Cambridge IGCSE Physics · Syllabus 0625 · 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 IGCSE 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. Describe any image with three pairs of words: enlarged, same size or diminished; upright or inverted; real or virtual.
Common mistakes with Focal Length
- 13. “A converging lens always produces a real image.” Supplement 3.2.3.6 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?