Lenses and visible light (physics only) – GCSE Physics exam questions with mark scheme

1Grade 1–3FoundationTriple only4 marks

The diagram shows the symbols for two different lenses, A and B.

(a) Which lens, A or B, is a concave lens? [1]

(b) Rays of light parallel to the principal axis pass through a convex lens. After the lens the rays all pass through one point.

Give the name of this point. [1]

(d) Which type of lens, convex or concave, always produces a virtual image? [1]

(a) B(1)
(b) principal focus (allow: focal point; do not accept 'focus' alone)(1)
(c) focal length(1)
(d) concave(1)
Answer: (a) B (b) principal focus (c) focal length (d) concave
Examiner tip: Learn the symbols: a convex lens has arrowheads pointing outwards at the ends, a concave lens has them pointing inwards.

2Grade 6–7Triple only5 marks

A projector uses a convex lens to form an image of a small picture on a screen.

The picture is 24 mm tall. The image on the screen is 1.8 m tall.

(a) Calculate the magnification produced by the lens. [3]

(b) Explain why magnification has no unit. [1]

(a) 1.8 m = 1800 mm (allow: 24 mm = 0.024 m / 2.4 cm and 180 cm — both heights in the same unit)(1)
(a) magnification = 1800 ÷ 24 (allow: 1.8 ÷ 0.024)(1)
(a) 75 (allow: 75 with no unit; an answer of 0.075 or 0.75 scores 0 for this mark)(1)
(b) it is the ratio of two lengths / both heights have the same unit so the units cancel(1)
(c) real and it is formed on a screen (allow: the light rays actually meet / pass through the image; no mark for 'real' alone)(1)
Answer: (a) 75 (b) it is a ratio of two heights in the same unit, so units cancel (c) real — it can be formed on a screen
Examiner tip: Convert both heights to the same unit before dividing: 1.8 ÷ 24 = 0.075 is the classic wrong answer.

3Grade 6–7Triple only6 marks

The diagram shows a scale ray diagram for an object placed in front of a convex lens. Two rays from the top of the object have been drawn. Point P is on the principal axis.

(a) Use the diagram to determine the focal length of the lens. [2]

(b) Describe the image formed by the lens. [3]

(a) P is the principal focus / the focal length is the distance from the centre of the lens to P (may be implied by a correct answer)(1)
(a) 4 cm (allow: 4.0 cm; must have unit)(1)
(b) real(1)
(b) inverted (allow: upside down)(1)
(b) diminished (allow: smaller than the object / magnification less than 1; ignore 'closer to the lens')(1)
(c) 0.5 (allow: ½; image height 2 cm ÷ object height 4 cm; do not accept 2 or a negative value)(1)
Answer: (a) 4 cm (b) real, inverted, diminished (c) 0.5
Examiner tip: When describing an image, give all three properties: real or virtual, upright or inverted, and magnified or diminished.

4Grade 6–7HigherTriple only3 marks

The diagram shows rays of light parallel to the principal axis passing through a concave lens. The dashed lines show where the rays appear to come from.

(a) Give the name of point X. [1]

(b) Explain why a concave lens always produces a virtual image. [2]

(a) (virtual) principal focus (allow: focal point)(1)
(b) the rays spread out / diverge after passing through the lens (so the rays never actually meet)(1)
(b) the image is where the rays only appear to come from (when traced back) so it cannot be formed on a screen (allow: the image is formed by rays extended backwards)(1)
Answer: (a) principal focus (b) the lens makes rays diverge, so they never meet; the image is only where the rays appear to come from, so it is virtual
Examiner tip: A virtual image is formed where rays appear to come from — say that the real rays never meet, not just that the image is 'not real'.

5Grade 6–7Triple only5 marks

A jeweller uses a convex lens as a magnifying glass to look at a scratch on a ring.

