Cambridge O Level Physics · Syllabus 5054 · Waves
Focal Length
What is Focal Length?
Focal length is the distance from the optical centre of a lens to its principal focus; a more powerful lens has a shorter focal length.
This definition is part of the Waves chapter in Cambridge O Level Physics.
Focal Length in context
A converging lens brings a parallel beam together at the principal focus; a diverging lens spreads a parallel beam so that it appears to come from a principal focus on the incident side. The distance from the optical centre to the principal focus is the focal length. Where real refracted rays actually meet, the image is real and can be caught on a screen; where only their backward extensions meet, the image is virtual and cannot. Linear magnification is the ratio of image length to object length.
Common mistakes with Focal Length
- 13. “A converging lens always produces a real image.” Why wrongEvery magnifying glass disproves it. With the object inside the focal length the emerging rays still diverge, so they never cross. CorrectReal image only when the object is further from the lens than F. Inside F the image is virtual, upright and magnified. Say this“With the object inside the focal length, the emerging rays diverge and the image is virtual, upright and magnified.” Check\(f = 10\) cm and the object is 6 cm from the lens. Real or virtual? Upright or inverted?
Questions students ask about Focal Length
What is the difference between a real image and a virtual image formed by a lens?
A real image is formed where refracted rays actually cross, so it can be caught on a screen; this happens when the object is further from a converging lens than the focal length. A virtual image is formed only where the backward extensions of diverging rays appear to meet, so no light actually reaches it and it cannot be projected onto a screen; this happens when the object is inside the focal length, as in a magnifying glass.