Lenses; visible light (physics only) — AQA GCSE Physics

SPEC 4.6.2.5 + 4.6.2.6 • LENSES & VISIBLE LIGHT

Lenses & Visible Light

Two halves: lens ray diagrams + magnification, then how visible light reflects, transmits, and gives colour. Module 6 ends here.

🔬 PHYSICS-ONLY · NOT IN COMBINED SCIENCE · PAPER 2 · BOTH TIERS

Why This Lesson Matters

The lens half is one of the most reliable sources of standard-demand marks on Paper 2 — but specific recall failures cost students. The visible-light half is conceptually short but every term has its own spec wording.

<50%

Could identify "focal length" from a labelled lens diagram in a recent paper

50%

Scored zero on comparing convex/concave images — most failed to use the word "virtual"

20%

Drew the correct ray but forgot the arrowhead on the inverted image

What You'll Walk Away With

Both lens types and their image rules · how to construct a ray diagram step-by-step · the magnification calculation (with the "no units" trap) · specular vs diffuse reflection · how colour, filters and transparency work.

Lens Definitions & Symbols

Every lens question starts here. Get the four definitions right and the symbols correct on ray diagrams.

Definitions (Spec 4.6.2.5)

TermDefinitionConvex lens(converging)A lens that refracts parallel rays of light to bring them to a focus at a single point (the principal focus)Concave lens(diverging)A lens that refracts parallel rays of light to spread them out, as if they came from a single point (the virtual principal focus)Principal focus (F)The point where parallel rays converge (convex) or appear to come from (concave) after passing through the lensFocal length (f)The distance from the lens to the principal focus

Standard Ray Diagram Symbols (Spec Mandated)

In AQA ray diagrams, lenses are drawn as vertical lines with arrows at each end:

Convex: arrows OUTWARD (away from lens) at both ends

Concave: arrows INWARD (toward lens) at both ends

Convex Lens — Object Beyond f

The standard case: object placed beyond the focal length. The lens converges the light and forms a real, inverted image on the opposite side.

Construction — Two Rays from Top of Object

Ray 1 (parallel): draw a ray parallel to the principal axis. After the lens, it refracts through the far focal point F.

Ray 2 (central): draw a ray straight through the centre of the lens — undeviated.

Image position: where the two rays cross on the right of the lens.

Image type: real, inverted, formed on opposite side of the lens (used in cameras, projectors, the human eye).

Convex Lens — Object Inside f (Magnifying Glass)

If the object is placed closer to the lens than the focal length, the rays diverge after the lens (they don't converge). The brain projects them backwards to form a virtual image.

What You'd See

Rays diverge after passing through the lens — they never meet on the right-hand side

Extending the diverging rays backwards shows they appear to come from a position behind the object

The image is virtual, upright, enlarged, on the same side as the object

This is exactly how a magnifying glass works — small text in the focal-length region is enlarged virtually

Watch Out

A common student error is "convex always gives a real image". Not true. Convex gives a real image only when the object is beyond the focal length. Object inside f → virtual image. Both cases are legitimate exam questions.

Concave Lens — Always Virtual

A concave (diverging) lens spreads light outward, no matter where the object is. The image is always virtual, upright, and diminished.

Construction

Ray 1 (parallel): hits the lens, then diverges as if coming from the near focal point (the F on the same side as the object). The dashed line back to F shows this virtual origin.

Ray 2 (central): straight through the centre, undeviated.

Image position: the rays don't meet on the right. Extend the dashed line of Ray 1 backwards — the image is where it meets Ray 2.

Image type: virtual, upright, diminished. Always — for any object position.

Image Rules — Convex vs Concave

This table is the single most useful piece of recall for lens questions. Memorise the three rows.

SetupImage typeOrientationSizeWhere

Convex , object beyond fRealInvertedLarger/same/smaller depending on distanceOpposite side

Convex , object inside fVirtualUprightEnlargedSame side

Concave , any object positionVirtualUprightDiminishedSame side

The Compare-and-Contrast Trick

When asked for one similarity between a convex (object beyond f) and a concave image: both are "diminished" — if both objects are placed appropriately. One difference: convex is real and inverted; concave is virtual and upright. The mark scheme requires the word "virtual" — "not real" does NOT score.

Magnification — Equation & Worked Example

The single equation in this topic. Given on the Physics equation sheet.

