Lenses and Optical Instruments

Lenses and Optical Instruments

Introduction
In chapter 2, we looked at refraction of light at a plane boundary between media of different optical densities. Have you ever asked yourself what would happen if the refractive boundary between media is curved rather than a plane surface? These curved transparent glass materials are called lenses and have a valuable property of forming images of objects situated in front of them. In this chapter, you will understand that lenses refract light to form images and these lenses can be applied in different optical instruments.

3.1: Lenses
You always see people wearing on spectacles. What is the nature of the glasses used? Where else are such glasses applied?

3.1.1: Types of Lenses
Activity 3.1 Understanding the types of lenses
Key question: Describe the features of lenses What you need: Two different lenses, ICT resource.
Initial question: What is a transparent material?
What to do (In groups):
1.Hold and touch the surfaces of the lenses provided.
2.Use them to look at different objects.
3.Identify the features on each of the lenses.
4.Draw sketches to represent the shapes of lenses provided.

Conclusion and Application

Suggest the differences between the two lenses.
Discuss the nature of the images formed by the lenses
Which of the lenses is convex or concave? Explain.
Mention some of the applications of lenses.

Investigating the path followed by light through lenses
Activity 3.2 Key question: Describe how light behaves when it passes through lenses.
What you need: notebooks and pens, ICT resource.
Initial question: What is meant by a light beam?

What to do (In groups):

1. Analyse the pictures shown in Figure 3.1. (a) and (b).
2. Identify the lenses shown in Figure 3.1 (a) and (b).
   ! Conclusion and Application
   3.Identify the difference in the way light passes through the lenses.
   4.Which of the two lenses is a converging or diverging lens. State why?
   5.Explain the behaviour of light as it passes through the lenses.
   State any two applications of
   (a) Converging lenses.
   (b) Diverging lenses.

3.1.3: Terms used in lenses
Activity 3.3 Exploring the terms used in lenses
Key question: Explain the terms used in lenses.
What you need: ICT resource, reference text books.
Initial question: What is a light ray?
What to do (In groups);

1. Research about the following;
   (a) centre of curvature
   (b) principal focus
   (c) focal length
   Conclusion and Application
   (d) optical centre
   (g) focal plane
   (e) principal axis
   (h) aperture
   (f) radius of curvature (i) paraxial rays
   2.Write down symbols which represent the terms.
   3.State the meaning of each of the terms used in lenses.
   4.How many principal foci do lenses have?
   5.What is the difference between the principal focus of a convex lens and concave lens?

6. Draw ray diagrams and on them show the terms in step (1
7. Discuss the relationship between the radius of curvature and the focal length.
   3.1.4: Formation of Images by Lenses
   Locating and describing images formed by lenses
   Activity 3.4
   Key question: Describe the nature of images formed by lenses.
   What you need: ICT resource, Note book, pencil, ruler, graph book, relevant
   text book.
   Initial question: Differentiate between a real and virtual image.
   What to do (In groups);
8. Using or textbook, make research about;
   (a) Principal rays used in location of images in lenses.
   (b) Formation of images by lenses.
   Conclusion and Application
9. State the principal rays used to locate images in lenses.
10. Use the principal rays to determine the position and nature of images formed by the lenses when the object is placed;
    (a)At infinity
    (b)Beyond the centre of curvature.
    (c)At the centre of curvature.
    (d)Between the centre of curvature and principal focus.
    (e)At the principal focus.
    (f)Between the principal focus and the optical centre.

Self Check

1. A convex lens has a focal length of 12 cm and a real object 6 cm tall is placed 18 cm from the centre of the lens. By means of an accurate scale diagram, find the position, size and nature of the image formed.
2. A concave lens has a focal length of 2 cm and real object 1.0 cm tall is placed at 3 cm from the centre of the lens. By means of an accurate scale diagram, find the position, size and the nature of the image formed.
3. An object 2 cm high is placed 12 cm away from a convex lens of focal length 6 cm. By using an accurate drawing on graph paper, find the position, height and type of image.

3.1.5: Focal length of a lens
It is possible to determine the focal length of a lens. Why do you think this is important?
Activity 3.5 Determining the focal length of a convex lens
Part (a): Distant object method
Key question: Determine the focal length of a convex lens.
You will need: Convex lens lens holder, window, screen, metre rule, table
Initial question: What is focal length?
What to do (In groups):

1. Use the convex lens to focus an image of a distant object on the screen.
2. State the characteristics of the image formed?
3. Determine the focal length of the convex lens.
4. Using ray diagrams, show how the image is formed.
   Part (b): Illuminated object method
   What you need: ICT resource, reference textbooks, Convex lens, Lens holder,
   plane mirror, wire gauze, metre rule, source of light and a white screen.
   What to do (In groups);
5. Use the ICT resource or textbooks to find out how the focal length of a convex lens is determined using an illuminated object.
6. Using the available materials, plan and carry out the experiment by organis ing the set up as shown in Figure 3.2.

