Real images in a convex lens

The idea

For a convex lens, an object at infinity, beyond 2F1, at 2F1 or between F1 and 2F1 gives a real, inverted image at F2 (point-sized), between F2 and 2F2 (diminished), at 2F2 (same size) or beyond 2F2 (enlarged).

A lens and a screen

A convex lens stands between a lit candle and a small screen. A student moves the candle slowly closer to the lens and keeps moving the screen to find the sharp image.

As the candle comes closer, what happens to the place where the sharp image forms?

• It moves farther from the lens, and the image grows
• It moves closer to the lens, and the image shrinks
• It stays where it was

Four object positions

In every picture F is 20 cm from the lens. The red ray runs parallel to the axis and the purple ray goes through the optical centre. Open every tab.

The image moves out

From infinity to between F₁ and 2F₁ the image moved from F₂ out beyond 2F₂, and it grew from a point to a big image. It was real and inverted each time.

For a convex lens, an object at infinity, beyond 2F₁, at 2F₁ or between F₁ and 2F₁ gives a real, inverted image at F₂ (point-sized), between F₂ and 2F₂ (diminished), at 2F₂ (same size) or beyond 2F₂ (enlarged) respectively.

• Object | Image | Size | Nature
• At infinity | At F₂ | Highly diminished, point-sized | Real and inverted
• Beyond 2F₁ | Between F₂ and 2F₂ | Diminished | Real and inverted
• At 2F₁ | At 2F₂ | Same size | Real and inverted
• Between F₁ and 2F₁ | Beyond 2F₂ | Enlarged | Real and inverted

Notes

A convex lens forms a real, inverted image for an object at infinity, beyond 2F₁, at 2F₁ and between F₁ and 2F₁: at F₂, between F₂ and 2F₂, at 2F₂ and beyond 2F₂.

Check yourself

Which position of the object gives a real image of the same size with a convex lens?

The parallel rays of the Sun fall on a convex lens. (The object is at infinity.) Where is the image?

An object is between F₁ and 2F₁ of a convex lens. Its image is

An object beyond 2F₁ moves towards 2F₁. What happens to its real image?

• At F₁. An object at F₁ gives an image at infinity.
• At 2F₁ — correct. Yes!
• Beyond 2F₁. An object beyond 2F₁ gives a diminished image between F₂ and 2F₂.
• At 2F₂, the same size. That is the image of an object at 2F₁.
• Between F₂ and 2F₂, diminished. That is the image of an object beyond 2F₁.
• At F₂, point-sized, real and inverted — correct. Yes!
• beyond 2F₂, enlarged, real and inverted — correct. Yes!
• between F₂ and 2F₂, diminished, real and inverted. That is the image for an object beyond 2F₁.
• on the same side as the object, virtual and erect. That image is formed for an object between F₁ and the optical centre.
• It moves towards the lens and gets smaller. It is the other way round: the image moves from between F₂ and 2F₂ out to 2F₂ and grows.
• It moves away from the lens and gets larger — correct. Yes!
• It stays at F₂. The image stays at F₂ only for an object at infinity.