NCEA Level 2 Physics 91170 Waves: past papers, practice and marking
- NCEA Level 2
- Physics
- AS91170
- 4 credits
- External exam
About 91170
AS91170, Demonstrate understanding of waves. Mirrors, lenses, refraction, total internal reflection, pulses, diffraction and interference: the 4-credit external, in the six question types it keeps reusing.
What 91170 covers
- Curved mirrorsConcave and convex: ray diagrams with arrows, image description, 1/f = 1/dₒ + 1/dᵢ, magnification, real and virtual images
- LensesConverging and diverging: ray diagrams, magnifying glass, projectors and cameras, negative f and negative dᵢ
- RefractionBending towards or away from the normal, n₁ sin θ₁ = n₂ sin θ₂, speed and wavelength in a medium, frequency unchanged
- Total internal reflectionThe two conditions, critical angle (angle of refraction 90°), optical fibres and cladding, prisms
- Waves at boundariesWave equation v = fλ, f = 1/T, electromagnetic and mechanical waves, water waves entering shallow water, reflected and transmitted pulses, superposition
- Diffraction and interferenceSpreading through a gap and around obstacles, two-point source interference, path difference, nodal and antinodal lines
91170 past papers
Open a 91170 past paper beside your own Ako page, then get it marked, part by part.
Work through a past paper All NCEA Level 2 Physics past papers
Practice questions
Question 1: Curved mirrors
A bike lamp has a small LED in front of a concave reflector of focal length 2.5 cm.
- State where the LED should be placed to give a parallel beam.(up to Achieved)
- Explain why placing the LED at the focal point produces a parallel beam.(up to Merit)
Check your answer
Part a: what gets the marks
- The LED goes at the focal point, 2.5 cm in front of the reflector, because rays from F reflect parallel to the axis.
A full answer: At the focal point, 2.5 cm in front of the reflector.
Part b: what gets the marks
- Rays travelling parallel to the axis reflect through F, and rays are reversible, so light leaving the LED at F reflects parallel to the axis and forms a parallel beam.
A full answer: A concave mirror reflects rays travelling parallel to the axis through the focal point. Light paths are reversible, so every ray that leaves the focal point is reflected parallel to the axis. All the light leaves in one direction as a parallel beam, so it does not spread out and stays bright further down the road.
What gets each grade
Question 2: Curved mirrors
A bike lamp has a small LED in front of a concave reflector of focal length 2.5 cm. A faulty bracket holds the LED 0.50 cm beyond the focal point.
- A faulty bracket holds the LED 0.50 cm beyond the focal point. Use 1/f = 1/dₒ + 1/dᵢ to find where the reflected light converges.(up to Merit)
- Discuss how the beam behaves with the LED beyond F, and how it would behave if the LED were moved inside F instead.(up to Excellence)
Check your answer
Part a: what gets the marks
- Object distance from the mirror: dₒ = 2.5 cm + 0.50 cm = 3.0 cm
- Rearrange the mirror equation: 1/dᵢ = 1/f − 1/dₒ = 1/(2.5 cm) − 1/(3.0 cm) = 0.400 cm⁻¹ − 0.333 cm⁻¹ = 0.0667 cm⁻¹
- Image distance: dᵢ = 1/(0.0667 cm⁻¹) = 15.0 cm in front of the reflector
A full answer: dₒ = 2.5 cm + 0.50 cm = 3.0 cm.
1/dᵢ = 1/(2.5 cm) − 1/(3.0 cm) = 0.0667 cm⁻¹, so dᵢ = 15.0 cm: the light converges 15.0 cm in front of the reflector.
Part b: what gets the marks
- With the LED beyond F the reflected rays converge to a real image 15.0 cm in front of the reflector and then spread out, so the beam is not parallel.
- With the LED inside F (for example dₒ = 2.0 cm, dᵢ = −10 cm) the image is virtual, so the reflected rays diverge straight away and the beam spreads out from the start.
A full answer: Beyond F, the reflected rays converge to a real image 15.0 cm in front of the lamp, then cross over and spread out, so the beam is bright close up and quickly becomes wide and dim. Inside F (for example dₒ = 2.0 cm gives dᵢ = −10 cm), the image is virtual: the reflected rays already diverge as if from a point behind the reflector, so the beam spreads from the start. Only at F do the rays leave parallel, which is why the LED position matters.