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Lenses

**1. Explain the following terms:**

(a) radius of curvature of a lens surface

(b) power of a lens

2. Which has the largest radius of curvature – a lens of long focal length or a lens of short focal length?

3. (a) One lens has a power of +20D and another –20D . Explain the difference between the two lenses.

(b) what is the power of a convex lens that has a focal length of 0.1 m

(c) what is the power of a convex lens that has a focal length of 25 cm

(d) what is the focal length of a lens that has a power of +1.5 and what type of lens is it

(e) what is the power of a concave lens that has a focal length of 25 cm

4. Calculate the image position and the magnification for the following:

(a) an object placed 12 cm from a lens of focal length +10 cm

(b) an object placed 25 cm from a lens of focal length +15 cm

(c) an object placed 20 cm from a lens of focal length +40 cm

(d) an object placed 20 cm from a lens of focal length –40 cm

5. Sketch the variation of the separation of the object and image (u+v) against the object distance (u) for a convex lens where v is the image distance.

6. The minimum value of the separation of the object and image (u+v) for a certain convex lens is found to be 60 cm. What is the focal length of the lens?

7. Draw a diagram showing a plane wave before and after passing through a convex lens.

8. Describe the images of a real object formed by:

(a) a convex lens and

(b) a concave lens

Your answer should distinguish between objects at:

(a) a greater distance than 2f

(b) between 2f and f and then

(c) less than f.

9. When a camera is used to take a photograph of a nearby object the lens has to be moved relative to the film. Using carefully drawn diagrams show why this is, and calculate the size and direction of movement when a camera with a lens of focal length 30 mm is used to photograph first an object 100 m away and then one 25 cm away.

10. How do you explain why a camera with a fixed lens can be used satisfactorily for most objects?

(a) radius of curvature of a lens surface

(b) power of a lens

2. Which has the largest radius of curvature – a lens of long focal length or a lens of short focal length?

3. (a) One lens has a power of +20D and another –20D . Explain the difference between the two lenses.

(b) what is the power of a convex lens that has a focal length of 0.1 m

(c) what is the power of a convex lens that has a focal length of 25 cm

(d) what is the focal length of a lens that has a power of +1.5 and what type of lens is it

(e) what is the power of a concave lens that has a focal length of 25 cm

4. Calculate the image position and the magnification for the following:

(a) an object placed 12 cm from a lens of focal length +10 cm

(b) an object placed 25 cm from a lens of focal length +15 cm

(c) an object placed 20 cm from a lens of focal length +40 cm

(d) an object placed 20 cm from a lens of focal length –40 cm

5. Sketch the variation of the separation of the object and image (u+v) against the object distance (u) for a convex lens where v is the image distance.

6. The minimum value of the separation of the object and image (u+v) for a certain convex lens is found to be 60 cm. What is the focal length of the lens?

7. Draw a diagram showing a plane wave before and after passing through a convex lens.

8. Describe the images of a real object formed by:

(a) a convex lens and

(b) a concave lens

Your answer should distinguish between objects at:

(a) a greater distance than 2f

(b) between 2f and f and then

(c) less than f.

9. When a camera is used to take a photograph of a nearby object the lens has to be moved relative to the film. Using carefully drawn diagrams show why this is, and calculate the size and direction of movement when a camera with a lens of focal length 30 mm is used to photograph first an object 100 m away and then one 25 cm away.

10. How do you explain why a camera with a fixed lens can be used satisfactorily for most objects?