Focal length of a convex lens

Objectives

• Form a sharp real image with a convex lens.
• Measure object and image distances.
• Find the focal length from the lens formula.

Theory

A convex lens forms a real, inverted image on a screen when the object is further from the lens than the focal length. Object distance u, image distance v and focal length f are related by the lens formula 1/f = 1/u + 1/v (distances measured as positive lengths for a real object and real image). A graph of 1/v against 1/u is a straight line of gradient −1 whose intercepts are both 1/f.

1/f = 1/u + 1/v

Procedure

1. Move the screen until the image is sharp, then record the position.
2. Record sharp images for at least four different object distances.
3. Calculate f for each pair, or plot 1/v against 1/u.

Precautions

• Move the screen slowly through the sharpest position from both sides.
• Keep lens, object and screen at the same height.

Subject

Physics

Grade

10–12

Chapter

Light (Nepal National Curriculum (CDC), grade 10)

Difficulty

Medium

Duration

25 min

Safety level

Low

Version

1.0

Apparatus

• Optical bench
• Convex lens and holder
• Illuminated object
• Screen