Convex Lens Viva Questions – Finding Focal Length
Convex Lens Viva Questions – Finding Focal Length using Graphs
To find the focal length of a convex lens by plotting graphs between u and v or between 1/u and 1/v
A. Basic Understanding
Q1. What is a convex lens?
Answer: A convex lens is thicker at the centre; therefore, it converges parallel light rays.
Q2. What type of images can a convex lens form?
Answer: A convex lens forms real as well as virtual images; hence, it has wide applications.
Q3. What is the principal axis of a lens?
Answer: The principal axis passes through the optical centre; therefore, it acts as the reference line.
B. Conceptual Questions
Q4. What do you mean by focal length of a convex lens?
Answer: The focal length is the distance between the optical centre and the principal focus; thus, it measures lens power.
Q5. Why does a convex lens converge light rays?
Answer: Because refraction bends rays toward the normal, they meet at a point; hence, convergence occurs.
Q6. How does image distance change when object distance decreases?
Answer: As the object moves closer, the image distance increases; therefore, the image shifts farther away.
C. Formula and Graph-Based Questions
Q7. State the lens formula.
Answer: The lens formula is 1/f = 1/v + 1/u; therefore, it connects u, v, and f.
Q8. Which graph gives focal length directly?
Answer: The graph between 1/u and 1/v gives focal length directly; hence, it offers better accuracy.
Q9. Why is the u–v graph curved?
Answer: Because u and v vary nonlinearly, the graph bends; therefore, it does not form a straight line.
D. Image Formation
Q10. Where does the image form when the object lies beyond 2F?
Answer: The image forms between F and 2F; consequently, it appears diminished.
Q11. What is the nature of image formed when the object lies at infinity?
Answer: The image forms at the focus; thus, it is real and highly diminished.
E. Accuracy and Observation
Q12. Why should readings be taken for different object positions?
Answer: Different readings reduce random error; therefore, accuracy improves.
Q13. Why does the 1/u–1/v graph give better results?
Answer: Because the graph is linear, it reduces graphical error; hence, precision increases.
F. Errors and Precautions
Q14. What error arises due to improper alignment?
Answer: Improper alignment changes image position; therefore, distance values become incorrect.
Q15. Why must parallax be avoided?
Answer: Parallax shifts the apparent reading; hence, it causes measurement error.
G. Applications
Q16. Where are convex lenses commonly used?
Answer: Convex lenses are used in cameras and microscopes; therefore, they help focus light.
Q17. Why do convex lenses act as magnifying glasses?
Answer: Because they form enlarged virtual images; consequently, objects appear bigger.
Final Understanding
Q18. What key relationship does this experiment verify?
Answer: It verifies the lens formula; thus, it helps determine focal length accurately.
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