Topic 6 - Waves

What is a transverse wave?

Vibrations are perpendicular to direction of energy transfer (e.g. light, water waves)

What is a longitudinal wave?

Vibration are parallel to direction of energy transfer (e.g. sound waves)

Define wavelength (λ)

Distance between identical points on waves - Crest-to-crest

Define frequency (f)

Number of waves per second

Measured in (Hz)

Define period (T)

The time it takes for one complete wave to pass a point

Define amplitude

Maximum displacement from undisturbed position

What is the wave speed equation?

v = f × λ

What is the speed of light in a vacuum?

3 × 10⁸ m/s

What is the speed of sound in air?

340 m/s

How to measure wave speed with a ripple tank?

Measure the wavelength: Use a metre rule to measure across 10 waves and then divide by 10

Measure the frequency: Count how many waves pass a point
in one second

Use equation: wave speed = wavlength x Frequency

How does the medium affect wave speed in solids?

Sound: Faster in solids > liquids > gases; because particles are tightly packed, allowing vibrations to transfer more quickly.

What is reflection?

Wave bounces off surface

What is specular reflection?

Smooth surface → clear reflection

What is diffuse reflection?

Rough surface → scattered rays

What is refraction?

Change in speed causes change in direction

What happens when a wave enters a slower medium?

Bends towards the normal

How to investigate reflection and refraction?

Use light box, protractor, mirror/block; measure angle of incidence/reflection/refraction; trace rays

What are sound waves?

Longitudinal, travel through vibrations

Needs a medium

What is the range of human hearing?

20 Hz – 20,000 Hz

Why is hearing range limited?

Limited by size/shape of eardrum and cochlea sensitivity

WOrk plus how we hear

What is ultrasound?

High frequency sound waves with a frequency greater than 20 000 Hz, above limit of human hearing

Uses of ultrasound

Medical imaging, industrial testing

Explain why ultrasound can be used to measure the thickness of the layer of fat

-(ultrasound) waves reflected
-at boundary

Explain how ultrasonic waves are used to produce the image of an unborn baby.

Partly reflected when they hit a boundary between 2 different media

Time taken for reflected wave (to return) is used to produce the image

What are seismic waves?

Waves from earthquakes used to study Earth’s internal structure

What are P-waves?

Longitudinal, travel through solids & liquids

What are S-waves?

Transverse, only through solids

Explain why the study of seismic waves provides evidence for the structure of the Earth's core.

S-waves cannot travel through liquids

S-waves are not detected on the opposite side of the Earth, indicating the outer core is liquid

EM waves properties

• All electromagnetic waves are transverse

• All electromagnetic waves have the same speed in a vacuum.

• Can be reflected, refracted, and diffracted

Order of EM spectrum (low to high frequency)

Radio → Microwaves → IR → Visible → UV → X-rays → Gamma

Raging martians invade venus using x-ray guns

Gamma rays

• Kills cancer cells → used in cancer treatment, sterilising equipment

• Causes: Mutation, killings living cells - Ionising radiation

X-rays

Very high frequency - absorbed by bone

• Medical imaging

• Mutations, cancer (ionising radiation)

Ultraviolet

• Fluorescent lamps, Sun beds

• Skin aging, cancer

Infrared

Cooking food, night vision

Microwaves

Cooking, satellite communication

Radio waves

Broadcasting, radio signals

Vsible light

Enables sight and is used in cameras, and fibre optics for communication

What causes different colours in visible light?

Each colour has a specific range of wavelengths.

What happens when light hits an object?

It is absorbed, transmitted, or reflected, depending on the object.

Why does a red object look red?

It reflects red light and absorbs other colours more strongly.

What do black and white objects do with light?

• Black absorbs all visible wavelengths (appears black).

• White reflects all visible wavelengths (appears white).

What do colour filters do?

They only let certain wavelengths (colours) through and absorb the rest

What happens when white light passes through a red filter?

It will appear red, because the red filter only lets red light through, and white paper reflects all colours that hit it.

What if red light hits a blue object?

The object will appear black, because blue objects only reflect blue. Since there’s no blue in red light, it absorbs all the red and reflects nothing.

Transparent

Translucent

Opaque

• Transparent: Let all light through clearly (e.g. glass).

• Translucent: Let some light through, but scatter it (e.g. frosted glass).

• Opaque: Block all light; no transmission (e.g. wood).

Convex Lens (Converging):

• Shape: Thicker in the middle, thinner at edges

• Effect: Converges light rays to a point (principal focus)

• Uses: Magnifying glasses, cameras, projectors, human eye lens

What do convex lenses do to parallel rays?

They converge them to the principal focus.

Concave Lens (Diverging):

• Shape: Thinner in the middle, thicker at edges

• Effect: Diverges light rays, rays appear to come from a point

• Uses: Glasses for short-sightedness

What do concave lenses do to parallel rays?

They diverge them; rays appear to come from the principal focus on the same side.

What is the focal length?

The distance from the centre of the lens to the principal focus.

What is a real image?

Formed when rays meet; can be projected on a screen.

Produced by: Convex lenses

What is a virtual image?

Formed when rays appear to meet; cannot be projected.
Produced by: Convex and Concave lenses

Equation for magnification

Magnification = image height / object height

No units because it a ratio

What is a black body?

A perfect absorber and emitter of radiation

Perfect black body radiation

the object absorbs all of the radiation incident on it

Why is a perfect black body the best possible emitter of radiation?

● It is a perfect absorber since it absorbs all radiation incident on it ● A perfect absorber is also a perfect emitter

What happens to the quantity of infrared radiation emitted by an object as temperature increases?

The hotter the object, the more infrared radiation it will emit.

Dark, Matt Surfaces:

• Good absorbers and emitters of radiation (e.g., black surfaces).

• Absorb all colours of visible light, which is why they appear black when all wavelengths are absorbed.

Light, Shiny Surfaces:

• Poor absorbers and emitters of radiation (e.g., white or reflective surfaces).

• Reflect most visible light, making them appear white and emitting less radiation.

What can be said about the rates of emission and absorption for a body at constant temperature?

The body is absorbing and emitting radiation at the same rate.

What can be said about the rates of emission and absorption for a body increasing in temperature?

The body is absorbing radiation faster than it is emitting it.

What affects Earth's temperature?

1. The Earth's rate of absorption and emission of radiation
2. The amount of reflection of radiation into space

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