u Method (Continued) 7. Plot a graph from the results in the table (make sure that the axes are 1
u (cm) v (cm)
__ u
1
__ v
1 1
__ u and 1
__ u + 1
__ v = 1
__ f
__ v ).
f (cm)
Characteristic graph
1 u
NOTE
When using the graph to find value for f: • where the line cuts the 1 • where the line cuts the 1
__ u axis, 1
__ v axis, 1
__ v = 0. Therefore 1
__ u = 0. Therefore 1
__ u = 1
__ v = 1
__ f (from formula)
__ f .
Average the values for f from both axes to find the final value for f.
1 v
M3.4b Result
• As one value decreases, the other value increases. • The set of 1
__ u and 1
__ v values will add up to an
approximate reciprocal focal length.
Conclusion The focal length of a convex lens is found using the following formula:
__ u + 1
1
__ v = 1
__ f
u = object distance (m), v = image distance (m), f = focal length (m)
Errors
The following errors should be considered when conducting this experiment:
• parallax in measuring u and v • image not being in sharp focus • an unsecured or unsteady screen • the vertical and horizontal cross-threads being at different distances from the light source.
Precautions
• Find the average of your measurements and verify them with your lab partners.
• Double-check if the image is well resolved with your lab partners.
• Attach a hard backing to screen and hold it in a retort stand.
• Make sure the cross-threads are held taut in front of ray box.
Possible exam questions 1.
How would you choose the optimum object distance for a clear, resolved image of the cross-threads?
2. How would you keep the screen steady? 3. Why is v the most inaccurate of the values?
