# WAVES

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WAVES

## 1. Describing waves

### 1.1. amplitude, A

1.1.1. maximum vertical distance from its equilibrium position.

1.1.2. unit: cm, centimetre

### 1.2. period, T

1.2.1. time taken to complete one complete cycle.

1.2.2. unit: s, seconds

### 1.3. frequency, ƒ

1.3.1. number of complete cycles per second

1.3.1.1. ƒ = 1/T

1.3.2. unit: Hz, Hertz

### 1.4. wavelength, λ

1.4.1. horizontal length between two crests/two troughs

1.4.2. unit: same as length

### 1.5. wave speed, v

1.5.1. speed of travelling wave

1.5.1.1. v = λ/T

1.5.1.2. so, v = λƒ

1.5.2. unit: same as speed

## 2. Wave behaviour

### 2.1. reflection

2.1.1. occurs when the waves return after it encounters an obstacle

2.1.2. direction of propagation of waves change after undergoing reflection

2.1.3. angle of reflection = angle of incidence

2.1.4. characteristics of reflected rays

2.1.4.1. speed : unchanged

2.1.4.2. wavelength : unchanged

2.1.4.3. frequency : unchanged

2.1.5. reflection of light

2.1.5.1. regular & diffuse reflection

2.1.5.1.1. parallel beam of light falls on a plane mirror, it is reflected as a parallel beam and regular reflection

2.1.5.1.2. diffuse reflection happens because, unlike a mirror, the surface of an object is not perfectly smooth

2.1.5.2. real and virtual images

2.1.5.2.1. real image: produced on a screen; formed by rays that actually pass through it

2.1.5.2.2. virtual image: cannot be formed on a screen; produced by rays which seem to come from it but do not pass through it

2.1.5.3. characteristics of reflected images

2.1.5.3.1. lateral inversion

2.1.5.3.2. same size as object

2.1.5.3.3. same distance away from mirror

2.1.5.3.4. virtual image

### 2.2. refraction

2.2.1. occurs when waves propagate from a less dense to a denser medium

2.2.2. incident & refracted rays propagate in different directions

2.2.3. angle of the incidence > angle of refraction

2.2.4. characteristics of refracted rays

2.2.4.1. speed : less in denser medium

2.2.4.2. wavelength : shorter in denser medium

2.2.4.3. frequency : unchanged

2.2.5. refraction of light

2.2.5.1. light is refracted because its speed changes when it enters another medium

2.2.5.2. higher refractive index, greater bending effect

2.2.5.3. refractive index = speed of light in vacuum / speed of light in medium

2.2.5.4. law of refraction; snell's law

2.2.5.4.1. when light is refracted , an increase in the in angle of incidence, 𝒊 produces an increase in the angle of refraction, 𝒓.

2.2.5.4.2. their sines are always in proportion

2.2.5.4.3. refractive index = n

2.2.5.5. critical angle, c

2.2.5.5.1. a certain angle of incidence which causes the angle of refraction to be 90°

2.2.5.6. total internal reflection

2.2.5.6.1. angles of incidence greater than c cause the refracted ray to disappear and all the incident light is reflected inside the denser medium

2.2.5.7. examples of refraction of light: lenses

2.2.5.7.1. concave

2.2.5.7.2. convex

### 2.3. diffraction

2.3.1. occurs when waves bend round the sides of an obstacle/spread out as they pass through a gap

2.3.2. propagation depends on width of gaps; smaller gaps = larger change of direction, spread is bigger

2.3.3. characteristics of diffracted rays

2.3.3.1. speed : unchanged

2.3.3.2. wave length : unchanged

2.3.3.3. frequency : unchanged

2.3.3.4. amplitude : smaller in diffracted rays

## 4. Properties of Waves

### 4.1. Mechanical waves

4.1.1. caused by a disturbance or vibration in matter.

