An image is described by three properties: real or virtual, upright or inverted, and magnified or diminished. For a convex lens, those three change as the object moves closer to or further from the lens.
This lesson follows drawing basic ray diagrams and is part of light and optics.
How does object position change the image?
For a convex lens of focal length f, the object position decides everything. The table lists the usual cases.
| Object position | Image position | Real or virtual | Upright or inverted | Size |
|---|---|---|---|---|
| Beyond 2F | Between F and 2F | Real | Inverted | Diminished |
| At 2F | At 2F | Real | Inverted | Same size |
| Between F and 2F | Beyond 2F | Real | Inverted | Magnified |
| Inside F | Same side as object | Virtual | Upright | Magnified |
Why it changes: as the object moves closer, the rays leave the lens less bent and meet further away. The image grows, until at F the rays leave parallel and no image forms.
Worked example: a lens and a screen
A convex lens has f = 12 cm. A student places a small arrow 18 cm from the lens and finds a sharp image on a screen. Describe the image.
Step 1. Compare the object distance with f and 2F. Here 2F = 24 cm, so 18 cm is between F (12 cm) and 2F (24 cm).
Step 2. Use the table. The image is real, inverted and magnified, and it sits beyond 2F on the far side.
Step 3. Check the clue in the question. The image appears on a screen, which can only happen with a real image. So the description agrees.
A full answer is: “The image is real, inverted and magnified, formed beyond 2F on the opposite side of the lens.”
The mistake that costs marks
Students write “the image is big” or “the image is upside down” without saying real or virtual. Write all three properties in the standard words.
| Step | Wrong | Right |
|---|---|---|
| Size | Big | Magnified |
| Direction | Upside down | Inverted |
| Type | (missing) | Real |
| Screen clue | Ignored | Image on a screen means real |
Another slip is mixing up the two distances. The object distance is measured from the lens to the object, and the image distance from the lens to the image.
Virtual images with a magnifying glass
A magnifying glass is a convex lens used with the object inside the focal length. Rays from the object spread after the lens, so they do not meet.
Your eye traces the rays back and sees a larger, upright, virtual image behind the object. No screen will show it.
Check yourself
An object is 40 cm from a convex lens of focal length 15 cm. Describe the image with all three properties.
Answer
2F = 30 cm, so 40 cm is beyond 2F.
The image is real, inverted and diminished. It forms between F and 2F, that is, between 15 cm and 30 cm from the lens on the far side.
What to study next
The next lesson compares how convex and concave lenses are used in cameras, spectacles and other devices. Continue with comparing lens applications. You can also vary the object distance in the lens and image-formation explorer.
To practise describing images with a teacher, see online one-to-one Science tuition.