No Brain Too Small  PHYSICS   Level 2 Physics: Waves - Refraction - Answers The Mess that is NCEA Assessment Schedules…. Level 2 Physics: AS 91170 replaced AS 90254. In 90254, from 2004 to 2011, there was an Evidence column with the correct answer and Achieved, Merit and Excellence columns explaining the required level of performance to get that grade. Each part of the question (row in the Assessment Schedule) contributed a single grade in either Criteria 1 (Explain stuff) or Criteria 2 (Solve stuff). From 2003 to 2008, the NCEA shaded columns that were not relevant to that question (Sorry haven’t had time to do 2004 yet). In 91170, from 2012 onwards, the answers/required level of performance are now within the Achieved, Merit and Excellence columns. Each part of a question contributes to the overall Grade Score Marking of the question and there are no longer separate criteria. There is no shading anymore. At least their equation editor has stopped displaying random characters over the units. And in 2013, with 91170, we are back to an Evidence column with the correct answer and Achieved, Merit and Excellence columns explaining the required level of performance to get that part. Each part of a question contributes to the overall Grade Score Marking of the question.

Question Evidence Achievement Merit Excellence

2016(2) (a)

Correct diagram drawn.

(b) Property of a wave that does not change: Frequency v = f As the wave enters medium B, it slows down. Since wavelength wave speed, if the wavelength reduces (since frequency remains the same), the (wave) speed must reduce.

Correct property stated (frequency)

OR

wave speed is less.

Correct property stated AND correct explanation of reduced wave speed.

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(c) n1

n2

v2

v1

2

1

v2

3.3

0.20.3

v2 3.3 0.20.3

v2 2.2 m s–1

Correct formula and substitution.

Correct wave speed with units.

2016(3) (a) 

Water and glass have different refractive indices, so bend light differently and the boundary of the water and glass is visible.

Different refractive indices.  

(b)  n1 sin1 n2 sin2

Air-water interface:

1.00sin 32 1.33sinr

sinr sin1 1.00sin 321.33

r 23.48

Water-glass interface:

1.33sin23.48 1.52sinr

sinr sin1 1.33sin23.481.52

r 20

(This can be combined into a single equation.)

Correct substitution in Snell’s Law.

Correct answer.

 

 No Brain Too Small  PHYSICS   

2014(2) (c)

n1

n2

v2

v1

2

1

1.01.367

2

6.5107

2 6.5107

1.3672 4.76 107 m

f v

3.0 108

6.5107

f 4.6 1014 Hz

NB Cons. error

Correct wavelength. OR Frequency.

Correct wavelength. AND Frequency.

2014(3) (c)

Smaller wavelength OR correct direction (At right angles to each other.)

Smaller wavelength AND correct direction.

(d) Pulse is inverted – reflects off a fast slow interface (light-heavy; fixed.) (Pulse travels further in fast light spring in same time.)

Correct diagram with inverted pulse and further along than reflected pulse. OR Explanation for inverted pulse.

Correct diagram. AND Explanation for inverted pulse.

 No Brain Too Small  PHYSICS   

2013(3) (a)

Either correct refracted waves OR wave direction OR wave direction at right angles to refracted waves.

Correct refracted waves AND wave direction.

(b) Frequency remains same. Wavelength must decrease.

Correct.

Question Achievement Merit Excellence

2012(1) (d)

air 9056 34 (Using 56° = 9.6 cm)

nair sinair nglass singlass

1.0sin34 1.5singlass

glass 21.8

glass 21.8 AND Distance in glass = 8.0 / cos21.8○ = 8.6 cm

2012(2) (a)

20° Correct (90 – 70)

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Question Evidence Achievement Merit Excellence

2011(1) (d)

= 90 – 50 = 40° Correct answer.

(e) n1 sin1 n2 sin2

n2 1.0 sin65

sin 40 1.410

sinC 1

1.410C 45

Correct substitution in Snell’s Law.

Correct except for ONE error. Correct answer.

(accept 1.4, 46○)

 No Brain Too Small  PHYSICS   

2011(2) (f)

n1

n2

v2

v1

1.341.41

2.1108

v1

vf 2.1108 1.41

1.34vf 2.2 108 m s1

Correct substitution

OR ONE error

Correct answer.

2 sig. figs. Correct sig. figs.

2010(3) (a)

1All correct.

(b) (i)& (ii)

The reason for the ray not being able to pass through to air as the angle of incidence in water is greater than critical angle. Hence the ray undergoes total internal reflection.

2Correctly calculates angle of refraction in water (61°).

OR correctly finds critical angle

(48.75°).

2Correct angle of 61° AND EITHER critical angle for

water-air interface (48.75°). OR Applies Snell’s law in order

to calculate angle of refraction in air (ERROR).

