Deck 3: Refraction, Refractive Index & Polarisation

What is refraction?

The change in direction of a wave as it passes from one medium to another due to a change in wave speed.

What causes refraction?

A change in wave speed as the wave moves between media of different optical densities.

What happens to wave speed and wavelength when entering a more optically dense medium?

Wave speed and wavelength decrease, but frequency remains the same.

Does frequency change during refraction?

No, frequency remains constant.

Define refractive index.

n = c / v, where c is the speed of light in a vacuum and v is the speed of light in the material.

State Snell’s Law.

n₁sinθ₁ = n₂sinθ₂

What is the critical angle?

The angle of incidence at which the angle of refraction is 90°.

How do you calculate the critical angle?

sinθc = n₂ / n₁, where n₁ > n₂.

What is total internal reflection?

When light is completely reflected back into the denser medium at an angle greater than the critical angle.

What conditions are needed for total internal reflection?

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

Where is total internal reflection used?

In optical fibres and endoscopes.

Why is TIR useful in optical fibres?

It keeps light confined within the core, allowing data transmission with minimal loss.

What is polarisation?

The process of restricting the oscillations of a transverse wave to one direction.

What types of waves can be polarised?

Only transverse waves can be polarised.

Give an example of polarised light in real life.

Glare reduction in sunglasses.

What is an application of polarisation?

TV and radio signals; reducing glare in photography.

How can polarisation demonstrate the transverse nature of light?

If light can be polarised, it must oscillate perpendicular to the direction of travel.

What stays the same when light enters a denser medium?

Frequency.

What increases as light enters a less dense medium?

Wave speed and wavelength.

Why do optical fibres need a cladding with lower refractive index?

To ensure total internal reflection occurs within the core.