No Brain Too Small PHYSICS

Transcription

1 Level Physics: Waves Total Internal Reflection - Answers The Mess that is NCEA Assessment Schedules. Level Physics: AS 970 replaced AS In 9054, from 004 to 0, 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 (Explain stuff) or Criteria (Solve stuff). From 003 to 008, the NCEA shaded columns that were not relevant to that question (Sorry haven t had time to do 004 yet). In 970, from 0 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 03, with 970, 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 06(3) (c) The angle of incidence has to be greater than the critical angle. This is true for a ray going from a medium of higher refractive index (optical density) to one of lower refractive index. ONE condition correctly stated. BOTH conditions correctly stated. (i) TWO of: correct path of beam correctly labelled angle of incidence use of Snell s Law. ALL of: correct path of beam correctly labelled angle of incidence use of Snell s Law including realisation that r 90. ALL correct n correctly calculated to 3 decimal places. All correct but error.

2 (ii) sinq sinq Water-air interface: sin sin90.00 sin () (a) Total internal reflection. Correct answer Correct ray diagram (not 90 ). (b) sinq sinq sin47.0sin90 sin Correct formula and substitution incorrect answer. Correct mathematical calculation and answer. 03() sinq sinq.5sinc sin90 c sin Error if:.5sin45º sinθ is used The angle of incidence at the glass/air interface is greater than the critical angle, so it will totally internally reflect at an angle of reflection of 45 o. ONE of: Correct critical angle error sin(angle).06 Correct explanation. TWO of: Correct critical angle error sin(angle).06 Correct explanation. All of: Correct critical angle error sin(angle).06 Correct explanation.

3 Question Achievement Merit Excellence 0() (a) (b) Correct angle marked for critical angle Outgoing ray at 90 The angle of incidence has to be greater than critical angle. This is true for a ray going from a medium of higher refractive index (optical density) to one of lower refractive index. Correct angle marked for critical angle. Refracted ray at 90. Total Internal Reflection. The angle of incidence has to be greater than critical angle. This is true for a ray going from a medium of higher refractive index (optical density) to one of lower refractive index. (c) sinq c sinq sin4.0sin90.0sin90 sin

4 Question Evidence Achievement Merit Excellence 0() (e) sinq sinq.0 sin65 n sin sinq C.40 q C 45 Correct substitution in Snell s Law. Correct except for ONE error. Correct answer. (accept.4, 46 ) 009() sin sin sin C sin 90 sin C sin c.33 c 49 Correct derivation of formula. Correct critical angle. Correct critical angle and answer. NO. The angle of incidence is greater than the critical angle, so the ray reflects. 007() sin sin.6 sin c c sin.6 c 39 Correct formula substitution. Correct working answer.

5 006() (f) Rays should travel from an optically dense to an optically less dense medium. The angle of incidence should be greater than critical angle. One fact given. Both aspects explained. (g) nsin nsin.0 sin 90 c sin ( ).33 Correct formula and substitution. Correct sf. Critical angle calculated correctly. c sf 005() Critical angle is the angle of incidence that gives an angle of refraction of (almost) 90. Correct description of critical angle or angle of refraction of 90 0 or ray refracts along the boundary. (e) Correct formula selected and θ 90 o. Correct method answer.

6 (f) Two rays correctly drawn, extended back to locate image. Image laterally inverted. Two rays from Robbie reflected from the boundary. Two internally reflected rays from the same part of Robbie drawn correctly Arrows from Robbie. Two internally reflected rays from the same part of Robbie drawn correctly Arrows from Robbie Image is laterally inverted (i.e. upside down with head and feet in correct position).