(a) Where must the ring be placed so that the lens acts as a magnifying glass? [1]

(b) The image seen through a magnifying glass is a virtual image. What is meant by a virtual image? [1]

Calculate the actual length of the scratch in millimetres. [3]

(a) between the lens and the principal focus (allow: closer to the lens than the focal length / less than one focal length from the lens)(1)
(b) an image that cannot be projected onto a screen (allow: the light rays only appear to come from the image / the rays do not actually pass through it)(1)
(c) object length = 2.7 ÷ 6 (allow: 6 = 2.7 ÷ object length)(1)
(c) = 0.45 (cm) (allow: 27 mm ÷ 6 for this and the previous mark)(1)
(c) 4.5 (mm) (an answer of 0.45 mm scores 2 marks; 16.2 or 162 scores 0)(1)
Answer: (a) between the lens and its principal focus (b) cannot be formed on a screen; rays only appear to come from it (c) 4.5 mm
Examiner tip: Check which height you are finding: the scratch is the object, so divide the image length by the magnification, then convert to mm.

6Grade 4–5FoundationTriple only5 marks

An actor wears a white shirt, a red tie and black trousers. In white light each item appears its own colour.

The stage lights are changed so that only one colour of light shines on the actor.

Colour of light	Shirt	Tie	Trousers
red	(i)	red	black
green	(ii)	(iii)	black

(a) Complete the table by giving the colours (i), (ii) and (iii). [3]

(b) Explain why the tie appears the colour you gave for (iii). [2]

(a)(i) red(1)
(a)(ii) green(1)
(a)(iii) black(1)
(b) the tie (only) reflects red light / red wavelengths(1)
(b) the tie absorbs green light (so no light is reflected from it) (allow: there is no red light to reflect; ignore 'the tie absorbs all light' unless linked to green light)(1)
Answer: (a) (i) red (ii) green (iii) black (b) the tie only reflects red light; it absorbs the green light, so no light reflects from it and it appears black
Examiner tip: An opaque object only reflects its own colour; if that colour is not in the light shining on it, it looks black.

7Grade 4–5Triple only4 marks

(a) On a still day a clear image of the trees around a lake can be seen in the water. On a windy day no clear image can be seen.

Explain why. [2]

(b) A bathroom window is made from frosted glass. Light passes through the window, but people outside cannot see clearly into the bathroom.

Explain why. Use the words transparent or translucent in your answer. [2]

(a) still water is a smooth surface so gives specular reflection (allow: light is reflected in a single direction)(1)
(a) waves make the surface rough so the light is reflected in many different directions / diffuse reflection (allow: scattered)(1)
(b) frosted glass is translucent (do not accept 'transparent' for frosted glass)(1)
(b) the light is scattered as it passes through (so no clear image is formed) (allow: light is transmitted but in many directions; ignore 'light is absorbed')(1)
Answer: (a) Still water is smooth: specular reflection (single direction) forms an image; ripples make it rough: diffuse reflection scatters light (b) frosted glass is translucent: it transmits light but scatters it, so no clear image
Examiner tip: Transparent materials let light through without scattering; translucent materials let light through but scatter it, so you only get one mark for naming the word.

8Grade 6–7Triple only6 marks

A student has a convex lens, a lamp with a small object (an arrow cut into a card) in front of it, and a white screen.

Describe a method the student could use to investigate how the magnification produced by the lens depends on the distance between the object and the lens.

