CCEA A Level Physics 2.2 Refraction and 2.3 Lenses Exam Questions

About CCEA A Level Physics 2.2 Refraction and 2.3 Lenses

Refraction and lenses form the geometric optics section of Unit AS 2: Waves, Photons and Astronomy. You need Snell's law and refractive index, total internal reflection and the critical angle, and their use in step-index optical fibres and flexible endoscopes. The lens work covers ray diagrams for converging and diverging lenses, the lens equation, linear magnification, an experiment to verify the lens equation, lens power in dioptres and the correction of short sight (myopia) and long sight (hypermetropia). Examiners like critical angle calculations, the role of the cladding in a fibre, lens equation problems with sign conventions and explaining which lens corrects each eye defect.

What you need to know

• Refractive index and Snell's law n = c/v for a medium. Snell's law: n1 sin θ1 = n2 sin θ2. Light bends towards the normal when it slows down entering a denser medium.
• Total internal reflection Happens when light travels from a higher to a lower refractive index and the angle of incidence exceeds the critical angle. sin c = n2/n1, which becomes sin c = 1/n for a material to air.
• Step-index optical fibres A glass core surrounded by cladding of lower refractive index. Light is totally internally reflected at the core-cladding boundary. Cladding protects the core surface and stops signals crossing between touching fibres.
• Flexible endoscopes Use an incoherent bundle of fibres to carry light into the body and a coherent bundle (fibres in the same relative positions at both ends) to carry the image back, allowing keyhole examination of internal organs.
• Lens equation and magnification 1/u + 1/v = 1/f using the real is positive convention, so a diverging lens has a negative focal length and virtual images have negative v. Linear magnification m = v/u = image height / object height.
• Power and eye defects Power P = 1/f with f in metres, unit dioptre (D). Myopia (short sight) is corrected with a diverging lens; hypermetropia (long sight) with a converging lens.

Common mistakes in 2.2 Refraction and 2.3 Lenses

• Using sin c = 1/n at a core-cladding boundary; use sin c = n(cladding)/n(core).
• Forgetting that total internal reflection requires light to travel towards the lower refractive index.
• Leaving out the sign for a diverging lens; its focal length and power are negative.
• Mixing up the bundles in an endoscope; only the image bundle needs to be coherent.
• Giving power in dioptres while using f in centimetres; convert to metres first.

Try three 2.2 Refraction and 2.3 Lenses exam-style questions