Waves 1

Made with help from @beanny3004's flashcards: https://knowt.com/flashcards/7e598737-637e-4f97-ac54-0b748931397a

Progressive wave

A wave in which oscillations (or peaks & troughs / compressions & rarefactions) move through the medium as energy is transferred

Displacement

distance from the equilibrium position in a particular direction; a vector, so it can have either a positive or a negative value

Amplitude

Maximum displacement from the equilibrium position (can be positive or negative)

Wavelength

minimum distance between two points in phase on adjacent waves, for example, the distance from one peak to the next or from one compression to the next

Period of oscillation (time period)

The time taken for one oscillation

Frequency

The number of oscillations per unit time

Wave speed

the distance travelled by the wave per unit time

Wave speed equation

v = fλ

wave speed = frequency x wavelength

Releationship between frequency and time period

f = 1/T

Phase

The position of a certain point on a wave cycle, (units are radians, degrees or fractions of a cycle)

Phase difference

• How far ‘out of step’ the oscillations at two points on the same wave or two points on similar waves are from each other

• Difference is normally measured in degrees/radians, but sometimes in fractions of a cylce or wavelength

Transverse wave sketch (include the direction of oscillations, the direction of energy transfer, equilibrium position, wavelength, amplitude and peak & trough)

Longitudinal wave sketch (include the direction of oscillations, the direction of energy transfer, wavelength, amplitude and compressions & rarefactions)

Transverse wave

A wave in which oscillations (of medium particles) are perpendicular to the direction of energy transfer

Longitudinal wave

A wave in which oscillations of the medium particles are parallel to the direction of energy transfer

Describe the difference between longitudinal and transverse waves.

• In a transverse wave oscillations are perpendicular to the direction energy transfer

• In a longitudinal wave oscillations are parallel the direction of energy transfer

Range of wavelengths of radio waves

10^-1 to >10⁶ metres

Range of wavelegnths of microwaves

10^-3 to 10^-1 metres

Range of wavelengths of infrared radiation

7×10^-7 to 10^-3 metres

Range of wavelengths of visible light

4×10^-7 to 7×10^-7 metres

Range of wavelengths of ultraviolet

10^-8 to 4×10^-7 metres

Range of wavelengths of x-rays

10^-13 to 10^-8 metres

Range of wavelengths of gamma rays

<10^-16 to 10^-10 metres

Diffraction

The spreading out of a wave front as it passes through a gap or around an obstacle. Maximum diffraction will occur when the gap the wave passes through is the same size as the wavelength of the incident wave

Reflection

When a wave changes direction at a boundary between two different media so that the wave remains in the original medium

Law of reflection

The angle of incidence is equal to the angle of reflection

Refraction

The change in direction of a wave when it passes from one medium to another

Law of refraction

n₁sin(θ₁)=n₂sin(θ₂)

Snell’s law

n sin(θ) = k

where n is the refractive index of material, θ is the angle between the normal and the incident ray, and k is a constant

In phase

When particles are oscillating perfectly in step - they have 0 phase difference

In anti-phase

When particles are oscillating completely out of step - they have a phase difference of π^c / 180^o

Refractive index

• How much a material slows down light - it is a ratio between the speed of light through a vacuum (3.00 x 10⁸ms^-1) and the speed of light through the material (in ms^-1)

• It has no units as it is a ratio

• n = c/v

Critical angle

• The angle of incidence at the boundary between two media that will produce an angle of refraction of 90°

• Sin(C)=1/n

Total internal reflection

The reflection of all light hitting a boundary between two media back into the original medium when the light is travelling through the medium with the higher refractive index and the angle of incidence at the boundary is greater than the critical angle

Conditions required for TIR

• 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.

• The angle of incidence must be above the critical angle. This angle depends on the refractive index of the medium.

Intensity

• The power transmitted per unit area

• Intensity = power / area = P / A

• Has unit Wm^-2

What is the relationship between intensity and amplitude?

Intensity is proportional to amplitude²: I ∝ A²

Electromagnetic wave

• Transverse wave with oscillating electric and magnetic field components at right angles to each other

• They don’t need a medium to propogate

• They have a speed of 3×10⁸ms^-1 in a vacuum

Electromagnetic spectrum

full range of frequencies of EM waves, from gamma rays to radio waves

Wave profile

displacement-distance graph of a wave (‘snapshot’ of the wave)

Unpolarised wave

Transverse wave in which the oscillations occur in many planes

Partially polarised wave

• Transverse wave in which there are more oscillations in one particular plane, but the wave is not completely plane polarised

• This occurs when transverse waves reflect off a surface

Plane polarised wave

A transverse wave in which the oscillations are limited to only one plane - the convention is to use the electric field

Polarisation (transmission) axis

The axis (direction) which waves are allowed through a polarising filter

How a polarising filter works

• Contains many long chain molecules that are aligned in the same direction

• The molecules absorb light aligned in that direction

• Therefore the orientation of the molecules is PERPENDICULAR to the polarisation axis. Diagrams usually show the polarisation axis - DO NOT CONFUSE THE TWO

Describe how you can use a polarising filter to determine if a beam of light is polarised. State clearly what you should observe.

Rotate the polarising filter and look for dim and bright light alternating every 90°

How does changing the wavelength affect diffraction

Assuming the wavelength is still comparable to the size of the gap:

• Shorter wavelength means less diffraction

• Longer wavelength means more diffraction, until the wavelength is greater than or equal to the gap. After this it diffracts the same amount

How does the movement of air molecules creates compressions and rarefactions in a sound wave travelling through air?

• The air molecules oscillate parallel to the direction of the energy transfer.

• This creates areas of high and low pressure which are compressions and rarefactions respectively