Practise Lenses. 14 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
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Lenses form images by refracting light. What is meant by the focal length of a lens?
The distance from the lens to the principal focus.
Where must the stamp be placed for the lens to act as a magnifying glass?
Closer to the lens than the principal focus.
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
Lenses form images by refracting light.
(a) How is a concave lens shown in a ray diagram? Tick (✓) one box.[1]
A vertical line with arrowheads pointing inwards at each end
A vertical line with arrowheads pointing outwards at each end
A dashed vertical line
A rectangle
(b) What is meant by the focal length of a lens?[1]
(c) A lens forms an image of an insect. The image is 36 mm tall. The insect is 12 mm tall. Calculate the magnification. Use the equation: \(\text{magnification} = \dfrac{\text{image height}}{\text{object height}}\)[2]
(d) Explain why magnification has no unit.[1]
Show the answer and mark scheme
(a)Answer: A vertical line with arrowheads pointing inwards at each end
(b)Answer: The distance from the lens to the principal focus.
the distance from the lens to the principal focus
(c)Answer: 3
36 ÷ 12
3
(d)Answer: It is a ratio of two heights in the same unit.
it is a ratio of two lengths measured in the same unit (so the units cancel)
Question 2Medium6 marks
Hassan uses a convex lens as a magnifying glass to look at a postage stamp.
(a) Where must the stamp be placed for the lens to act as a magnifying glass?[1]
(b) Give two words that describe the image Hassan sees.[2]
(c) The letters printed on the stamp are 2.5 mm tall. The magnification of the lens is 4.0. Calculate the height of the image of the letters. Use the Physics Equations Sheet.[2]
(d) Explain why the image cannot be projected onto a screen.[1]
Show the answer and mark scheme
(a)Answer: Closer to the lens than the principal focus.
between the lens and the principal focus / closer to the lens than the focal length
(b)Answer: Any two of: virtual, upright, magnified.
virtual
upright / the right way up
magnified / larger than the object
(c)Answer: 10 mm
4.0 = image height ÷ 2.5
10 (mm)
(d)Answer: It is virtual: the rays only appear to come from it.
it is a virtual image: the rays of light do not actually meet there, they only appear to come from the image
Question 3Hard6 marks
Compare the ways in which convex lenses and concave lenses refract light and the images they produce. You may use ray diagrams in your answer.[6]
Show the answer and mark scheme
Answer: Level-of-response answer comparing converging/diverging of rays, real/virtual images, orientation and size of the images.
both lenses form images by refracting light
a convex lens brings parallel rays of light to a focus at the principal focus (converges light)
a concave lens spreads parallel rays out (diverges light) so they appear to come from the principal focus
the image formed by a convex lens can be real or virtual; a concave lens always forms a virtual image
convex lens with the object further away than F: real, inverted image (larger or smaller depending on the distance), e.g. in a camera or projector
convex lens with the object closer than F: virtual, upright, magnified image (magnifying glass)
concave lens: image is always virtual, upright and smaller than the object, on the same side as the object
ray diagram symbols: convex lens drawn with outward-pointing arrows, concave lens with inward-pointing arrows
magnification = image height ÷ object height; greater than 1 is possible for a convex lens but not for a concave lens
Marked with levels of response: the full level descriptors are in the app.