Refraction at a plane surface Flashcards – AQA A-level Physics (7408)

What equation defines the refractive index $n$ of a substance in terms of the speed of light in a vacuum $c$ and in the substance $c_s$?

$n = \frac{c}{c_s}$

How does the speed of light in a substance change as its refractive index increases?

It decreases.

What is the approximate refractive index of air?

$1$

Under what conditions does a light ray change direction at a plane boundary between two media?

When it enters the second medium at an angle to the normal and its speed changes.

From what reference line are the angles of incidence and refraction measured?

The normal to the boundary.

What is Snell’s law for refractive indices $n_1$ and $n_2$ and angles of incidence and refraction $\theta_1$ and $\theta_2$?

$n_1\sin\theta_1 = n_2\sin\theta_2$

How does a light ray bend when it enters a medium with a larger or smaller refractive index?

A larger index bends it towards the normal; a smaller index bends it away from the normal.

Why do different wavelengths of light travel at different speeds in an optical fibre?

The fibre material’s refractive index depends on wavelength.

What happens to an optical pulse containing a range of wavelengths as it travels through a fibre?

It spreads out in time because its wavelengths travel at different speeds.

Why does material dispersion limit the information transmission rate over a given distance?

Broadened neighbouring pulses can overlap, making them harder to distinguish.

What is modal dispersion in an optical fibre?

The broadening of a light pulse because different permitted paths, or modes, take different times to reach the end.

Why do rays at different angles arrive at different times in a step-index fibre?

They follow paths of different lengths.

How can modal dispersion limit the rate at which pulses are distinguished over a given distance?

Broadened neighbouring pulses can overlap, making them harder to distinguish.

How does absorption attenuate light in an optical fibre?

It transfers energy from the light to the fibre material, reducing the signal’s intensity.

What effect can absorption have on long-distance transmission in an optical fibre?

The weakened signal becomes harder to detect, limiting transmission distance unless the signal is restored.

How do absorption and dispersion differ in their effects on an optical signal?

Absorption reduces signal strength; dispersion broadens pulses in time.