3.10.1.1
Converging and diverging lenses bend light to form images
Medical Physics | AQA A-Level Physics
Key Definition
Converging (convex) lens: A lens that is thicker at the centre and refracts parallel rays to meet at the principal focusThe point where rays parallel to the principal axis converge (converging lens) or appear to diverge from (diverging lens) after passing through the lens..
Key Definition
Diverging (concave) lens: A lens that is thinner at the centre and refracts parallel rays so they appear to diverge from a virtual principal focusThe point where rays parallel to the principal axis converge (converging lens) or appear to diverge from (diverging lens) after passing through the lens..
- A real imageAn image formed where light rays actually converge. It can be projected onto a screen. is formed where light rays actually converge. It can be projected onto a screen.
- A virtual imageAn image formed where light rays appear to diverge from. It cannot be projected onto a screen. is formed where light rays appear to diverge from. It cannot be caught on a screen.
- Three rules for drawing ray diagrams through a thin converging lens:
- A ray parallel to the principal axis refracts through the principal focus.
- A ray through the optical centre passes straight through, undeviated.
- A ray through the principal focus refracts to emerge parallel to the principal axis.
- For a diverging lens, parallel rays diverge as if coming from the virtual focus on the same side as the incoming light.
- The key part is that converging lenses can form both real and virtual images, but diverging lenses always form virtual, upright, diminished images.
Common Mistake
HIGH
Students often: Confuse which type of lens is converging and which is diverging.
Instead: Remember that a converging lens is fat in the middle (convex), while a diverging lens is thin in the middle (concave).
Instead: Remember that a converging lens is fat in the middle (convex), while a diverging lens is thin in the middle (concave).