Describe the action of thin converging and thin diverging lenses on a parallel beam of light.
Key Concepts
A thin lens is a lens whose thickness is small compared with its focal length.
Two principal types:
Converging (convex) lens – thicker at the centre.
Diverging (concave) lens – thinner at the centre.
The focal length \$f\$ is positive for converging lenses and negative for diverging lenses.
Principal axis: the line passing through the centre of the lens and its optical centre.
Action on a Parallel Beam of Light
Converging (Convex) Lens
A parallel beam of light incident on a thin converging lens is refracted so that the emergent rays converge to a real focal point \$F\$ on the opposite side of the lens.
The distance from the lens to \$F\$ is the focal length \$f\$ (positive).
All rays parallel to the principal axis meet at \$F\$ after refraction.
The emergent beam is convergent; if a screen is placed at \$F\$, a sharp point of light is formed.
Suggested diagram: Parallel rays striking a convex lens and converging at the focal point on the far side.
Diverging (Concave) Lens
A parallel beam of light incident on a thin diverging lens is refracted so that the emergent rays appear to diverge from a virtual focal point \$F'\$ on the same side of the lens as the incoming light.
The distance from the lens to \$F'\$ is the magnitude of the focal length \$|f|\$ (negative sign for calculations).
Extended backward, the emergent rays intersect at \$F'\$.
The emergent beam is divergent; no real focus is formed on the far side.
Suggested diagram: Parallel rays striking a concave lens and appearing to diverge from a virtual focal point on the incident side.
Comparison Summary
Lens Type
Shape
Focal Length \$f\$
Effect on Parallel Beam
Image of Distant Object
Converging
Convex (thicker centre)
Positive (\$f>0\$)
Rays converge to a real focal point on the opposite side
Real, inverted, reduced image at the focal plane
Diverging
Concave (thinner centre)
Negative (\$f<0\$)
Rays diverge as if originating from a virtual focal point on the same side
Virtual, upright, reduced image located between lens and focal point
Important Points for Exams
Remember the sign convention: \$f>0\$ for converging, \$f<0\$ for diverging.
For a parallel beam, the image formed by a converging lens is at the focal point; a diverging lens produces a virtual image at the focal point.
Use ray diagrams to justify the behaviour of each lens type.
When answering, state whether the focal point is real or virtual and on which side of the lens it lies.