Equation (Sheet)

magnification = image height ÷ object height

Magnification is a ratio — it has no units

Magnification > 1 → image is enlarged

Magnification < 1 → image is diminished

Image height and object height must be in the same units (both mm OR both cm)

Worked example: A convex lens is used as a magnifying glass to read small text. The text height is 1.2 mm. The image height seen through the lens is 4.8 mm. Calculate the magnification produced by the lens. 2 marks

ESUA Solution

Emagnification = image height ÷ object heightSmagnification = 4.8 ÷ 1.2Uno units (ratio)Amagnification = 4.0

Examiner Alerts — Lenses

Three recurring traps. Each one is a wasted easy mark.

✗ Loses marks

"magnification = 4 cm" or "= 4 m"Adding units to a ratio. Magnification is dimensionless.

✓ Full marks

"magnification = 4.0" (no units)Just a number. State sig figs to match the data.

✗ Loses marks

"the image is not real"Mark scheme rejects "not real" — examiners want the specific physics word.

✓ Full marks

"the image is virtual"The correct physics term. Use it explicitly.

✗ Loses marks

Draws ray diagram with no arrowhead on imageMissing arrowhead → cannot tell if image is upright or inverted → mark withheld.

✓ Full marks

Draws image as a clear arrow with arrowhead showing orientationArrowhead pointing DOWN = inverted; pointing UP = upright.

Visible Light Spectrum (Spec 4.6.2.6)

Visible light occupies a tiny slice of the full EM spectrum — but every colour within it has its own narrow band of wavelengths and frequencies.

RED ~700 nm

ORANGE

YELLOW

GREEN

CYAN

BLUE

VIOLET ~400 nm

Key Facts

Visible spectrum: roughly 400 nm (violet) to 700 nm (red)

Red has the longest wavelength and lowest frequency

Violet has the shortest wavelength and highest frequency

Each colour is a narrow band — not a single value

White light is a mixture of all visible wavelengths combined — pass it through a prism to separate the colours

Recall Trap

"White light is one frequency" is wrong. It's a mixture. Similarly "violet has a longer wavelength than red" is the reversal — easy to mix up, examiners test it.

Specular vs Diffuse Reflection

The same law of reflection (angle of incidence = angle of reflection, from the normal) applies in both cases. The difference is the surface.

Key Difference

Specular: reflection from a smooth surface in a single direction — parallel rays in produce parallel rays out. Gives a clear image (mirror).

Diffuse: reflection from a rough surface causes scattering — parallel rays in are reflected in many different directions. No clear image (paper, wall).

The law of reflection still applies at each point. It's just that on a rough surface, the normal direction varies along the surface, so each ray reflects in a different direction.

Why Objects Have Colours

The colour of an opaque object (one that doesn't transmit light) is determined by which wavelengths it reflects. Other wavelengths are absorbed.

The Rule (Spec 4.6.2.6)

The colour of an opaque object is determined by which wavelengths of light are more strongly reflected. Wavelengths that are not reflected are absorbed.

Object appearsWhich wavelengths reflect?Which wavelengths absorb?

RedRed light (long wavelength)All other colours absorbed

GreenGreen lightAll other colours absorbed

WhiteAll visible wavelengths reflected equallyNone (or very little)

BlackNone reflectedAll wavelengths absorbed

Quick Application

A red apple under white light appears red because it reflects red and absorbs all other colours. Under blue light only, the same apple would appear black — there's no red light to reflect, and the blue is absorbed.

Colour Filters

A colour filter works by absorbing some wavelengths and transmitting others. The filter's colour is the colour it lets through.

The Filter Rule

A red filter transmits red wavelengths; absorbs everything else

A blue filter transmits blue wavelengths; absorbs everything else

Pass white light through a red filter → only red light comes out

Pass green light through a red filter → no light comes out (green is absorbed)

Mini-puzzle:

Answer: Black. The blue filter only lets blue light through. The red rose reflects only red light and absorbs blue. So no light reaches your eye — the rose appears black.

Filter Logic Chain

For any "object + filter" question: ask (1) what light reaches the object? (2) what does the object reflect? (3) does that reflected light pass back through the filter? If yes → that colour. If no → black.

Transparent · Translucent · Opaque

Three terms describing how an object handles transmitted light.

