Light and Sound
5 marks · NormalThe diagram below shows a prism, like those used in binoculars, and a glass block. Both rely on total internal reflection.
Describe and explain what happens to a ray of light inside a prism when it strikes the surface at an angle greater than the critical angle. Draw a labelled diagram to support your answer.
Calculate the angle of refraction if a ray of light enters glass from air at 35° and the refractive index of glass is 1.52.
Binoculars use prisms for total internal reflection, but sound waves cannot be 'bent' this way. Suggest one reason for this difference.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) — · must show diagram: prism, ray at surface, angle > c, reflected inside, surface labels
(b) 22° degrees · must show n=sin i/sin r; rearrange for r
(c) Sound needs a medium to travel and cannot be totally internally reflected like light because it is a longitudinal wave and transmission depends on particle vibration rather than interface refraction. · must show reason relates to differences between longitudinal and transverse waves and material boundaries
Must show: Diagram shows internal ray, total reflection, labelled critical angle; correct calculation using n=sin i/sin r; valid physical explanation
Common mistakes: Using wrong side for TIR; calculation errors in angles
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Correct labelled diagram showing ray reflection inside prism at angle > c.
B1 (a) · 1 mark · Explanation describes total internal reflection when angle > critical angle.
M1 (b) · 1 mark · Correct substitution in n=sin i/sin r, rearranged for r. (follow-through)
A1 (b) · 1 mark · Correct answer r≈22°. Accept 21–23° for rounding. (follow-through)
B1 (c) · 1 mark · Reason relates to sound needing a medium and its inability to exhibit TIR at boundaries like light.
Worked solution
a) When the incident angle is greater than the critical angle, all the light reflects back into the prism (total internal reflection).
Diagram: prism, incident ray > critical angle, TIR at surface, reflected ray inside.
c) Sound waves are longitudinal and need particle vibration in a medium; they cannot be totally internally reflected because their transmission does not depend on refraction between media like light (transverse) waves.