Light and lenses: GCSE Physics
Reflection, refraction, colour, convex and concave lenses, ray diagrams and magnification
The short version
Angle of incidence equals reflection. Refraction is caused by speed change. Convex lenses converge; concave diverge. Magnification = image height / object height.
Key idea
i = angle of incidence, r = angle of reflection, both from the normal
Grade 2–3: Light bounces off a mirror with matching angles, like a snooker ball off a cushion.
Grade 5–6: I measure from the normal and I can say i = r.
Grade 9: I can describe a virtual plane-mirror image and I never confuse this r with refraction's r.
Go a bit deeper
Along the normal: speed changes, direction does not
Grade 2–3: Light bends when it goes from air into water or glass.
Grade 5–6: I can say towards the normal into glass and away when it leaves.
Grade 9: I can use n = sin i / sin r if given, and I never say light always bends towards the normal.
Worked example
Worked example
Grade 2–3: Light bounces off a mirror with matching angles, like a snooker ball off a cushion. Grade 5–6: I measure from the normal and I can say i = r. Grade 9: I can describe a virtual plane-mirror image and I never confuse this r with refraction's r.
Lessons in this topic
- Reflection of light
- Refraction
- Colour and filters
- Convex lenses
- Concave lenses and magnification
- Required practical: light
In the app each lesson is video (when we have a real YouTube id) then intro, learn and summary steps.
What the examiner wants
Draw reflection/refraction and lens ray diagrams, describe required practicals, and calculate magnification including inverted real images.
Try these questions
Cover the answers and have a go first. Worked solutions are underneath.
The law of reflection: angle of incidence equals
A plane-mirror image is
When light goes from air into glass it generally
A rectangular glass block makes the emergent ray
Common mistakes
- Angles are from the normal. Do not write that light "needs a mirror to travel". Diffuse reflection is still i = r on each tiny facet - the facets point many ways.
- Do not mix up i = r (reflection) with refraction. Light does not "always go to the normal". Leaving glass, it goes away from the normal.
- A blue filter does not turn white light into "cold" light by cooling it. Black is the absence of reflected/transmitted visible light in this model, not a pigment you add as a third primary of light.
- A real image is inverted in the simple convex cases examiners love. Do not say all convex images are virtual.
Answers and working
Frequently asked questions
Is GCSE Light and lenses on Combined Science and Triple?
Light and lenses is Triple-only Physics. Combined Science students can skip it.
Which exam board is this Light and lenses page for?
The ideas are board-agnostic for AQA, Edexcel, OCR, WJEC and CCEA. Paper lengths and question numbers differ — check your specification.
Is Light and lenses Foundation or Higher?
The core ideas appear on both tiers. Higher papers add extra application and some HT-only detail, marked in lessons with a Higher badge.
Learn, try, get feedback, fix the mistake
Open the stepped lesson, then the quiz with hints. Required-practical and calculation questions use the same method you will need in the paper.
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