How do converging and diverging lenses form images, and what is the power of a lens?
Refraction and lenses: refraction in a glass block (core practical), converging and diverging lenses, real and virtual images, and the power of a lens linked to focal length.
A focused answer to Edexcel GCSE Physics 5.4 to 5.6 and 5.9 (separate physics), covering refraction in a glass block (core practical), how converging and diverging lenses refract light, real and virtual images, ray diagrams, and the power of a lens linked to its focal length and shape.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
Edexcel statements 5.4 to 5.6 and the core practical 5.9 (separate physics) want you to investigate refraction in a rectangular glass block, to use ray diagrams to show the refraction of light by converging and diverging lenses, to explain how lenses produce real and virtual images, and to relate the power of a lens to its focal length and shape.
The refraction core practical
This practical lets you see that light bends towards the normal entering the glass (slowing down) and away from the normal leaving it (speeding up), and that it emerges parallel to its original direction (just shifted sideways) from a rectangular block. Repeating for several angles of incidence shows that a larger angle of incidence gives a larger angle of refraction. Measuring carefully from the normal is the key skill.
Converging and diverging lenses
To draw a ray diagram you use rays whose paths are known: a ray parallel to the axis passes through (or appears to come from) the principal focus, and a ray through the centre of the lens carries straight on. Where the rays meet is the image. A real image forms where rays actually cross; a virtual image forms where they only appear to come from.
Power and focal length
A fat, strongly curved converging lens bends light a lot, so its focal length is short and its power is high; a thin, gently curved lens has a long focal length and a low power. The relationship is inverse: a shorter focal length means a more powerful lens. This is why strong reading glasses use fatter, more curved lenses.
How Edexcel examines this
These statements are separate-physics only. The core practical is examined as a method question worth four to six marks, where the mark scheme rewards drawing around the block, tracing the incident and emerging rays to find the path inside, drawing the normal, and measuring both angles from the normal, with repeats. Lens questions ask you to complete or interpret a ray diagram, classify the image as real or virtual, upright or inverted, magnified or diminished, and state which lens type produces it; accuracy in the standard rays (parallel ray through the focus, ray through the centre undeviated) is rewarded. The focal length and power link is a frequent two or three mark question: define the focal length as the lens-to-principal-focus distance and state that a shorter focal length (a more curved lens) gives a higher power. Examiners penalise reversing this relationship, so anchor it with the idea that a fatter lens bends light more. You may also be asked to explain why a magnifying glass (a converging lens used close to an object) gives a magnified virtual image.
Try this
Q1. State what is meant by the focal length of a lens. [1 mark]
- Cue. The distance from the centre of the lens to the principal focus.
Q2. State the type of image always formed by a diverging lens. [1 mark]
- Cue. A virtual image (upright and smaller).
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20204 marksDescribe a core practical to investigate refraction of light through a rectangular glass block. State the measurements taken.Show worked answer →
Place a rectangular glass block on paper and draw around it. Shine a ray of light into one face at an angle and mark the incident ray, then mark where the ray leaves the block to trace the refracted path inside (2 marks for tracing the incident and refracted rays). Remove the block, draw the normal at the point of entry, and measure the angle of incidence and the angle of refraction from the normal with a protractor (2 marks). Repeat for several angles of incidence. Markers reward tracing the ray through the block, drawing the normal, and measuring both angles from the normal, ideally with repeats for different incidence angles.
Edexcel 20223 marksA converging lens has a focal length of . State what is meant by the focal length, and explain how the power of the lens depends on the focal length.Show worked answer →
The focal length is the distance from the centre of the lens to the principal focus, the point where rays parallel to the axis converge after passing through the lens (1 mark). The power of a lens is greater for a shorter focal length: a more strongly curved (fatter) converging lens bends light more, has a shorter focal length, and therefore a higher power (2 marks). Markers reward defining the focal length as the lens-to-principal-focus distance and stating the inverse relationship that a shorter focal length means a higher power (a more powerful lens).
Related dot points
- Reflection and total internal reflection: the law of reflection, specular versus diffuse reflection, the critical angle, and the conditions for total internal reflection.
A focused answer to Edexcel GCSE Physics 5.1 and 5.2 (separate physics), covering the law of reflection, the difference between specular and diffuse reflection, the critical angle, the conditions for total internal reflection, and its uses in optical fibres and prisms.
- Colour and filters: how the colour of an opaque object depends on the wavelengths it reflects and absorbs, and how a colour filter transmits some colours and absorbs others.
A focused answer to Edexcel GCSE Physics 5.3 (separate physics), covering why opaque objects appear a particular colour through reflection and absorption of different wavelengths, why objects look black or white, and how colour filters transmit and absorb light.
- Reflection, refraction, transmission and absorption: what happens to waves at a boundary, why refraction changes the direction and speed of a wave, and the wavefront explanation.
A focused answer to Edexcel GCSE Physics 4.9 and 4.10, covering what happens to waves at a boundary between materials (reflection, refraction, transmission and absorption), why refraction changes a wave's direction and speed, and the wavefront explanation of refraction.
- The electromagnetic spectrum: the seven groups in order, the shared properties of EM waves, and the trends in wavelength, frequency and energy across the spectrum.
A focused answer to Edexcel GCSE Physics 5.7 to 5.13, covering the seven groups of the electromagnetic spectrum in order, the shared properties of electromagnetic waves, the trends in wavelength, frequency and energy, and the limited range our eyes can detect.
- Uses and dangers of EM waves: the practical uses of each group, the harm high-frequency waves can cause to cells, and how the use links to the wave's properties.
A focused answer to Edexcel GCSE Physics on the uses and dangers of electromagnetic waves, covering the uses of radio, microwave, infrared, visible, ultraviolet, X-ray and gamma radiation, the harm caused by high-frequency waves, and how each use links to the wave's properties.
Sources & how we know this
- Pearson Edexcel GCSE (9-1) Physics (1PH0) specification — Pearson (2016)