كيف يحسب الرقم الهيدروجيني pH للمحاليل؟ How to calculate pH and pOH?

يحسب الرقم الهيدروجيني بالعلاقة: pH = -log[H⁺]، وللمحاليل المائية عند 25°C: pH + pOH = 14. إذا كان pH 7 فالمحلول قاعدي. [الصيغة الرمزية الموزونة: pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴]

Theme C — Wave Behaviour

C.3.3 Diffraction of Waves — IB Physics HL Revision Notes & Topical Questions

5 min read 9 parts Lesson 19 of 37 1 quiz

📚 IB Physics HL Hub / Topical Revision

Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution

pH = -log[H⁺]  |  pOH = -log[OH⁻]  |  pH + pOH = 14  |  [H⁺][OH⁻] = 10⁻¹⁴

Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14

Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.

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Lesson summary

C.3.3 Diffraction of Waves — IB PHYSICS SL notes: Explain diffraction at a gap or around an obstacle and predict how the spreading depends on the ratio of wavelength to gap width.Diffraction — the spreading of a wave after it passes through a narrow gap or around an obstructionPa…

Observation
Gap much larger than λ
Gap about equal to λ
Gap smaller than λ
Unchanged quantities
Edge effect

Part 1 of 7What diffraction is

Diffraction

Diffraction the spreading of a wave after it passes through a narrow gap or around an obstruction
Particles travelling in straight lines would give a sharp shadow, but waves bend into the geometrical shadow region.

All waves

All waves water, sound, light, microwaves and radio waves all diffract

Evidence

Evidence diffraction was important evidence for the wave nature of light, and later of electrons

Observation

Observation plane waves sent towards a barrier with an adjustable gap in a ripple tank show it

Part 2 of 7Diffraction at a gap

Result

Result the waves spread out and the wavefronts become curved
The amplitude is smaller because the barrier on either side absorbs wave energy.

Gap much larger than λ

Gap much larger than λ there is very little spreading
The waves continue as an almost plane beam, with slight bending at the edges and a clear shadow.

Gap about equal to λ

Gap about equal to λ the spreading is greatest
The wave emerges with almost semicircular wavefronts.

Gap smaller than λ

Gap smaller than λ the spreading is still large but the transmitted amplitude becomes very small

Trend

Trend as the gap grows relative to λ, the curvature of the wavefronts becomes less pronounced

Unchanged quantities

Unchanged quantities wavelength, frequency and speed are the same before and after the gap
Keep the spacing of the wavefronts constant when drawing.

Part 3 of 7Diffraction around a barrier

Edge effect

Edge effect waves curve around the edge and spread into the region behind the object

Wavelength dependence

Wavelength dependence the greater the wavelength, the greater the diffraction

Barrier larger than λ

Barrier larger than λ there is some diffraction, much of the wave is reflected and a shadow region forms behind the barrier

Barrier about equal to λ

Barrier about equal to λ there is more diffraction and a smaller shadow region

Barrier smaller than λ

Barrier smaller than λ there is almost no shadow, because the wave passes round the barrier

Part 4 of 7Explanation using Huygens

Wavelets

Wavelets each point of the wavefront in the opening acts as a source of secondary wavelets

Middle of the gap

Middle of the gap the wavelets combine to give a plane wavefront

Edges

Edges there are no neighbouring wavelets to complete the envelope, so the wavefront curves into the shadow

Narrow gap

Narrow gap only a few sources act, so the curved edge dominates and the beam looks like a spreading circular wave

Part 5 of 7Everyday consequences

1Sound

Sound wavelengths of centimetres to metres are comparable to doorways, so sound is heard around corners
Audible sound has λ from about 17 mm to 17 m.

2Light

Light wavelengths of about 5 × 10⁻⁷ m are tiny compared with everyday gaps, so ordinary shadows look sharp
Diffraction still occurs at the edges.
Visible diffraction needs sizes of a few micrometres to a fraction of a millimetre.

3Radio

Radio long-wave radio of hundreds of metres diffracts round hills and buildings
Short-wave signals such as television and mobile phones need a clearer line of sight.

4Microwave oven

Microwave oven the door mesh has holes much smaller than the 12 cm wavelength, so microwaves are blocked while light passes through

5X-rays

X-rays their wavelengths match the spacing between atoms, so crystals diffract them

Part 6 of 7Resolution of instruments

Aperture limit

Aperture limit light through a circular aperture of diameter b spreads by an angle of about λ/b

Blurring

Blurring two close objects blur together when their angular separation is of that order

Improvement

Improvement larger telescope mirrors and shorter wavelengths give finer detail

Electron microscopes

Electron microscopes they resolve far smaller structures than optical microscopes because of the very small electron wavelength

Part 7 of 7More from the specification

Diffraction of Waves

The spreading of wavefronts as they pass through an aperture or around an obstacle, becoming most pronounced when the aperture width is comparable to the wavelength.

Refraction and Refractive Index

When light or wave energy passes across the boundary between two media, the change in wave speed and refractive index alters the direction of propagation according to Snell's law.

Constructive and Destructive Interference

Superposition of two coherent wavefronts results in constructive interference when their path difference equals an integer number of wavelengths,
and destructive interference when it equals a half-integer wavelength.

Phase Boundary Changes

Waves reflecting from the surface of an optically denser medium with a higher refractive index experience a phase change of pi radians (180 degrees), shifting interference fringe conditions.

Superposition

Resultant displacement is the sum of the displacements of the individual waves.

Coherent

Constant phase difference and same frequency.

Transmission

Sending electricity along power cables from the power station to consumers.

Critical angle

Incline angle at which sliding just begins.

Optically dense

Having a high refractive index

Phase difference

How far one oscillation is ahead of or behind another.

Interference

Effect of the overlap of waves.

Refraction

Change in direction of light when it changes speed between media.

Boundary

Surface between two media at which waves are partially reflected

Constructive

Waves in phase giving larger amplitude.

Medium

Material through which light travels.

Destructive

Waves in antiphase giving reduced amplitude.

Normal

Line at 90° to a surface.

n1 sin θ1

n1 sin θ1 n2 sin θ2

What the symbols mean: n = refractive index (no unit) · i or θ₁ = angle of incidence · r or θ₂ = angle of refraction (both measured from the normal)

Key terms

Term	Meaning
Diffraction	Spreading of waves on passing through a gap or around an obstacle.
Gap width, b	Width of the aperture compared with the wavelength.
Geometrical shadow	Region where a straight-line (ray) model predicts no wave.
Huygens wavelets	Secondary sources used to explain spreading at a gap.

Avoid these mistakes

Saying the wavelength changes in diffraction — it does not.

Thinking diffraction happens only at narrow gaps — it always happens but is noticeable only when the gap is comparable to λ.

Drawing wavefronts beyond a gap with a smaller spacing than before — the spacing is unchanged.

Believing light does not diffract because shadows look sharp — its wavelength is just very small.

Assuming a wider gap gives more spreading — a narrower gap relative to λ gives more.

Last updated: 15 October 2026