3.Record your results in a suitable table including (U+ V) and (UV)
4.Plot a graph of (UV) against (IJ+ V)
5.Determine the slope of the graph.
6.Compare and discuss the two values obtained in part (a) and (b).
7.Outline the importances of determining focal length of lenses.
The distance between the image and the convex lens obtained in the activity 3.5(a) is known as the focal length.

3.2: Optical instruments
Very many devices have been developed to enable us see distant or very tiny objects. For instance, which instrument would you use to see bacteria in a blood sample or view stars in the sky? What do these instruments use that makes them do such work?
3.2.1: Magnifying glass (Simple Microscope) A magnifying glass (Figure 3.3) is a very common instrument in everyday life. Where have you ever seen it? What is it used for? Have you ever wondered how it works?
Carry out activity 3.6 and learn more about magnifying glasses.
Figure. 3.3 Magnifying glass

Investigating how a magnifying glass works
Activity 3.6
Key question: Describe how magnifying glasses work
What you need: magnifying glass, hair on your head, newspaper
Initial question: What type of lens is used in a magnifying glass?
What to do (In groups);

1. Hold and touch the lens of the magnifying glass.
2. Use the magnifying glass to look at the hair, and letters on the newspaper.
   Conclusion and Application
3. Which type of lens is used in magnifying glasses?

4. Explain how a magnifying glass forms magnified images ?
5. Sketch a ray diagram to show the formation of the image.
6. Suggest uses of magnifying glasses in everyday life.
   3.2.2: The Lens Camera
   Whenever you are to go for outings, parties or any other event, one of the things to put
   in mind are the photographs to capture . You already know that you will need a lens camera as in Figure 3.4 to take the photographs. How are these cameras able to take magnificent photographs?

Activity 3.7
Investigating the lens camera
Key question: Describe the working principle of the lens camera?
What you need: ICT resource, relevant text books, lens camera (if available)
Initial question: What is a camera?
What to do: (In groups);

1. Make research to know about the lens camera.
2. Use the camera if available to take pictures around class.
   Conclusion and Application
   3.What are the essential parts of a lens camera?
   4.State the nature of images formed by a camera.
   5.Mention the functions of the different parts of the camera.
   6.Using ray diagrams, describe how the camera forms images.
   7.Mention the uses of cameras in everyday life.

3.2.3: The Human Eye
We always use our eyes to see different objects around us. How is the eye able to see such objects?

Activity 3.8 Describing the structure and function of the parts of the human eye
Key question: Describe the working principle of a human eye?
What you need: ICT resource, relevant text book.
Initial question: What is meant by light?
What to do: (In groups);

1. Look at your friend’s eye and critically observe its external shape.
2. Let the friend look at different objects in class as you observe his or her eye.
3. Brainstorm about the internal parts of the eye.
   Conclusion and Application
4. Sketch a diagram of the eye and name its external parts.
5. How does the eye behave as one tries to see objects.
6. Figure 3.5 shows the structure of the human eye. Label and give the functions of the parts A to I.

7.Describe how the human eye forms images.
8.HOW is the lens camera similar to the human eye?
9.State the differences and similarities between the lens camera and the human eye
Eye Defects
Like any other organ of the body, eyes are also liable to different problems which affect their vision. For instance what do you think happens to your eye sight as you grow older? In activity 3.9, you will explore more about eye defects.

Activity 3.9 Describing the eye defects

Key question: Describe how eye defects are corrected?
What you need: ICT resource, Note book and pen
Initial question: What are the types of eye defects?
What to do (In groups):

1. Analyse the situation of two students (John and Shamim) in the paragraph below and identify the problems with their eyes. in senior three class, there are two students with the following eye problems; John can see clearly on the chalkboard when he sits on the, : front desk, whereas Shamim can only see clearly the words written on the chalkboard when she sits at the back.
2. Share with the rest of the class.
3. Suggest the causes of the problems.
4. Giving reasons. Which of the diagrams in Figure 3.6 describes the conditions of the eyes of each student.

5. Using ray diagrams describe how each of the eye defects in part (4) can be corrected.
   ±.2.5: Telescopes
   Although our eyes can see a number of objects, they have certain limits in which they can see the features of objects clearly especially when objects are very far. Think about how difficult it is to distinctly view objects such as stars! In such cases you will need a telescope (Figure 3.7).

Activity 3.10 Finding out the types and uses of telescopes
Key question: Explain the types of a telescope?
What you need: Internet resource, relevant textbooks Initial question: What is a telescope?
What to do (In groups):

1. Using the internet or text books research about the telescope.
   Concluston and Application
2. How many lenses are used in telescopes?
3. Mention the types of telescopes.
4. Suggest the uses of telescopes

Chapter 3 Summary
In this chapter you have learnt that:
There are two types of lenses which include; convex and concave lenses.
The concave lens is a diverging lens and a convex lens is a converging lens.
The terms applied in lenses include: Principal focus, centre of curvature, radius of curvature and many others.

How to locate images formed by lenses.

The different methods used to determine the focal length of a lens that include the distant object and the illuminated object methods.
The different optical instruments such as the magnifying glass, lens camera, human eye, and the telescope.

How the different optical instruments work and their uses in everyday life.
The difference and similarities between the human eye and the lens camera.