Background and Objectives 

:

For the topic of  “Images Formed By Lenses”, a number of laboratory experiments/activities are recommended for the Form Five Science syllabus. Six of them are as below:

1.       Investigate the characteristics of images formed by biconvex lenses.

2.       Obtain the approximate focal length of a biconvex lens.

3.       Determine the focal length of a biconvex lens.

(a)  using an illuminated object and plane mirror.

            (b) using measurement of object and image distances.

4.       Determine the relationship between the object distance, image distance and focal length of a biconvex lens.

5.       Build a compound microscope.

6.       Build a simple astronomical/terrestrial telescope.

The conventional set-ups for these experiments have the following problems:

• The need for a dark room that some schools do not have.

• Images formed are not in line with the lens and the object, increasing errors in data collection and difficulty in finding images.

• Images formed are not sharp because light does not only pass through near the optical centre of the lens and thus increases error.

• The lens is moved instead of the object moving. This loses the sense of the relative change of image distance with change in object distance.

• Difficult to obtain satisfactory enlarged images for the compound microscope and telescope. 

The conventional set-ups for these experiments/activities are as below:

1.   To investigate the characteristics of images formed by biconvex lenses

3b. To determine the focal length of a biconvex lens using measurement

      of object and image distances [Refer point 3(b) above]

4.   To determine the relationship between the object distance, image distance and focal length of a biconvex lens.

 (Refer 5-D1)

2.   To obtain the approximate focal length of a biconvex lens.

(Refer 5-D2)

3a. To determine the focal length of a biconvex lens using an illuminated object and plane mirror.

 (Refer 5-D3)

5.   To build a compound microscope

(Refer 5-D4)

6.   To build a simple astronomical/ terrestrial telescope

(Refer 5-D5)

Problems arising from these set-ups result in the experiments either not done or students not getting accurate data. Much time is also lost trying to locate images.

Activities involving the compound microscope and telescope are almost not done as it is very difficult to obtain satisfactory enlarged images with these set-ups. As such students only go through the theory without the opportunity to actually try their hand in building these optical instruments and seeing the effect for themselves.

The Lens Kit eliminates all of the limitations above.

Benefits for teaching and learning process

:

The Lens Kit :

·        does not need a dark room.

·        uses minimal apparatus that are cheap and easily available.

·        has ruler on tube that reduces parallax error when reading the object and image distances.

In addition :

·        Object, lens and image are ensured to be in line to reduce errors and increase ease of obtaining images.

·        Images formed are sharp because only light near optical centre of the lens is allowed to pass through.

·        The object moves instead of the lens, thereby retaining the relative sense of  movement of the image with the change in object distances.

·        Images for the compound microscope and telescope are easily obtained, also very sharp and clear.

It is hoped that with this kit, teachers and students will have more opportunities to explore images formed and apply their theoretical knowledge to build for themselves functional microscopes and telescopes easily. This will also allow more time to discuss and analyze data obtained instead of spending all the time trying to get data.

Apparatus/ materials

:

1. Two PVC pipes a meter long each with slits of about 1 inch (or 2.54 cm) along each pipe with connectors.  

2. A meter rule is placed along each slit.  

3. Cardboards tubes from the core of toilet rolls or tapes as holders for lenses, screens, mirrors, objects etc.  

4. A torchlight as light source  

5. Biconvex lenses ( f = 5 cm, 10 cm, 15 cm, 30 cm, 50 cm)  

6. Battery (size D)  

Construction of teaching aids

:

The components :  

1. The tubes, connectors and meter rule (Refer 5-D6)  

2. Paper tube holder (Refer 5-D7)  

3. Cardboard screen, lens holders, mirror holders, objects (Refer 5-D8)  

4. Light source (Refer 5-D9 and 5-D10)  

The set-up :

1. Place the object/screen/lens/mirror onto the tube holder. Shown here, the screen is placed onto the tube holder. (Refer 5-D11)

2. Place the torchlight into the holder for the light source. (Refer 5-D12)

3. Connect the tubes, if necessary using the connector. (Refer 5-D13)

4. Slide the holders into the tube. Place the object, lens, screen, light source etc in positions as required. An example is given in the diagram. (Refer 5-D14)

The Lens Kit set-ups for experiments/activities are as below :

1. To investigate the characteristics of images formed by biconvex lenses.

3b. To determine the focal length of a biconvex lens using measurement of object and image distances.

