Louis La Minh Hoang

In my recent physics revision, I focused on convex and concave lenses, with special attention on the Bessel method, which is a more advanced and practical way of determining the focal length of a lens. This topic goes beyond simple ray diagrams and requires both careful reasoning and precise calculations.

We started by revisiting the basic behavior of lenses. Convex lenses converge light and can form real images, while concave lenses diverge light and usually form virtual images. Although these ideas are familiar, the problems became more challenging when we had to apply them in experimental and analytical contexts rather than just theory.

The core of the lesson was the Bessel method, which applies to convex lenses. Instead of measuring object and image distances directly, we fix the distance between the object and the screen. When this distance is greater than four times the focal length, there are two distinct lens positions where a sharp image can be formed. Understanding why these two positions exist required a deep grasp of image formation and symmetry in lens behavior.

Using geometry and algebra, we derived the Bessel formula for focal length, which depends on the fixed distance between the object and screen and the separation between the two lens positions. The challenge was not memorizing the formula, but understanding the reasoning behind it and applying it correctly in complex numerical problems. Small errors in measurement or logic could completely change the final result.

We also discussed how concave lenses cannot directly form real images on a screen, which means the Bessel method cannot be applied to them in the same way. This contrast helped clarify the fundamental differences between converging and diverging lenses and strengthened my conceptual understanding.

Overall, this revision made lens theory feel much more rigorous and structured. The Bessel method showed how physics connects theory with real experiments, and how careful mathematical reasoning is essential even for topics that seem simple at first glance.