A student measures the focal length of a concave (diverging) lens by placing an…
PHYSICS · Multi-part · CBSE · Physics · Light — Reflection and Refraction · Light — Reflection and Refraction
Problem
A student measures the focal length of a concave (diverging) lens by placing an illuminated object in front of the lens and locating the apparent position of the virtual image using a parallax "no-parallax" method. By convention, virtual image distances are negative. The measured data are provided below.
Trial
so (cm)
si (cm)
1
30.0
−20.0
2
35.0
−23.0
3
40.0
−27.0
4
50.0
−34.0
The manufacturer marks the lens as having a focal length of −80cm.
(a) For each trial, calculate the focal length of the lens, being careful with sign conventions.
(b) Determine the average focal length obtained from all four trials and calculate its percent difference from the manufacturer’s value.
(c) Discuss whether the variability among the four focal-length values is primarily due to random error, systematic error, or a combination of both. Support your answer with quantitative or qualitative reasoning based on the data.
(d) Suppose another student performing the same experiment forgot to assign negative signs to the virtual image distances. Explain qualitatively how this mistake would affect the calculated focal lengths and the inferred lens type.