4. To determine the relationship between the object distance, image distance and focal length of a biconvex lens. (Refer 5-D14)

Procedure :

i.  Slide in the light source, big object, lens and screen as shown in the above diagram.

ii. Make sure that the lens is at position zero centimetre on the meter rule.

iii. Change the object distance by sliding the object together with the light source.

iv.  Slide the screen until a sharp image is obtained.

v.  Read the image distance from the meter rule.

2. To obtain the approximate focal length of a biconvex lens. (Refer 5-D15)

Procedure :

i.   Slide in the lens and the screen.

ii.  Direct the tube to a bright distant object outside.

iii. Slide the screen to obtain a clear sharp image.

iv. The distance between the lens and screen is the approximate focal length of the lens.

3a.  To determine the focal length of a biconvex lens using an

        illuminated object and plane mirror. (Refer 5-D16)

Procedure :

i. Slide in the light source, small object, lens and mirror as shown in the above diagram.

ii.  Slide the lens and mirror together until the image of the small object is sharp and clear beside the object.

iii. The distance between the small object and the lens is the focal length of the lens.

5.  To build a compound microscope. (Refer 5-D17)

Procedure :

i.  Slide in the screen, small object, objective and eyepiece into the tube.

ii.  Adjust the position of the objective and eyepiece to get a clear sharp image.

Note:

Small object can be replaced with onion cells from fresh onions.

The opaque and transparent screen is optional. It is used to detect the position of the image formed by the objective.

6. (a)  To build a simple astronomical telescope. (Refer 5-D18)

Procedure :

i.   Slide in the objective and the eyepiece.

ii.  Direct the tube to a bright distant object.

iii.  Slide the eyepiece until a sharp, clear inverted image is seen.

Note:

The opaque and transparent screen is optional. It is used to detect the position of the image formed by the objective.

6. (b)  To build a simple terrestrial telescope. (Refer 5-D19)

Procedure :

i.   Slide in the objective, erecting lens and the eyepiece.

ii.  Direct the tube to a bright distant object.

iii. Slide the erecting lens and eyepiece until a sharp, clear upright image is seen.

Note:

The opaque and transparent screen is optional. It is used to detect the position of the images formed by the objective and erecting lens.

Implementation of the teaching

:

1. To investigate the characteristics of images formed by biconvex lenses.

Place the object, lens and screen in the tube. Fix the lens at the centre of the two connected tubes. Use the torchlight as the light source for the object. Images formed should be bright and sharp in normal lighting in the lab. If there is a directional light from one side of the lab, turn the tube so that the slit is against the light. Images will also be easily obtained as students slide the screen in the tube. Have students change the object distance and observe the characteristics of the images formed. Students can record their observations and discussions made based on their observation.

2.  To obtain the approximate focal length of a biconvex lens.

Place the lens and screen into the tube. Direct the tube towards a bright object a distance away. Move the screen until a sharp image is seen. The distance between the lens and the screen is the approximate focal length of the lens.

3a. To determine the focal length of a biconvex lens using an illuminated object and plane mirror

Place object with light source, lens and mirror into the tube. Move the object or lens until an image is formed next to the object. This is a simple experiment. The difficulty in the conventional set-up is the alignment of the different components. Usually one or more of the components is not aligned causing difficulty in finding the image. The alignment using the tube is so good that sometimes the image is formed on top of the object. In that instance, the mirror may need to be adjusted a little to see the image. Students can be asked to explain why this arrangement can be used to determine the focal length of the lens.

4.   To determine the relationship between the object distance, image distance and focal length of a biconvex lens.

Place the object, lens and screen in the tube. The object distance is changed and the image distance is obtained. An experiment can be done here and graphs drawn can be used to determine a formula for the relationship between the object distance, image distance and focal length of the lens. In the tube, sharp images can be obtained just by sliding along the tube. Images are still clear and sharp more than 1 meter from the object. Data obtained are accurate, thereby allowing better interpretation of the graphs and determining the lens formula.

5.  To build a compound microscope.

6   To build a simple astronomical/terrestrial telescope.

A simple project can be done here. Students are to build the compound microscope and the telescope using materials given or requested. This allows students to use all the theory that they know of lenses and  images and apply it here. One of the things that students can see here is the image of the first lens that will become the object for the second lens using the opaque and transparent screen. Most students find it difficult to imagine that an image for one lens becoming the object for the second lens.

Suggestion for modification

: