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A student carried out an experiment to measure $f$, the focal length of a converging lens - Leaving Cert Physics - Question 3 - 2021
Question 3
A student carried out an experiment to measure $f$, the focal length of a converging lens. (i) Draw a labelled diagram of the apparatus used in this experiment. (i... show full transcript
Worked Solution & Example Answer:A student carried out an experiment to measure $f$, the focal length of a converging lens - Leaving Cert Physics - Question 3 - 2021
Step 1
(i) Draw a labelled diagram of the apparatus used in this experiment.
Answer
To draw a labelled diagram of the apparatus used in this experiment, include the following components:
- Object: A crosswire or a ray box.
- Converging Lens: Clearly labeled with appropriate geometric features.
- Screen: Where the image will be formed.
- Ray Diagrams: Show at least two rays from the object through the lens to the screen.
Step 2
(ii) On your diagram, indicate and label the object distance u and the image distance v correctly.
Answer
On the diagram, label the object distance as which is the distance from the object to the lens. Label the image distance as which is the distance from the lens to the image formed on the screen.
Step 3
Step 4
(iv) How did the student know that the correct image distance had been found?
Answer
The student knew the correct image distance had been found by observing that the image formed was in sharp focus when viewing through the lens and adjusting the distance on the measuring instrument until a clear image was acquired.
Step 5
Step 6
(vi) The student placed an object 16 cm in front of the converging lens. It produced an image at a distance of 48 cm from the lens. Calculate the focal length of this lens.
Answer
To calculate the focal length , use the previously stated formula: Given:
- Substituting into the formula: rac{1}{f} = rac{1}{16} + rac{1}{48} To combine, find a common denominator (48): rac{1}{f} = rac{3}{48} + rac{1}{48} = rac{4}{48} = rac{1}{12} Thus, .
Step 7
(vii) Why will this experiment not work if the object is placed very close to the lens?
Answer
If the object is placed very close to the lens, typically less than the focal length, the rays of light will diverge. This results in a virtual image that cannot be projected onto the screen, making it impossible to obtain a visible image.