Name: _________________________________
Lab Partner: ___________________________
4.6 – Converging Lenses Lab
Optical Bench
Materials:
 meter stick
 2 support legs
 converging lens & holder
 short candle & holder
 screen (white works best)
 long piece of chart paper
 piece of masking tape
 marker
 matches/lighter (share)
Part 1: Find the focal length of your lens
The focal length is where an image will clearly form for an object far away. For this activity, your object will be a
window.
Procedure:
1. Use a window in the room as an object to focus on your screen. Stand far away from the window.
2. Place the lens about half a metre from your screen and move the screen between the lens and window.
Find the point where the light from the window focuses onto the screen (you may need to tilt the lens
and/or screen a little). Your image will be inverted (upside down).
3. Use the meter stick to find the distance from the lens to the screen at that focused point. This is your
focal point, because the light’s rays focus at that point. Record it below.
Focal Length (f) of the lens: …………………………………….
Part 2: Creating Ray diagrams
Procedure:
1.
2.
3.
4.
5.
6.
7.
8.
9.
Lay out the chart paper and draw three 1 m long lines on the paper using your marker, spaced well apart.
Draw one dot 10 cm from the end of each line. Mark it as your optical centre, O.
From that dot measure the distance you calculated as your focal length and mark it as your focal point, F.
From the dot, measure twice the focal length, and mark it as 2F.
Repeat on the opposite side of the lens, but mark F’ and 2F’.
Repeat for all 3 lines.
Label the top line 1st, the middle one 2nd and the one on the bottom 3rd.
Draw a large, converging lens with its centre at O.
For the 1st diagram, draw an object 4 cm high (an arrow) beyond 2F. Draw the 3 rays from the top of the
object as you’ve practiced and find where the image forms. Record its properties below.
Location: ………………………..
Orientation : …………………….
Size: ……………………………..
Type: …………………………….
10. For the 2nd diagram, draw an object 4 cm high (an arrow) between 2F and F. Record do. Draw the 3
rays from the top of the object as you’ve practiced and find where the image forms. Record its properties
below.
Location: ………………………..
Orientation : …………………….
Size: ……………………………..
Type: …………………………….
11. For the 3rd diagram, draw an object 4 cm high (an arrow) between F and the lens (closer to the lens
than F is better). Record do. Draw the 3 rays from the top of the object as you’ve practiced and find
where the image forms. Record its properties below.
Location: ………………………..
Orientation : …………………….
Size: ……………………………..
Type: …………………………….
Part 3: Set up the Optical Bench
Procedure:
1. Place the two support legs on the meter stick to hold it up.
2. Place the converging lens in a holder at the 10 cm line of the meter stick.
3. Stick a small piece of masking tape on the meter stick one focal length away from the lens (you may
need to refer to Part 1 of your lab to find the focal length). Write F on the masking tape.
4. Place another piece of masking tape at twice the focal length from the lens and label it 2F.
5. Repeat steps 3 – 4 on the other side of the lens, but mark F’ and 2F’ on the tape instead.
Part 4: Viewing Reflected Images
Procedure:
1st Ray Diagram  Object beyond 2F
1. Place the optical bench over top of your 1st ray diagram and place the
candle where the object is.
2. Light the candle. Move the screen along the meter stick until you see a
clear image. Ideally it’s where the image formed in the ray diagram you
drew before.
3. Describe the image that forms on the screen (if one forms).
Image Formed
L – ……………………..
O – ……………………..
S – ……………………..
T – ……………………..
2nd Ray Diagram  Object between 2F and F
4. Place the optical bench over top of your 2nd ray diagram and place the
candle where the object is.
5. Light the candle. Move the screen along the meter stick until you see a
clear image. Ideally it’s where the image formed in the ray diagram you
drew before.
6. Describe the image that forms on the screen (if one forms).
3rd Ray Diagram  Object between F and the lens
7. Place the optical bench over top of your 3rd ray diagram and place the
candle where the object is.
8. Light the candle. Move the screen along the meter stick until you see a
clear image. Ideally it’s where the image formed in the ray diagram you
drew before.
9. Describe the image that forms on the screen (if one forms).
Image Formed
L – ……………………..
O – ……………………..
S – ……………………..
T – ……………………..
Image Formed
L – ……………………..
O – ……………………..
S – ……………………..
T – ……………………..
10. Put away all materials from where you got them in the classroom.
Discussion:
1. Did your ray diagrams help you find the image of your candle? If so, which ray diagrams worked best
(1st, 2nd or 3rd)?
2. Where must you place a candle if you want to form an image that’s upright and bigger than the object
(i.e. between which 2 points)?
3. Where must you place a candle if you want to form a smaller image (i.e. between which 2 points)?
4. Between which 2 points does an image form, but it’s virtual (i.e. could not be projected onto your
screen)?
5. Which of the images from the 3 ray diagrams you set up could not be projected onto your screen?