Each optical device uses a lens because of the image it forms. The lens type and the object position decide which image you get, and the device is built around that image.
This lesson is part of light and optics and uses the table from explaining image formation.
How do convex and concave lenses differ in use?
A convex lens brings light together. A concave lens spreads light apart. Applications follow from that one difference.
| Device | Lens | Object position | Image formed | Why it works |
|---|---|---|---|---|
| Camera | Convex | Beyond 2F | Real, inverted, diminished | Fits a scene onto a small sensor |
| Magnifying glass | Convex | Inside F | Virtual, upright, magnified | Makes small print look larger |
| Projector | Convex | Between F and 2F | Real, inverted, magnified | Enlarges a slide onto a screen |
| Spectacles for short sight | Concave | Distant objects | Virtual, upright, diminished | Spreads light before the eye |
Worked example: explaining one device
Question: Explain why a convex lens is used in a camera.
Sentence 1: the lens type. A convex lens bends rays inwards so that they meet and form a real image.
Sentence 2: the object position. The scene is far away, beyond 2F, so the image forms between F and 2F.
Sentence 3: the image. The image is real, inverted and diminished. A real image can be captured on the sensor, and the small size fits the sensor.
A three-sentence pattern of type, position and image covers most device questions.
Comparing two devices
Question: Compare a camera and a magnifying glass. The two use the same type of lens but form different images.
A camera has the object beyond 2F, whereas a magnifying glass has the object inside F. The camera forms a real, inverted, diminished image that can be recorded, but the magnifying glass forms a virtual, upright, magnified image that can only be seen by the eye.
This single answer names the lens, the position and the image for both devices, which is what the mark scheme looks for.
The mistake that costs marks
The common slip is to say only what the device does, such as “a camera takes photos”. That states the use but not the science.
| Weak answer | Stronger answer |
|---|---|
| A magnifying glass makes things big | A convex lens with the object inside F forms a magnified, upright, virtual image |
| A camera has a lens | A convex lens forms a real, inverted, diminished image on the sensor |
| Glasses help you see | A concave lens spreads light so a distant image falls on the retina |
If a question names the device, always add the lens type and the image properties.
Check yourself
A slide projector uses a convex lens. Explain where the slide is placed and why the picture on the screen is upright although the image is inverted.
Answer
The slide is placed between F and 2F of the lens. The lens forms a real, magnified and inverted image on the screen.
The slide is loaded upside down, so the inverted image appears the right way up to the audience.
What to study next
Some devices use more than one lens, so their diagrams need careful reading. Continue with interpreting optical-device diagrams. Use the lens and image-formation explorer to check your device answers.
To go through device questions with a teacher, see online one-to-one Science tuition.