Waves and optics - AQA A-Level Physics

Transverse waves

Particle oscillations are perpendicular to the direction of energy transfer

Longitudinal waves

Particle oscillations are parallel to the direction of energy transfer

Progressive waves

Waves that transfer energy through a medium

Mechanical waves

Particle oscillations in a physical medium

Polarisation

Passing a transverse wave through a polaroid filter, allowing oscillations to only occur in one plane perpendicular to the direction of wave propagation.

Why can’t longitudinal waves be polarised?

Because oscillations are strictly parallel to wave propagation

Law of reflection

angle of incidence = angle of reflection

Refraction

The change in direction of a wave when part of the wave-front speeds up or slows down before the rest of it.

Diffraction

The spreading out of a wave as it passes through a gap

How can maximum diffraction be obtained?

By making the gap size similar to the wavelength

Superposition

The displacement of two waves is combined as they overlap each other.

Phase difference

The difference in the point of a cycle between two points on a wave or two waves.

Path difference

The difference in distance travelled between two coherent waves from their sources to the point they meet at.

How could you tell that two waves will constructively interfere by their path difference?

If the path difference is equal to a multiple of their wavelength

Constructive interference

Reinforcement - path difference is a whole number of wavelengths

Stationary waves

Produced by two progressive waves of the same frequency and amplitude travelling in opposite directions superposing.

Coherent

Having the same frequency, amplitude and a constant phase difference

Anti-node

Maximum displacement

Nodes

No displacement

Destructive interference

Cancellation - path difference is an odd number of half wavelengths

Describe the interference pattern seen when passing monochromatic light through a double slit

A pattern of light and dark fringes, with equidistant fringe separation

Describe the interference pattern seen when passing white light through a double slit

A central white maximum and coloured fringes, with the shortest wavelength (violet) closest to the maximum and the longest wavelength (red) furthest away

Describe the fringe pattern seen when passing monochromatic light through a single slit

A wide, intense central maxima and light fringes of decreasing intensity

Diffraction grating

A plate of many closely spaced slits

Describe the fringe pattern produced by passing monochromatic light through a diffraction grating

A pattern of light and dark fringes, similar to the pattern seen in a double slit arrangement, but sharper and brighter

How can we find the maximum n order number in a pattern from a diffraction grating

Take θ as 90° and round down to the nearest whole number

What is grating spacing equivalent to?

1/number of slits per m (d=1/n)

Refractive index

Measure of bending of a light ray when it passes from one medium into another

What is the refractive index of air?

1

Snell’s law

n1 sin θ1 = n2 sin θ2

Critical angle

The angle of incidence in a dense medium, which results in the angle of refraction in the less dense medium being 90°

What are the two conditions for total internal reflection to occur?

• The incident ray must be travelling into a less optically dense medium

• The angle of incidence must be greater than the critical angle

What is the effect on the critical angle in a medium with a greater refractive index?

A smaller critical angle

Why is it important to minimise signal absorption in an optical fibre?

To minimise the reduction in ampltiude and signal intensity, which may distort the information in the final pulse

What does dispersion in an optical fibre cause?

Pulse broadening, which is where the received rays are broader than the original transmitted signal. These broader signals can overlap, causing loss of data.

What is material dispersion in an optical fibre?

The dispersion of the different wavelengths of light due to them travelling at different speeds, causing pulse broadening.

What is modal dispersion in an optical fibre?

Light pulses from monochromatic light will spread out due to each light pulse having a different incident angle and therefore also having a different angle of reflection.