Question 10
(a) (i) Define critical angle as used in optics.
(ii) State two conditions necessary for total internal reflection to occur.
(iii) List three practical applications of total internal reflection.
(b) State two effects of refraction.
(c) (i) Define progressive waves.
(ii) A plane progressive wave is represented by the equation y = 0.5 sin (1000 πt -
)
(α) frequency of the wave;
(ꞵ) wavelength of the wave;
(γ) speed of the wave.
Observation
Part (a): Performance was fair. (i) Many candidates stated that the angle of incidence gives an angle of refraction of 90o without taking into cognizance of how optically dense the media are. (ii) Most candidates were able to state the conditions correctly. (iii) Some candidates were able to list fibre optics and fish eye view correctly while others who mentioned binoculars and periscopes failed to qualify them with prism. No candidate =s mentioned transmission of radio signals.
Part (b): Performance was good as many candidates were able to respond correctly to this question.
Part (c): (i) Majority of the candidates failed to state that the disturbance enables energy to be transferred from its source to another. They only stated that the wave travels from one point to another.
(ii) candidates performance was fair.
The expected answer is:
(a) (i) Definition of critical angle
The angle of incidence in the optically denser medium for which the angle of refraction in the less dense medium is 90°.
(ii) Conditions necessary for total internal reflection to occur
- Light rays must travel from a dense medium to a less dense medium
- Angle of incidence in the dense medium must be greater than the critical angle.
(iii) Practical applications of total internal reflection
- Prism periscopes
- Prism Binoculars
- Fibre optics
- Fish eye view
- Transmission of radio signals
(b) Effects of refraction
- straight objects placed in liquids appear bent (on the interface or boundary)
- mirages
- dispersion of white light/formation of rainbow
- apparent displacement of objects placed in liquids/apparent shadows of pools base
(c) (i) Definition of progressive waves
A disturbance which travels through a medium that enables energy to be transferred from its source to another (without the particles of the medium transferred).
OR
Transfer of energy and momentum from the source of disturbances.
(ii()a) Calculation of frequency of wave
y = 0.5 sin (1000pt – 100p
)
= a sin (2πft – 2p
)
2pf = 1000p
f = 500 Hz
(b) Calculation of wavelength of wave
= 
l = 
= 0.34 m
(g) Calculation of speed of wave
v = fλ
= 500 x 0.34