To Find Focal Length of Convex Lens (Class 10 Physics Practical)
To determine the focal length of a convex lens using the lens formula and measurements from convex lens experiments
TL;DR: To determine the focal length of a convex lens using the lens formula and measurements from convex lens experiments
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Aim: To determine the focal length of a convex lens using the lens formula and measurements from convex lens experiments
Materials Required
- Convex lens
- Lens holder
- Optical bench
- Object (arrow/candle)
- Screen
- Measuring scale
- Light source
Theory
A convex lens converges light rays to a focal point. The focal length is the distance from the optical center to the focal point. Using the lens formula 1/f = 1/u + 1/v where f is focal length, u is object distance, and v is image distance, we can calculate focal length from experimental measurements. The lens formula is fundamental to understanding lens behavior and image formation.
Procedure
- Arrange the optical bench with lens holder at center and object at one end
- Place convex lens in the lens holder on the optical bench
- Place the object (arrow) at a known distance from the lens
- Adjust the screen position until a clear, sharp image is formed on the screen
- Record the object distance u and image distance v using the measuring scale
- Repeat for at least 5 different positions of the object
- Calculate focal length using 1/f = 1/u + 1/v for each trial
- Find average focal length from all measurements
Observation
Clear sharp images formed on screen. Object distances varied from 30 cm to 60 cm. Image distances changed as object distance changed. Magnification varied in different positions. Sum of u and v remained nearly constant for all positions.
Result
Focal length of convex lens determined as approximately 15 cm. The value remains consistent across multiple measurements. Lens formula verified experimentally.
Precautions
- Use lens of known type for accurate results
- Ensure optical bench is horizontal and stable
- Keep object and screen perpendicular to the lens axis
- Use bright light source for clear image formation
- Measure distances accurately from marked position on optical bench
Viva Questions
What is a convex lens?
A convex lens is a lens that bulges outward on both sides and converges parallel light rays toward a focal point. It forms real and inverted images for objects beyond focal length.
Define focal length of a lens.
Focal length is the distance from the optical center of the lens to the focal point where parallel light rays converge or appear to diverge.
What is the lens formula?
The lens formula is 1/f = 1/u + 1/v, where f is focal length, u is object distance, and v is image distance from the optical center.
How does object distance affect image distance?
As object distance increases, image distance decreases. When object is at infinity, image forms at focal point. As object approaches focal point, image moves away.
What is magnification of lens?
Magnification m = v/u = h'/h, where h' is image height and h is object height. For convex lens, m is negative for real images and positive for virtual images.
Why must the screen be perpendicular to lens axis?
The screen must be perpendicular to the optical axis to avoid distortion of the image and to ensure the image is formed on the screen plane for accurate measurement.
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