11.9 Total Internal Reflection

Question 1

When does total internal reflection occur?

Answer 1

It occurs at the boundary between two different media. When the light strikes the boundary at a large angle to the normal, it is totally internally reflected. All the light is reflected back into the original medium. There is no light energy refracted out of the original medium.

Question 2

The conditions for total internal refraction.

Answer 2

1) The light must be travelling through a medium with a higher refractive index as it strikes the boundary with a medium with a lower refractive index. For example: when light in glass meets air, but not

2) The angle at which the light strikes the boundary must be above the critical angle. This angle depends on the refractive index of the medium.

Question 3

How will you determine the refractive index from the critical angle? Describe an experiment.

Answer 3

Place the transparent block on a piece of white paper and draw its outline.

Shine a ray of light from inside the block towards the flat surface at an angle (i.e., light is going from denser to air).

Vary the angle of incidence and observe the refracted ray:

At first, the ray bends out into the air.

As the angle increases, the refracted ray gets closer to the surface.

At a specific angle, the refracted ray disappears and travels along the boundary — this is the critical angle.

Beyond this, you’ll see total internal reflection.

Use a protractor to carefully measure the critical angle (θc) where this boundary skimming occurs.

Use the formula:

n = 1 / sinθc

Question 4

What are optical fibres designed to do?

What are their uses?

Answer 4

Optical fibres are designed to totally internally reflect pulses of visible light travelling through them.

Transmitting data for fast broadband connections and images from inside patients during keyhole surgery.

Question 5

Describe the structure of an optical fibre.

Answer 5

A simple optical fibre has a fine glass core surrounded by a glass cladding with a lower refractive index.

Light travelling through the fibre is contained within the core because of total internal reflection at the core/cladding boundary.

Question 6

Why does total internal reflection occur in an optical fibre?

Answer 6

Total internal reflection occurs in an optical fibre because:

The core has a higher refractive index than the cladding,

Light inside the core hits the core–cladding boundary at an angle greater than the critical angle because the core is denser than the cladding.

This causes the light to be completely reflected back into the core,

So the light keeps bouncing along the fibre without escaping, allowing efficient signal transmission.