gen-define-the-focal-length-of-a-lens-and-give-one-short-worked-420d

Define the focal length of a lens and give one short worked example.

# Focal Length of a Lens **Definition:** The focal length of a lens is the distance from the optical center of the lens to the focal point (where parallel rays of light converge or appear to diverge from after passing through the lens). For a **converging (convex) lens**, parallel rays converge at the focal point. For a **diverging (concave) lens**, parallel rays appear to diverge from the focal point. --- ## Worked Example **Problem:** A converging lens has a focal length of 20 cm. An object is placed 30 cm from the lens. Find the image distance. **Solution:** Using the thin lens equation: $$\frac{1}{f} = \frac{1}{d_o} + \frac{1}{d_i}$$ Where: - f = focal length = +20 cm - d₀ = object distance = 30 cm - dᵢ = image distance = ? Substituting: $$\frac{1}{20} = \frac{1}{30} + \frac{1}{d_i}$$ $$\frac{1}{d_i} = \frac{1}{20} - \frac{1}{30} = \frac{3-2}{60} = \frac{1}{60}$$ $$d_i = 60 \text{ cm}$$ **Answer:** The image forms 60 cm on the opposite side of the lens from the object (a real, inverted image).