Cambridge IGCSE Physics 2012 (0625-11+13) - Waves

0625-11 MCQ
Candidates found Question 21 (31% correct) and Question 40 (35% correct) the most difficult on the paper.

Question 21

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Many candidates did not appear to understand what is meant by the term ‘wavefront’ and there was evidence of widespread guessing.

A wavefront is the locus of points having the same phase: a line or curve in 2d, or a surface for a wave propagating in 3d. 

Based on the definition, the answer must therefore be option 'C'. While 'A' being the direction of wave motion; 'B' being the wavelength interval of a wave; and 'D' being the reflecting surface.

Question 40

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In this question on nuclear structure more than half the candidates opted for 'B', either failing to notice that the question concerned a nucleus rather than an atom, or in the mistaken belief that nuclei contain electrons.

A nuclide (from nucleus) is an atomic species characterized by the specific constitution of its nucleus, i.e., by its number of protons Z, its number of neutrons N, and its nuclear energy state.

Noticed that candidates always fail to learn their definitions well, hence, led to poor understanding. This question-type can easily be handled using pure definition approach.

0625-13 MCQ
Question 21 (28% correct) and Question 24 (36% correct) were the most difficult on the paper.

Question 21

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A wavefront is the locus of points having the same phase: a line or curve in 2d, or a surface for a wave propagating in 3d. 

Based on the definition, the answer must therefore be option 'B'. While 'A' being the amplitude of wave motion; 'C' being the wavelength interval of a wave; and 'D' being the wave edge. 

This year's candidates were not taught well in waves section.

Question 24

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This question required knowledge that a ray of light travelling from water to a less dense medium (air) would be partially reflected, as well as being refracted. Most candidates did not know that some reflection would occur for angles of incidence below the critical angle.

Total internal reflection is a phenomenon that happens when a propagating wave strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface. If the refractive index is lower on the other side of the boundary and the incident angle is greater than the critical angle, the wave cannot pass through and is entirely reflected. The critical angle is the angle of incidence above which the total internal reflectance occurs. This is particularly common as an optical phenomenon, where light waves are involved, but it occurs with many types of waves, such as electromagnetic waves in general or sound waves.

When a wave crosses a boundary between materials with different kinds of refractive indices, the wave will be partially refracted at the boundary surface, and partially reflected. However, if the angle of incidence is greater (i.e., the direction of propagation or ray is closer to being parallel to the boundary) than the critical angle – the angle of incidence at which light is refracted such that it travels along the boundary – then the wave will not cross the boundary and instead be totally reflected back internally. This can only occur where the wave travels from a medium with a higher refractive index (n1) to one with a lower refractive index (n2). For example, it will occur with light when passing from glass to air, but not when passing from air to glass. 

This question demands a higher standard of care. Candidates are advised to read slightly beyond current syllabus. 

Exams Question Paper Source:

CIE - Cambridge IGCSE - Physics - 0625 

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(1)    IGCSE Physics Challenging Drill Questions (Physical Books) 
(2)    IGCSE Physics Challenging Drill Questions eBooks (full)