## Tuesday, 3 April 2012

### 7 Interesting SPM Paper 1 Questions on Waves (Yrs 2005 ~ 2011)

1. Yr 2011, P1, Q32 (pg. 287): Which statement is correct when water waves are reflected by a reflector?
A.     The velocity of water waves before and after reflection are the same
B.     The wavelength of water waves become longer after reflection
C.     The amplitude of water waves becomes smaller after reflection

Comments: The publisher gives A as the answer which is not right because on reflection, the direction of waves changes – that means the velocity before and after the reflection CANNOT be the same. The effects of reflection are that the speed, frequency and the wavelength remain the same – thus B cannot be the answer also. You should choose C as the answer because on hitting the reflector, not all the wave energy is reflected - some will be transformed into heat and sound; and damping occurs as the reflected waves move along – all these means the reflected waves carry less energy and the amplitude becomes progressively smaller over time.

1. Yr 2010, P1, Q25 (pg. 237): Which device is used to show that light is a transverse wave?
A. Glass Prism                          B. Double Slit
C. Diffraction Grating                D. Polaroid block

Comments: You cannot find the answer from your F4 or F5 textbooks. Neither can you do so from the normal reference books. To get perfect scores in physics, you need to “google” for additional knowledge sometimes! The answer: D.

3        Yr 2008, P1 Q 33 (pg. 147): Which is the correct relationship between the wavelength of an electromagnetic radiation and the energy it carries?
Wave length                 Energy carried
A.        Short                            High
B.        Short                           Low
C.        Long                            High
D.        Long                            Low

Comments: Again, this question is beyond Malaysian F4 & F5 physics textbooks and the normal physics reference books. But diligent students might already have known that the energy carried by photons of the electromagnetic radiation is given by the formula:
E = h f
Where, E = Energy,
h = Planck constant
f = frequency
Since, frequency f = Speed / wavelength and is therefore inversely proportional to wavelength with speed as a constant (3 x 10^8 m/s in vacuum or air approximately), short wavelength means high frequency means high energy (Answer: A); but, long wavelength also means short frequency and therefore low energy (Answer: D also). Answer: Both A and D.

4        Yr 2008, P1 Q34 (pg. 147): Diagram 19 shows the shape of a radio wave that is received by a simple radio receiver. What is the shape of the wave produced after it is passed through a diode?
A, B, C or D.             Answer: A

Comments: Students who have studied the chapter on “Electronics” (F5 Chapter 4) should know that the answer is A. For those who have not studied “rectification” in electronics and do not know what a diode is, be patient, you will cover the chapter in due course.

5        Yr 2006, P1 Q30 (pg. 55): The process of combining audio and radio frequency signals is known as:
A.     resonance
B.     damping
C.     modulation
D.     rectification

Comments: Surprisingly, "modulation" is not much talked about or discussed in any pages of Science Stream students' F5 Physics textbook BUT it talked about in greater details in pg. 189 of Arts Stream students' F5 Science textbook…A case of examiners got mixed-up?..hehehe :) Anyway, if this year, examiners ask: What is the device that combines a modulator and a demodulator (or, a demodulator and a modulator)? The answer is: “modem”. The amplitude modulated (AM) wave form is tested in next question.
The answer to this Q30: C. modulation

6. Yr 2005, P1 Q29 (pg. 9): Which graph represents amplitude modulated waves?
A, B, C or D.                    Answer: A

Comments: Again, amplitude modulated wave form is not shown in any pages of F5 Physics textbook of Science Stream students BUT in pg. 189 of F5 Arts Stream students’ Science textbook. What if in Yr 2012, the examiners rephrase this Q29 to ask about frequency modulated (FM) wave form instead? The answer should then be: C (where you notice the period T for each successive wave cycle progressively increases and then decreases and the modulation of the frequency continues). Click here to see the AM and FM wave forms in greater clarity.

1. Yr 2005, P1 Q35 (pg. 11): The graphs show the cross-sections of the water waves. Which wave has the greatest energy?
A, B, C or D.               Answer: C (for water waves)

Comments: Wave C has the highest amplitude (and thus the wave height). Wave D has the highest frequency. For water waves, energy is proportional to the square of its wave height – thus C is the answer for water waves. But if it were electromagnetic waves, the answer would be D because energy of photons of electromagnetic waves is proportional to frequency of the electromagnetic waves (E = h f, where E is energy; h is Planck’s constant and f is frequency of the waves).