What are thin lenses? A thin lens is a lens whose thickness is small compared to its focal length. Think of it like a magnifying glass that is so thin you can almost see through it. Thin lenses are used in glasses, cameras, microscopes, and many everyday devices.
| Scenario | Given | Result |
|---|---|---|
| Converging lens, object beyond focal point | \$f = 10\,\text{cm}\$, \$d_o = 30\,\text{cm}\$ | \$d_i = 15\,\text{cm}\$ (real, inverted) |
| Diverging lens, object anywhere | \$f = -12\,\text{cm}\$, \$d_o = 20\,\text{cm}\$ | \$d_i = -7.5\,\text{cm}\$ (virtual, upright) |
🔍 Example 1 – A book in front of a magnifying glass: The book is 25 cm from the lens, the lens has a focal length of 10 cm. Using the thin‑lens formula, the image forms at 16.7 cm on the other side. The image is real and inverted, but because the lens is close to the book, we see a magnified, upright virtual image when we look through the lens.
📐 Example 2 – A camera lens: A camera sensor is 2 cm from the lens. If we want a sharp image of a distant object (effectively at infinity), the lens must be positioned at its focal length, e.g., \$f = 2\,\text{cm}\$.
When we use a laser pointer (a single frequency of light), the lens behaves predictably because all rays have the same wavelength. This eliminates chromatic aberration, so the image is sharp and colour‑consistent. In contrast, white light contains many frequencies, which can spread out slightly after passing through a lens.