2Justifies the reason for the ray not being able to pass through to air as the angle of incidence in water is greater than critical angle. Hence the ray undergoes total internal reflection.

 No Brain Too Small  PHYSICS   

2009(2) (a)

Correct diagram showing both rays bending away from normal and image found by extending the rays.

The harbour bottom appears shallower. Image of harbour bottom is seen where the refracted ray appears to come from.

1Correct diagram showing both rays bending away from normal.

1Achievement plus apparent harbour floor drawn in closer to surface.

1Merit plus explanation that image of harbour bottom is seen where the refracted rays appear to come from.

(b) Frequency stays the same. Speed of light increases as it leaves the water.

1Both correct.

(c)

n1 sin1 n2 sin2

1.33sin 39 1sin2

sin2 0.8369

2 57

2Correct except for one mistake. 12 sig. fig.

2Correct answer.

2009(3) (a)

2Correct substitution and working, unit not required.

(c)

1Correct in shallow e.g. fronts parallel but closer together.

1Achieve plus correctly drawn waves in deep e.g. bent away from normal.

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(d)

(Accept 42°, can be solved using speed of wave.)

2Correct relative refractive index e.g. 0.756 or ratio of wavelengths.

2Correct answer except for one error.

2Correct answer.

2008(5) (a)

 

n1 sin1 n2 sin2

2 sin1 1sin 481.67

26.4o

n2 sin2 n3 sin3

3 sin1 1.67sin 26.41.472

30o

2Correct formula and correct substitution

OR angle of refraction in glass 23.6°.

1 Answer to 2sf.

2Angle of refraction in glass 26.4°

OR angle of refraction in turpentine 27°.

2Angle of refraction in turpentine 30°.

(b)

1Diagram shows light reflecting. 1Diagram shows light reflecting at an equal angle and refracting away from the normal when it exits into air.

(Allow second TIR)

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(c)

 

n1 sinc n2 sin2

1.67sinc 1.472sin90

c sin1 1.472sin901.67

c 61.8o

2Correct formula and substitution.

(Allow c = 36.8°)

2Correct answer.

2007(1) (a)

The refractive index is a measure of how much a material slows light down.

OR a measure of how much a material bends (refracts) light as it enters.

OR A measure of the optical density of a material.

1Correct answer.

(b) r = 90° – 65° = 25° 2Correct answer.

(c)

n1 sin1 n2 sin2

sin2 n1 sin1

n2

2 sin1 1 sin 251.60

2 15

2Correct formula AND substitution. Or substitution consistent with 1(b).

2Correct working AND answer. Or answer consistent with 1(b).

 No Brain Too Small  PHYSICS   

2007(2) (c)

n1 sin1 n2 sin2

sin2 n1 sin1

n2

n1

n2

v2

v1

sin2 v2 sin1

v1

sin2 0.35 sin 35

0.252 53

2Correct use of Snell’s Law OR speed ratio.

2Correct linking of formulae (symbols or values).

2Correct working AND answer.

2007(1) (a) and (b)

The right-hand end of the waves slow down before the left-hand end. This causes the wave front to swing towards the normal.

1Arrow drawn correctly to show refracted wave direction.

1Refracted wave fronts closer together and parallel to each other.

1Merit plus correct explanation.

(c) Frequency stays the same. Wavelength decreases as speed decreases as v = f.

1One correct statement. 1Both parameters correct.

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(d) f = 820

f = 0.40 Hz 2Correct.

(e) Hz, s–1 1Both correct.

(f) V = f = v

f

= 2.80.40

= 7.0 m

2Correct answer.

2006(2) (c)

1Ray drawn correctly through one medium

1Ray drawn correctly through both oil and water

(d) n1sin1 = n2sin2

2 = sin–1(1.00 sin40 1.50) = 25.4

n2sin2 = n3sin3

3 = sin–1(1.50 sin 25.4 1.33) = 29

2Correct formula and substitution for initial calculation.

22 calculated correctly

(Can be done without this step.)

2All correct.

(e) Rays drawn correctly to show refraction.

Image located.

1Rays bend away from normal. 2Correct rays and image.

 No Brain Too Small  PHYSICS   

2005(2) (a)

1 First deviation towards normal or second deviation away from normal.

1 First deviation towards normal and second deviation away from normal. Incident and emergent ray not parallel.

(b)

2 Correct formula selected and values substituted. Allow incorrect angles.

2 Correct formula selected and correct values substituted.

2 Correct method and answer.

(c) Absolute refractive index is a measure of how much light is slowed down (or bent) when it travels into a medium (from a vacuum).

1 Correct answer. Must mention light speed slows down or light bends or optical density or from a vacuum.

1 Correct answer. Must mention light speed slows down or light bends, or optical density, and from a vacuum.