TermTransmission behaviourExamples

TransparentTransmits most/all of the light passing through, with very little scattering — you can see clearly through itWindow glass · clear water · clean air

TranslucentTransmits light but scatters it — you can see brightness but not clear imagesFrosted glass · thin paper · tracing paper

OpaqueDoes not transmit light at all — light is either absorbed or reflectedWood · metal · stone · most objects

How This Connects to the Topic

The colour rules from the previous slides apply specifically to opaque objects. Transparent and translucent objects can have colour too (e.g. coloured glass), but their colour is determined by which wavelengths they transmit rather than reflect — same physics as a filter.

Practice Question

(a) A convex lens has parallel rays of light falling on it. The rays come to a focus at a point 8 cm from the lens. State the focal length of the lens. 1 mark

(b) An object 1.5 cm tall is placed in front of a convex lens. The image produced is 6.0 cm tall. Calculate the magnification produced by the lens. Use the Physics equation sheet. 2 marks

(c) A student looks at a red rose through a green filter under white-light illumination. State the colour the rose appears, and explain why. 3 marks

Part (a) — 1 mark

✓

Focal length = 8 cm. (The distance from the lens to the principal focus IS the focal length, by definition.)

Part (b) — 2 marks

E

magnification = image height ÷ object height

S

magnification = 6.0 ÷ 1.5

A

magnification = 4.0 (no units — it's a ratio)

Part (c) — 3 marks

MP1

The rose appears black.

MP2

The green filter only transmits green light — it absorbs all other wavelengths, including red.

MP3

The red rose reflects only red light and absorbs green. Either way around the chain — the green light reaching the rose is absorbed by it, OR the red reflected from the rose is absorbed by the filter — no light reaches the eye, so the rose appears black.

⚠ For a 3-mark question, the chain matters: (1) what the filter does · (2) what the object reflects · (3) what reaches the eye.

PEE Worked Example

A classic compare-and-contrast question. Apply PEE to score full marks.

Worked example: A student compares the image formed by a convex lens (with the object placed beyond the focal length) with the image formed by a concave lens. Give one similarity and one difference between the two images. 2 marks

P — Property (a similarity)

Both images can be diminished — depending on the exact object position with the convex lens, and always with the concave lens.

E — Evidence (a difference)

The convex lens (object beyond f) forms a real, inverted image. The concave lens forms a virtual, upright image.

E — Explain (why this works)

Convex lenses converge light — when the object is beyond f, the refracted rays meet on the opposite side of the lens, forming a real image that could be projected on a screen. Concave lenses diverge light — the rays spread out and only appear to come from a point behind the lens, forming a virtual image that cannot be projected.

Why PEE Wins Here

Compare-and-contrast questions require matched pairs: one property mentioned for both lenses. "Concave gives a virtual image" alone is only half a mark. Always pair: "convex gives real, concave gives virtual". One similarity + one paired difference = full marks.

Summary & Things to Remember

Lenses (4.6.2.5)

Convex (converging) — symbol: outward arrows. Object beyond f → real, inverted image. Object inside f → virtual, upright, enlarged (magnifying glass).

Concave (diverging) — symbol: inward arrows. Always virtual, upright, diminished image.

Ray construction: parallel ray + central ray. Where they meet (or appear to meet via dashed extension) = image position.

Magnification = image height ÷ object height. No units. Same length units top and bottom.

Visible Light (4.6.2.6)

Spectrum: ~400 nm (violet) to ~700 nm (red). Red = longest wavelength / lowest frequency.

Specular reflection: smooth surface, parallel rays out. Diffuse : rough surface, scattered.

Colour of opaque object: determined by which wavelengths it reflects. White reflects all; black absorbs all.

Filters: absorb certain wavelengths, transmit others. Red filter → only red light passes.

Transparent (clear) · translucent (scattered) · opaque (no transmission).

The Three Biggest Habit Wins

Use the word "virtual" — never "not real". Mark schemes are strict.

Magnification has no units. Don't add cm or m.

Always draw the arrowhead on a ray-diagram image — orientation = mark.

Module 6 Complete

This is the final lesson of Module 6 (Waves). You've covered transverse/longitudinal waves, RP8, reflection, sound, ultrasound, seismic waves, the EM spectrum (5 lessons), RP9 refraction, EM production + hazards, EM uses, black body radiation + RP10, and now lenses + visible light. Next: Module 7 — Magnetism & Electromagnetism.