Total Internal Reflaction

More Wave Properties

Define partial refraction.

When a wave reaches a boundary between two media, some of the wave energy is refracted into the second medium and some is reflected back into the first medium.

What determines the proportions of light that are reflected and refracted at a boundary?

The nature of the boundary and the angle of incidence.

What happens to a light ray travelling from an optically more dense medium to an optically less dense medium?

It bends away from the normal.

What happens to the angle of refraction as the angle of incidence increases when light travels from a more dense to a less dense medium?

The angle of refraction increases.

Define critical angle.

The largest angle of incidence at which a ray travelling from an optically more dense medium can still emerge into a less dense medium.

What is the symbol for critical angle?

θc.

What happens when the angle of incidence is less than the critical angle?

The light is refracted out of the medium, with some light also reflected.

What happens when the angle of incidence equals the critical angle?

The refracted ray travels along the boundary at 90° to the normal.

What is the angle of refraction when the angle of incidence equals the critical angle?

90°.

What happens when the angle of incidence is greater than the critical angle?

Total internal reflection occurs.

Define total internal reflection (TIR).

The complete reflection of a wave back into the same medium at a boundary.

What are the two conditions required for total internal reflection?

The wave must travel from a more optically dense medium to a less optically dense medium, and the angle of incidence must be greater than the critical angle.

Can total internal reflection occur when light travels from a less optically dense medium to a more optically dense medium?

No.

Can total internal reflection occur when the angle of incidence is equal to the critical angle?

No. The refracted ray travels along the boundary.

What happens to the wave energy during total internal reflection?

All of the wave energy is reflected back into the original medium.

What law does a totally internally reflected ray obey?

The law of reflection.

State the law of reflection.

The angle of incidence equals the angle of reflection.

From which line are the angles of incidence and reflection measured?

The normal.

State Snell's law.

n₁ sin θ₁ = n₂ sin θ₂.

How can Snell's law be used to derive the critical angle equation?

At the critical angle θ₁ = θc and the refracted angle θ₂ = 90°, so n₁ sin θc = n₂ sin 90°.

What is sin 90°?

1.

What is the general equation for critical angle?

sin θc = n₂/n₁, where n₁ is the refractive index of the more optically dense medium and n₂ is that of the less optically dense medium.

What is the critical angle equation when light travels from a material into air?

sin θc = 1/n.

How can refractive index be calculated from the critical angle for a material-to-air boundary?

n = 1/sin θc.

How can the critical angle be calculated if the refractive index is known?

θc = sin⁻¹(1/n), when the second medium is air.

What happens to the critical angle as refractive index increases for a material-to-air boundary?

The critical angle decreases.

What does a small critical angle mean about a material's refractive index?

It has a relatively large refractive index.

What does a large critical angle mean about a material's refractive index?

It has a relatively small refractive index.

How can total internal reflection be used to redirect light?

Light can be reflected at internal surfaces so that it changes direction without leaving the material.

How can prisms use total internal reflection?

Light strikes an internal surface at an angle greater than the critical angle and is completely reflected.

Why can prisms be useful for directing light?

They can use total internal reflection to redirect light efficiently with very little loss of light.

What are optical fibres?

Thin transparent fibres that guide light along their length using repeated total internal reflection.

How does light travel through an optical fibre?

It repeatedly undergoes total internal reflection at the boundary inside the fibre.

Why must the angle of incidence inside an optical fibre be greater than the critical angle?

So that total internal reflection occurs and the light remains inside the fibre.

What happens if the angle of incidence in an optical fibre becomes smaller than the critical angle?

Some light can refract out of the fibre.

What are optical fibres used for?

Transmitting information, carrying images, decorative lighting and medical endoscopes.

How are optical fibres used in telecommunications?

Light pulses travel through fibres and carry information over long distances.

How are optical fibres used in medical endoscopes?

One group of fibres carries light into the body and another carries the reflected light back to form an image.

What is an endoscope?

A device using optical fibres that allows doctors to view inside the body without open surgery.

What is the purpose of fibres carrying light from a light source in an endoscope?

They illuminate the object or internal organ.

What is the purpose of fibres carrying reflected light in an endoscope?

They carry an image back to the observer.

How can total internal reflection be investigated experimentally?

Pass a ray of light through a semicircular glass block and gradually increase the angle of incidence at the flat surface.

What happens to the refracted ray as the angle of incidence is increased in a TIR experiment?

It bends increasingly away from the normal.

What happens to the reflected ray as the critical angle is approached?

Its intensity increases.

How is the critical angle identified experimentally?

It is the angle of incidence when the refracted ray travels along the boundary.

Why is a semicircular glass block useful when investigating total internal reflection?

A ray aimed through the centre meets the curved surface along a radius, so it enters normally and does not refract at that surface.

At which surface of a semicircular glass block is the critical angle measured?

At the flat surface as the light attempts to leave the glass.

How can the refractive index of glass be calculated from its measured critical angle?

n = 1/sin θc.

What safety precaution should be taken when using a ray box?

The ray box can become hot, so avoid touching hot parts.

What safety precaution should be taken when using glass blocks?

Do not knock glass blocks together because sharp chips could break off and cause injury.