Question 10
(a) Define diffraction.
(b)(i) Explain critical angle.
The diagram above illustrates a ray of light passing through a rectangular transparent plastic block.
(α) Determine the value of the critical angle.
(β) Calculate the refractive index of the block.
(c) A pipe closed at one end has fundamental frequency of 200 Hz. The frequency of the first overtone of the closed pipe is equal to the frequency of the first overtone of an open pipe. Calculate the:
(i) fundamental frequency of the open pipe;
(ii) length of the closed pipe;
(iii) length of the open pipe.
[speed of sound in air = 330 ms-1]
Observation
Part (a): Performance was average. Many candidates were able to define diffraction to score full mark.
Part (b)(i): Performance was fair as many candidates stop at definition. Candidates lost mark for not supplying additional information such as, beyond c, light undergoes total internal reflection; illustrate with a diagram etc.
(ii)(α) Performance was low. Many candidates supplied the correct formula but substituted wrongly to lost some mark.
(β) Performance was low.
Part (c): Most of the candidates had idea of what to do but some were using the given frequency for both the open and close pipe and close pipe thereby mixing up everything.
The expected answer is:
(a) Diffraction is the bending or spreading of a wave round an obstacle or as it passes through an opening
(b)(i) Critical angle
The angle of incidence in the dense medium for which the angle of refraction in the less dense medium is 90o
Correct additional information e.g diagram, equation
OR
It is the minimum angle of incidence above which a ray passing from a denser medium to a less dense medium will be totally internally reflected
(ii)(α) Critical angle = 90o – 44o = 46o
