal length of the lens being used.
CHAPTER 15 EXERCISE 15B
1. Draw a ray diagram to show how an image is formed in a concave lens.
2. Calculate the position, nature and image produced by an object 25 cm from a concave lens of focal length 20 cm.
3. A 15 cm high object is placed 30 cm away from a diverging lens of focal length 60 cm. Find the position, nature and height of the image produced.
4. An image a quarter the size of the object is formed in a concave lens of focal length 40 cm. Calculate the object and image distance.
