OCR A Level Physics A: Electromagnetic spectrum, refraction and total internal reflection (H556) – Revision Notes

Condensed recall for the final weeks. For full explanations and worked examples, use the study guide. These notes follow the OCR Advanced GCE in Physics A (H556) specification, Version 3.0 (March 2026), section 4.4.2 Electromagnetic waves, statements (a) to (e). Related pages: practice questions, the OCR A Level Physics hub, the printable checklist and the free 10-minute diagnostics.

4.4.2(a) Spectrum and properties

Lowest frequency first: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma.

Properties common to all EM waves:

• transverse: the two fields are perpendicular to each other and to the wave’s direction
• travel through a vacuum
• identical speed in a vacuum, c = 3.00 × 10⁸ m s⁻¹
• obey c = fλ in a vacuum
• transfer energy; can be reflected, refracted, diffracted, polarised and can interfere

Higher frequency means shorter wavelength, because c is fixed.

4.4.2(b) Orders of magnitude of wavelength

Radiation	Wavelength / m (order of magnitude)
Radio	10⁻¹ to 10³ and above
Microwave	10⁻³ to 10⁻¹
Infrared	10⁻⁶ to 10⁻³
Visible	about 4 × 10⁻⁷ to 7 × 10⁻⁷
Ultraviolet	10⁻⁹ to 4 × 10⁻⁷
X-ray	10⁻¹¹ to 10⁻⁹
Gamma	below about 10⁻¹¹

Boundaries overlap. Learn the order of magnitude and the order of the regions. Check any answer against this table: a wavelength of 10⁻² m cannot be an infrared wave.

Method for a wavelength or frequency calculation:

1. Convert prefixes first (MHz to 10⁶ Hz, GHz to 10⁹ Hz, nm to 10⁻⁹ m).
2. Use c = fλ.
3. Compare your answer with the table to name the region.

Reminder: f = 1.8 GHz gives λ = (3.00 × 10⁸) / (1.8 × 10⁹) = 0.17 m, a microwave. A wave of wavelength 250 nm has f = (3.00 × 10⁸) / (250 × 10⁻⁹) = 1.2 × 10¹⁵ Hz, ultraviolet.

4.4.2(c) Polarisation

Term	Meaning
Unpolarised	oscillations in all planes at right angles to the direction of travel
Plane polarised	oscillations in one plane only
Plane of polarisation (EM)	plane of the electric field oscillation

Key points:

• Longitudinal waves cannot be polarised, so polarisation is evidence of a transverse wave.
• A polarising filter transmits only the component along its transmission direction.
• Two filters: rotate one through 360° and the light goes bright, dark, bright, dark. Dark when the filters are crossed (at 90°).
• Microwaves: a metal grille of parallel rods. Signal is a minimum when the rods are parallel to the electric field and a maximum when at 90° to it.
• One filter in front of unpolarised light gives plane polarised light of reduced intensity.
• A single filter cannot reveal polarisation by eye. A second filter is needed as an analyser.

Must-know distinction: a polarising filter for light transmits along its direction. A metal grille for microwaves blocks along the direction of its rods. Rods parallel to the field remove the wave; rods across the field let it through.

4.4.2(d)(i) Refraction

Quantity	Result
Refractive index	n = c / v (no unit, n > 1 for materials)
Snell’s law	n₁ sin i₁ = n₂ sin i₂
Constant	n sin i is the same on both sides of a boundary
Into higher n	bends towards the normal; speed and wavelength decrease
Into lower n	bends away from the normal; speed and wavelength increase
Frequency	unchanged
Wavelength	λ in material = λ in vacuum / n

Method:

1. Draw the normal and measure both angles from it.
2. Write n₁ sin i₁ = n₂ sin i₂ with the medium for each angle labelled.
3. Rearrange for the unknown, taking care with inverse sine.
4. Check that the direction of bending matches the rule.
5. Quote units, with the same precision as the data.

Worked reminder: air to perspex (n = 1.49), i = 24.0°. sin r = 0.4067 / 1.49 = 0.2730, so r = 15.8°. Speed in the perspex = (3.00 × 10⁸) / 1.49 = 2.01 × 10⁸ m s⁻¹.

A second one: light of wavelength 600 nm in a vacuum enters glass of n = 1.50. Its wavelength is 600 / 1.50 = 400 nm. The frequency is unchanged.

