2021 A/L Physics Past Paper · Question 03 (English Medium)
Bending of light at a sharp edge
Bending of light when passing a sharp edge is due to
Answer and explanation
Answer: (4) diffraction.
What the question asks
Find the cause of the bending of light when it passes a sharp edge.
The light bends round the edge into the shadow region.
Relevant theory
Waves in a ripple tank passing through the centre of a gap continue straight on, but there is a tendency for the end of each wavefront to deviate into the geometrical shadow area behind the barrier.
Reading the options
Test each option against what happens at the edge.
The wave stays in the same medium, so its speed and wavelength do not change.
The wave end spreads into the shadow behind the edge.
That spreading of a wave past an edge or through a narrow opening is called diffraction.
Why the other options fail
Reflection sends light back from a surface. The light here goes on past the edge.
Refraction needs the light to enter a different medium. Here the medium stays the same.
Interference needs two or more overlapping waves. A single wave bends here.
Total internal reflection happens inside a denser medium at a boundary. There is none here.
Conclusion
Light that bends round a sharp edge into the shadow is showing diffraction.
The answer is (4).
Bending round an edge or spreading out of a narrow opening means diffraction.
Refraction needs a change of medium. Reflection sends the wave back.
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