Include any other equipment needed. [6]

place the object, lens and screen in a straight line along a metre rule (allow: on a bench with the lens in a holder)(1)
measure the height of the object with a ruler(1)
set the object at a measured distance from the lens (using a metre rule)(1)
move the screen until a sharp / focused image is formed on it(1)
measure the height of the image on the screen(1)
calculate magnification = image height ÷ object height (heights in the same unit)(1)
repeat for a range of (at least 5) different object distances(1)
object distances must be greater than the focal length (otherwise no real image forms on the screen)(1)
keep the same lens / same object throughout (control variables)(1)
carry out in a darkened room so the image is easier to see(1)
repeat each measurement and calculate a mean(1)
plot a graph of magnification against object distance(1)

Level	Marks	What it looks like
Level 3	5–6	A coherent method that could be followed to obtain valid results: positions object, lens and screen; focuses a sharp image; measures object and image heights and the object distance; calculates magnification; repeats for a range of object distances with at least one control or safety/accuracy point.
Level 2	3–4	A method that would produce some results: includes forming an image on the screen and measuring its height, and changing the object distance, but some steps are missing or not in a logical order (e.g. magnification not calculated or image not focused).
Level 1	1–2	Simple relevant statements, e.g. 'move the lens' or 'measure the image', with no clear method.
No relevant content	0	Nothing worthy of credit.

Answer: Line up object, lens and screen on a metre rule; measure object height; set measured object distance; move screen until image is sharp; measure image height; magnification = image height ÷ object height; repeat for several object distances (all beyond the focal length) with the same lens; darken the room; repeat and average; plot a graph.
Examiner tip: Many answers forget to say the screen is moved until the image is SHARP — without that, the image height measured is meaningless.

9Grade 4–5FoundationTriple only3 marks

A student uses a magnifying glass to look at an ant.

The ant is 6 mm long. The magnification produced by the magnifying glass is 4.

(a) Calculate the length of the image of the ant. [2]

(b) What type of lens is used in a magnifying glass? [1]

(a) image length = 4 × 6 (allow: 4 = image length ÷ 6)(1)
(a) 24 mm (allow: 2.4 cm; unit required for this mark)(1)
(b) convex (allow: converging)(1)
Answer: (a) 24 mm (b) convex lens
Examiner tip: Magnification = image height ÷ object height, so image height = magnification × object height — a magnifying glass makes the image bigger, so your answer must be more than 6 mm.

10Grade 6–7HigherTriple only7 marks

The table shows the range of wavelengths for some colours of visible light.

Colour	blue	green	yellow	orange	red
Wavelength in nm	450–495	495–570	570–590	590–620	620–700

The graph shows how the percentage of light transmitted by a colour filter depends on the wavelength.

(a) Determine the percentage of light of wavelength 610 nm that is transmitted by the filter. [1]

(b) Name the colour of the filter. Use the graph and the table to explain your answer. [2]

(c) White light passes through this filter and then through a blue filter. Explain what is seen after the blue filter. [2]

(d) Calculate the frequency of light with a wavelength of 650 nm.

speed of light = $3.0 \times 10^8$ m/s 1 nm = $1 \times 10^{-9}$ m [2]

(a) 40 (%) (allow: 38–42)(1)
(b) red and it transmits most light with wavelengths above 620 nm / wavelengths in the red range (allow: transmits wavelengths from about 600 to 700 nm)(1)
(b) it absorbs / does not transmit blue, green and yellow light (wavelengths below 590 nm) (allow: absorbs the other colours; allow: transmits no light below 590 nm)(1)
(c) no light / darkness (allow: black)(1)
(c) only red (and orange) light reaches the blue filter and the blue filter absorbs it (allow: the blue filter only transmits blue light, which has already been absorbed by the first filter)(1)
(d) $f = \dfrac{3.0 \times 10^8}{650 \times 10^{-9}}$ (allow: correct substitution into $v = f\lambda$)(1)
(d) $4.6 \times 10^{14}$ Hz (allow: $4.62 \times 10^{14}$; must have unit; a power-of-ten error scores 1 mark)(1)
Answer: (a) 40% (b) red — transmits wavelengths in the red range (above about 620 nm), absorbs blue, green and yellow (c) no light: the blue filter absorbs the red light (d) $4.6 \times 10^{14}$ Hz
Examiner tip: Two filters in a row only let through wavelengths that BOTH filters transmit — red then blue leaves nothing.

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