4.4.2(d)(ii) Practical: refraction and TIR (PAG6)

Equipment: ray box with single slit, rectangular block, semi-circular block, white paper, protractor, sharp pencil.

Refraction:

1. Draw round the block. Aim a thin ray at the block and mark two points on each ray.
2. Remove the block, join the points and draw the normal.
3. Measure i and r for at least six angles.
4. Plot sin i against sin r. Gradient = n. The line passes through the origin.

Critical angle (semi-circular block):

1. Aim the ray at the centre of the flat face through the curved surface, so it enters along a radius and is not deviated.
2. Increase the angle of incidence at the flat face until the refracted ray runs along the flat face. That angle is C.
3. Increase it further to see TIR. All the light stays in the glass and leaves through the curved face along a radius.

Reading the scale: protractor resolution gives an uncertainty of about half a degree on each angle, which matters most at small angles, where the percentage uncertainty is largest.

Improvements: thin ray, mark the centre of the beam, many readings, repeat and average C, use a larger protractor or a rotating table with a scale.

4.4.2(e) Critical angle and TIR

Item	Result
Critical angle	angle of incidence in the denser medium for which the refracted angle is 90°
Into air	sin C = 1/n
Between media	sin C = n₂ / n₁ (n₁ higher)
TIR needs	going from higher n to lower n AND angle of incidence greater than C
At the critical angle	refracted ray travels along the boundary

Worked reminder: n = 1.40 gives sin C = 0.7143, so C = 45.6°. For a glass-water boundary with n = 1.62 and 1.33, sin C = 1.33/1.62 = 0.8210, so C = 55.2°.

Must-know distinctions

• Normal versus surface: angles are always measured from the normal.
• Critical angle versus angle of refraction: C is an angle of incidence.
• Refraction versus TIR: refraction needs a transmitted ray; TIR has none.
• Speed versus frequency: speed changes, frequency does not.
• Polaroid versus grille: one transmits along its direction, the other blocks along its rods.

Quick self-test

1. State the order of the EM spectrum from lowest to highest frequency.
2. Calculate the wavelength of a 3.0 GHz radar signal.
3. Why can sound not be plane polarised?
4. Two polarising filters are crossed. What does an observer see and why?
5. Light goes from air into a plastic with n = 1.45 at i = 50.0°. Find r.
6. Find the speed of light in a material of n = 1.25.
7. What is the refractive index of a material in which light of frequency 5.0 × 10¹⁴ Hz has a wavelength of 3.0 × 10⁻⁷ m?
8. State two conditions for total internal reflection.
9. Calculate the critical angle for a material with n = 1.40 in air.
10. Light in glass of n = 1.60 meets a boundary with water of n = 1.33. Calculate the critical angle for this boundary.

Answers

1. Radio, microwave, infrared, visible, ultraviolet, X-ray, gamma.
2. λ = (3.00 × 10⁸) / (3.0 × 10⁹) = 0.10 m.
3. Sound is longitudinal. The oscillation is along the direction of travel, so there are no different planes at right angles to it.
4. Darkness. The first filter transmits oscillations along one direction, which has no component along the second filter.
5. sin r = sin 50.0° / 1.45 = 0.7660 / 1.45 = 0.5283, so r = 31.9°.
6. v = (3.00 × 10⁸) / 1.25 = 2.40 × 10⁸ m s⁻¹.
7. v = fλ = 5.0 × 10¹⁴ × 3.0 × 10⁻⁷ = 1.5 × 10⁸ m s⁻¹, so n = (3.00 × 10⁸) / (1.5 × 10⁸) = 2.0.
8. The light travels from a higher refractive index to a lower one, and the angle of incidence is greater than the critical angle.
9. sin C = 1/1.40, so C = 45.6°.
10. sin C = 1.33 / 1.60 = 0.8313, so C = 56.2°.

Where marks are usually lost

• Measuring angles from the surface.
• Forgetting to convert GHz, MHz or nm to base units.
• Saying “bends towards the normal” without linking it to the higher refractive index.
• Claiming the frequency changes on refraction.
• Using sin C = 1/n for a boundary between two materials, rather than n₂ / n₁.
• Describing TIR without the two conditions.
• In the practical, plotting i against r, or omitting that the line should pass through the origin.
• Mixing up which way a grille blocks microwaves.

Official syllabus

OCR, Advanced GCE in Physics A (H556), specification Version 3.0 (March 2026), section 4.4.2 Electromagnetic waves, for first assessment in 2017.