Thin lenses, ray diagrams and magnification
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The teaching part (2 min) is free to watch. A free account opens the whole lecture: 2 worked examples, 4 practice questions, 2 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- Seeing small things big
- Two kinds of lens
- The vocabulary
- Three rays
- A real image
- Where is the image?
Read the transcript of the teaching part
Last time
Before we start: last time, in Refraction and refractive index, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
Seeing small things big
A magnifying glass makes a tiny insect look huge. A camera shrinks a whole mountain onto a small sensor. The same piece of curved glass does both. It depends on where you put the object, and a few simple rays will tell you what you will see.
Two kinds of lens
A converging lens is thicker in the middle. It brings parallel rays together to a point, the principal focus. A diverging lens is thinner in the middle. It spreads parallel rays out, as if they came from a point behind the lens. The focal length f is the distance from the lens to the principal focus.
The vocabulary
The principal axis is the line through the centre of the lens, at right angles to it. The principal focus F is on each side of a converging lens, at a distance f from the centre. We often mark 2F as well, at a distance of twice the focal length.
Three rays
Ray one: parallel to the axis, so after the lens it goes through the principal focus on the far side. Ray two: through the centre of the lens, and it carries straight on without bending. Ray three: through the focus on the near side, so after the lens it goes parallel to the axis. Where two of these rays cross is the position of the top of the image.
A real image
Here the object is beyond twice the focal length. The rays from the top of the object cross on the far side, and a sharp image forms. The image is real, because the rays actually meet, so it can be caught on a screen. It is inverted and smaller than the object. This is how a camera works.
Where is the image?
Object beyond 2F: the image is real, inverted and smaller. Camera. Object at 2F: the image is real, inverted and the same size. Object between F and 2F: real, inverted and magnified. Projector. Object inside F: the rays do not meet, so the image is virtual, upright and magnified. Magnifying glass.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Thin lenses and images · Linear magnification · Thin lenses and ray diagrams.
Chapter 3.2: Light
- Reflection by a plane mirror 7 min
- Refraction and refractive index 5 min
- Critical angle and total internal reflection 5 min
- Thin lenses, ray diagrams and magnification 6 min
- Uses of lenses and correcting sight 5 min
Also for O Level Physics 5054
Which topics come up most in O Level Physics 5054 past papers, counted from every paper since 2004.