TwiddleLab / Physics / Light and Sight

Lenses Make Images

Lesson 5 of 10 · 7 min · Free, no account needed

What you will learn

• A converging lens brings parallel rays together at the focal point F. The distance from the lens to F is the focal length f.
• An object far away gives a small, upside-down real image (camera); between F and 2F, a big real image (projector); inside F, an upright, bigger virtual image (magnifier).
• A ray diagram uses two easy rays: parallel then through F, and straight through the centre. Image size ÷ object size = v ÷ u.

Start the lesson

How the lesson goes

1. A phone camera, a magnifying glass and a projector use the same kind of lens. How?

   First: where must the screen go to show this candle sharp?

2. Shine a parallel beam through the lens.

3. Move the candle. Make a camera, a projector and a magnifier.

   The dashed yellow line is your screen guess.

4. Converging lenses.

   A converging lens is thicker in the middle. Rays parallel to its axis meet at the focal point F; the focal length f is the distance from the lens to F.

   A real image forms where rays really meet: you can catch it on a screen. It is upside down.

   A virtual image is where rays only seem to come from: you see it through the lens. A magnifier makes one when the object is inside F.

   A ray diagram needs two rays: parallel then through F, and straight through the centre. Simplified: an ideal thin lens. The rule 1/f = 1/u + 1/v is in Practice.

5. Draw the ray diagram yourself.

   Two rays from the flame tip. Where they cross is the image.

6. Set up a projector for a wall 3 m away.

   The slide sits 12 cm behind the lens. Choose the focal length.

   A projector puts the slide just outside F. Then the rays meet far away: here 1/v = 1/11.54 - 1/12 gives v = 300 cm. The image is v ÷ u = 300 ÷ 12 = 25 times bigger, and upside down, which is why slides go in upside down.

7. Game: Focus the camera. Four subjects, against the clock.

   A blurry snap costs 3 seconds.

8. Math: the ray diagram hides two similar triangles.

   Move the candle and compare the ratios. Which one always matches the size ratio?

   The centre ray makes two triangles with the same angles (vertical angles at the lens centre, right angles at the axis). Similar triangles scale every side by the same factor, so image height ÷ object height = v ÷ u.

9. Where must the candle be for the lens to act as a magnifier?

   Inside F the lens cannot bring the rays together; it only makes them spread less. Your eye traces them back to a point behind the object, farther away and bigger: a virtual, upright image.

10. Your camera takes its photos.

Previous lesson: Total Internal Reflection: Light in a Pipe · Next lesson: Your Eye and Glasses · Part of the course: Light and Sight

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