4.1.1.1. matter: solid, liquid, gas; known as a medium

4.1.2. transmitted through a medium by vibrating particles and causing molecules to collide and transfer energy from one to the next

4.1.3. examples:

4.1.3.1. water waves

4.1.3.2. sound waves

4.1.3.2.1. nature of sound waves:

4.1.3.2.2. speed of sound:

4.1.3.2.3. loudness & amplitude

4.1.3.2.4. pitch & frequency

### 4.2. Electromagnetic waves

4.2.1. consist of 2 waves that oscillate perpendicular to one other

4.2.1.1. oscillating magnetic field

4.2.1.2. oscillating electric field

4.2.1.3. diagram:

4.2.2. do not need a medium to propagate; can travel through a vacuum as well as matter

4.2.3. examples:

4.2.3.1. light

4.2.3.2. microwaves

### 4.3. Transverse waves

4.3.1. wave in which oscillation of its particles of are perpendicular with the direction of wave

4.3.1.1. All EM waves = transverse waves

4.3.1.2. Transverse waves can be mechanical/electromagnetic in nature. example: water waves

### 4.4. Longitudinal waves

4.4.1. wave in which oscillation of its particles of are parallel with the direction of wave

4.4.1.1. All longitudinal waves = mechanical waves

4.4.2. series of compressions and rarefactions

## 5. EM Spectrum

5.1.1. waves of the EM field, propagating through space, carrying electromagnetic radiant energy

5.1.2. radiation - emission/transmission of energy in the form of waves/particles through space/medium

### 5.2. Properties of EM waves

5.2.1. travel through a vacuum at 300000 km/s (speed of light)

5.2.2. all are transverse waves

5.2.3. all transfer energy - source loses energy when it radiates EM waves, material gain energy when it absorbs them

### 5.3. Types of EM waves (up to down; from lowest wavelength to highest wavelength)

5.3.1. Gamma rays

5.3.1.2. produced when the nuclei of unstable atom decays (radioisotopes)

5.3.1.3. most penetrating and dangerous

5.3.1.4. uses: uranium-235 isotopes give out gamma rays used to kill cancer cells, cobalt-60 isotopes give out gamma rays which sterilise food/surgical instruments.

5.3.2. X-rays

5.3.2.1. can pass through many materials that absorb visible light.

5.3.2.1.1. short-wavelength X-rays are extremely penetrating

5.3.2.1.2. long-wavelength X-rays are less penetrating - can pass through flesh but not bone

5.3.2.2. dangerous - damage the living cells & can cause mutation and cancer.

5.3.2.3. uses: x-ray photos for dentistry & medicine to detect broken bones, security machines, can treat cancer by destroying abnormal cells if concentrated

5.3.3. Ultraviolet (UV)

5.3.3.1. invisible to the human eye

5.3.3.2. cause sun tan & produce vitamins in the skin but can penetrate deeper, causing skin cancer.

5.3.3.2.1. exposure to harmful UV rays can be prevented by: wearing protective clothing/sunscreen

5.3.3.3. causes materials to fluoresce - they absorb UV & convert its energy into visible light and glow.

5.3.3.4. uses: fluorescent lamps, UV lamp for scientific/medical purposes

5.3.4. Visible light

5.3.4.1. electromagnetic spectrum that can be detected by the human eye; light

5.3.4.1.1. sources of light:

5.3.4.1.2. nature of light:

5.3.4.2. dispersion of light

5.3.4.2.1. white light falls on a triangular glass prism; a spectrum is obtained.

5.3.4.2.2. prism separates the colours because the refractive index of glass is different for each colour

5.3.4.2.3. colours from lowest to highest refractive index: red, orange, yellow, green, blue, indigo, violet

5.3.4.3. monochromatic light - light of one wavelength and frequency

5.3.4.4. travel with the same speed of 3 × 108 m/s in air/vacuum but refracted by different amounts

5.3.5. Infrared (IR)

5.3.5.2. anything emits IR alone if it is hot but not glowing (below 500 °C)

5.3.5.3. uses: electric fire, a toaster or a grill

5.3.6.1. Microwaves

5.3.6.1.1. shortest wavelengths of all radio waves

5.3.6.1.2. uses

5.3.6.2. UVF & VHF

5.3.6.2.1. VHF = Very High Frequency (FM Radio) UHF = Ultra High Frequency (TV)

5.3.6.2.2. not reflected by the ionosphere

5.3.6.2.3. uses: local radio and for television.

5.3.6.3. LF, MF and HF

5.3.6.3.1. wavelengths of 2km to 10m

5.3.6.3.2. diffract around obstacles - can be received even if anything is in the way

5.3.6.3.3. reflected by ionosphere; makes long-distance radio reception possible