The real image of a converging lens
This page was translated from German with AI. It has not been checked by a native speaker. Original page in German
The presentation, the construction and the worksheet are in German. In the figures, \(g\) is the object distance and \(b\) the image distance (German: Gegenstandsweite, Bildweite).
Introduction: the projector
An animated projector. Watch where the light comes from and where the image appears.
The presentation
These are the slides from the lesson. Use the arrows at the bottom right or the arrow keys. You can fill in the boxes for your guess and your observation yourself. GeoGebra on the last slide only loads when you tap it.
The key points
In the experiment you project the screen of a phone onto the wall with a lens. This only works with the converging lens, not with the diverging lens.
The real image
A converging lens brings the light that leaves one point of the object together again in one point: the image point. This creates an image behind the lens that you can catch on a screen – a real image. It is upside down. A diverging lens does not create a real image.
Two special rays
The parallel ray runs parallel to the optical axis and goes through the focal point \(F\) behind the lens. The focal ray goes through the focal point \(F\) in front of the lens and runs parallel to the axis behind the lens.
Object distance and image distance
The object distance \(g\) is the distance from the object to the lens, the image distance \(b\) is the distance from the image to the lens.
Constructing with the set square
The construction to copy: a pencil draws step by step and the set square moves into place. Continue with the button „Weiter“ or the arrow key.
The worksheet
Copy each figure onto squared paper and construct the image. Afterwards you can open the solutions.
Show the worksheet solutions
Task 1: \(g = 9\) cm, \(b = 4.5\) cm. The image is 1 cm high, so smaller than the object.
Task 2: \(g = 5\) cm, \(b = 7.5\) cm. The image is 3 cm high, so bigger – like in a projector.
Task 3: \(g = 6\) cm, \(b = 6\) cm. The image is 2 cm high, so exactly the same size. If the object is twice the focal length away, \(b = g\).
Try it yourself
Task
A converging lens has the focal length \(f = 2\) cm. An arrow stands \(g = 6\) cm in front of the lens. Construct the image. How big is the image distance? Is the image bigger or smaller than the arrow?
Show solution
The rays cross at \(b = 3\) cm behind the lens. The image is half as big as the arrow and upside down.
Keep learning with a language model
You are my physics teacher in year 8. Our topic: the real image of a converging lens. I know the terms focal point F, focal length f, parallel ray, focal ray, object distance and image distance. Ask me five short questions or tasks, one after the other, for example: What happens to the image when the object moves closer to the lens? Always wait for my answer. Then tell me whether it is right and explain briefly why. Do not give away the answer beforehand.
How this was made: the idea, the plan of the lesson and the experiment are mine. I made the projector video with Blender. The image sequence that the projector throws onto the wall was generated by AI; I edited it afterwards.
A language model (LLM) built the slides, the construction, the worksheet and this page following my instructions. The translations were also made with AI; no native speaker has checked them.
Slides, worksheet, texts and prompt are licensed under CC BY 4.0: you may use, change and share them if you name the source.