The following errors should be considered when conducting this experiment:
• parallax in measuring real and apparent depth • locating the position of ‘no parallax’ • mirror not being at same height as liquid meniscus • not allowing for the thickness of the container.
Precautions
• Find the average of your measurements and verify them with your lab partners.
2.
• Decide on the position of no parallax by double-checking with your lab partners.
• Verify mirror height with your lab partners. • Measure the internal height of the container before adding liquid.
Possible exam questions 1.
How would impurities in the liquid affect the refractive index calculation?
What measurement has the greatest potential of error?
3.4 Measurement of the focal length of a converging lens Outline of experiment
screen
In this experiment, we will determine the focal length of a convex (converging) lens. The main measurements required are object distance (u) and image distance (v). Once these
distances have been gathered, we can use the formula 1 = 1
The graph of 1 and 1
__ f to calculate focal length.
__ u against 1
__ v can be plotted for each value set
__ f found by averaging where the graph cuts each axis.
Equipment required Convex lens, ray box, cross-threads, screen, ruler u Method
1. Before using the equipment in M3.4a, approximate the focal length of the lens by focusing a distant object onto a screen and measuring image distance (v). This distance will be the approximate focal length (due to distant parallel rays converging at the focal point).
2. Make all distances of u greater than this approximate focal length for the remainder of the experiment (this ensures all images will be real).
3. Set up the equipment as shown in M3.4a, making sure to fix the lens and screen securely. 4. Turn on the ray box and adjust the screen distance until a sharp image of the cross-threads is in focus.
5. Measure u and the corresponding distance v and record them in the table. 6. Repeat for different values of u and v at least four more times.
__ u + 1
__ v
v convex lens ray box u cross-threads M3.4a Measuring the focal length of a convex lens
