Total internal reflection and optical fibres — practice questions

Question 1 · difficulty L1 · recall

Total internal reflection can occur only when light travels

• A. from optically denser to optically rarer medium
• B. from rarer to denser medium only
• C. between equal-index media only
• D. normally onto any interface

Question 2 · difficulty L1 · understanding

The critical angle is the incidence angle in the denser medium for which the refracted ray

• A. returns exactly along the incident ray
• B. travels along the interface
• C. has zero speed
• D. is perpendicular to the interface in the rarer medium

Question 3 · difficulty L2 · understanding

For denser medium refractive index n1 and rarer medium n2, the critical angle C satisfies

• A. sin C=n1/n2
• B. cos C=n2/n1
• C. sin C = n2/n1
• D. tan C=n1-n2

Question 4 · difficulty L2 · application

Optical fibres guide light primarily by

• A. ordinary mirror reflection from metal coatings only
• B. diffraction at each turn
• C. absorption and re-emission
• D. total internal reflection at the core-cladding boundary

Question 5 · difficulty L3 · application

Why is the refractive index of fibre core chosen higher than that of cladding?

• A. To allow total internal reflection within the core
• B. To make light leave the core at every boundary
• C. To remove refraction entirely at the entrance
• D. To make the critical angle exceed 90°

Question 6 · difficulty L3 · application

If n1/n2 increases for a denser-to-rarer interface, the critical angle generally

• A. increases toward 90°
• B. decreases
• C. remains fixed
• D. becomes greater than 180°

Question 7 · difficulty L4 · analysis

A glass-air boundary has n_glass=1.5. The critical angle is closest to

• A. 30°
• B. 60°
• C. 42°
• D. 75°

Question 8 · difficulty L4 · analysis

A ray inside glass hits a glass-air interface at an angle below the critical angle. It will

• A. always undergo TIR
• B. stop at the interface
• C. reflect with 100% intensity by definition
• D. partly refract into air rather than undergo total internal reflection

Question 9 · difficulty L5 · analysis

Which practical device most directly exploits repeated total internal reflection?

• A. Optical fibre communication cable
• B. Simple opaque screen
• C. Resistive heater
• D. Galvanometer shunt

Question 10 · difficulty L5 · analysis

A ray in a fibre core is changed so its incidence at the core-cladding interface falls below the critical angle. The most likely result is

• A. stronger confinement by TIR
• B. increased leakage into the cladding
• C. zero refraction at the boundary
• D. automatic increase of core index