To Determine Focal Length of Concave Lens (Class 10 Physics Practical)
To determine the focal length of a concave lens using convex lens and lens formula
TL;DR: To determine the focal length of a concave lens using convex lens and lens formula
Written & reviewed by the Syllab.in Academic Team (CBSE/NCERT subject experts) · Updated
Aim: To determine the focal length of a concave lens using convex lens and lens formula
Materials Required
- Concave lens
- Convex lens
- Optical bench
- Lens holders
- Object (arrow)
- Screen
- Measuring scale
- Light source
Theory
Concave lens is diverging lens that cannot form real image. To find focal length, concave lens is combined with convex lens to form a converging system. The combined focal length is found experimentally, then concave focal length is calculated using formula: 1/F = 1/f1 + 1/f2.
Procedure
- Place convex lens and concave lens together in lens holder
- Arrange object at distance u from lens combination
- Adjust screen until sharp image is formed
- Record object distance u and image distance v
- Repeat for 5-6 different object positions
- Calculate combined focal length F using lens formula: 1/F = 1/u + 1/v
- Find focal length of concave lens using: 1/fc = 1/F - 1/fconvex
- Use known focal length of convex lens for calculation
Observation
Sharp real images formed for all positions. Combined lens acts as converging system. Image size changes with object position following lens laws.
Result
Focal length of concave lens successfully determined. Negative value confirms diverging nature of concave lens.
Precautions
- Ensure convex lens focal length is known accurately
- Position lenses close together at optical bench
- Use sharp light source for clear image
- Maintain horizontal optical bench for accuracy
- Measure distances carefully from marked positions
Viva Questions
Why is concave lens combined with convex lens in this experiment?
Concave lens alone cannot form real image on screen. Combining with convex lens makes the system converging and capable of forming real image.
What is the sign convention for focal length?
Convex lens has positive focal length, concave lens has negative focal length. This convention is used in lens formula calculations.
What does negative focal length indicate?
Negative focal length indicates diverging lens. Parallel rays appear to diverge from focal point after passing through diverging lens.
How to find focal length of concave lens mathematically?
Using formula 1/fc = 1/F - 1/fconvex, where F is combined focal length found experimentally and fconvex is known focal length of convex lens.
What types of images does concave lens form?
Concave lens always forms virtual, erect, and diminished image regardless of object position.
Why is it necessary to use known focal length of convex lens?
To calculate focal length of concave lens from combined focal length, we need to subtract convex focal length using lens combination formula.
More Class 10 Physics Practicals
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