K326 / K327 / 2027
Light overview

Topic 4 of 4

Real and virtual images

To classify an image, follow the rays first. Then compare the image's orientation and height with the object's.

A thin converging lens brings an axis-parallel beam to F. Rays from a nearby object point need not arrive parallel, so their meeting point can be elsewhere.

Actual convergence or apparent convergence?

Real image
Actual refracted rays from an object point meet at the image point. A screen placed there can show a sharp image.
Virtual image
The emergent rays diverge, but their backward extensions meet at an apparent image point. A screen at that apparent position cannot collect actual converging rays to form the image.
Upright or inverted
Compare orientation with the object. For the upright arrow examples, an image arrow below the axis is inverted; one above it is upright.
Magnified, same size or diminished
Compare image height with object height. A greater, equal or smaller height gives the corresponding description.

You can see both real and virtual images when the appropriate light enters your eye. A screen test concerns actual convergence, not visibility.

Read three completed diagrams

Each model uses a lens of focal length 8.0 cm and an upright object 2.0 cm tall. Identify which lines meet, then compare the image and object arrows.

Locate where rays, or their backward extensions, meet

Each completed diagram uses f = 8.0 cm and a 2.0 cm upright object. Horizontal and vertical distances have the same scale in all three panels. Use the supplied labels, not a ruler on your screen.

Blue and orange arrows represent the object and image. Green arrows show light travel. Dashed green lines are backward extensions.

A real, inverted, magnified image

A real, inverted, magnified imageThe upright object is 12 cm to the left of a converging lens of focal length 8 cm. Its height is 2 cm. The actual refracted rays meet 24 cm to the right, 4 cm below the axis. The image is real, inverted and twice the object height. One ray reaches the lens parallel to the axis and emerges through the far focus. A second ray through the optical centre continues undeviated. Equal scales apply horizontally and vertically, with 8 SVG units representing one centimetre in the supplied model.FFLensObject2 cmImage 4 cmScreen12 cm24 cm

The refracted rays meet at the image position. A screen placed there can receive a sharp image.

A real, inverted, diminished image

A real, inverted, diminished imageThe upright object is 24 cm to the left of the same lens and is 2 cm high. Actual refracted rays meet 12 cm to the right, 1 cm below the axis. The image is real, inverted and half the object height. Its small drawn height follows the same scale as the other panels. One ray reaches the lens parallel to the axis and emerges through the far focus. A second ray through the optical centre continues undeviated. Equal scales apply horizontally and vertically, with 8 SVG units representing one centimetre in the supplied model.FFLensObject2 cmImage 1 cmScreen24 cm12 cm

The refracted rays meet at the image position. A screen placed there can receive a sharp image.

A virtual, upright, magnified image

A virtual, upright, magnified imageThe upright object is 4 cm to the left of the same lens and is 2 cm high. The emerging rays diverge to the right. Their dashed backward extensions meet 8 cm to the left, 4 cm above the axis. The image is virtual, upright and twice the object height. The dashed extensions have no propagation arrows. One ray reaches the lens parallel to the axis and emerges through the far focus. A second ray through the optical centre continues undeviated. Equal scales apply horizontally and vertically, with 8 SVG units representing one centimetre in the supplied model.FFLensImage4 cmObject2 cm4 cm8 cmActual raysdiverge

Only the backward extensions meet. The apparent image is on the object's side of the lens; the emerging light travels to the right.

The first two images are formed by actual refracted rays and can be received on a screen at their positions. In the third, only backward extensions meet, giving an upright virtual image on the object's side.
First diagram: real, inverted and magnified
The actual rays meet 24 cm on the far side of the lens, 4 cm below the axis. The image is inverted and twice the 2 cm object height.
Second diagram: real, inverted and diminished
The rays meet 12 cm on the far side, 1 cm below the axis. The image is inverted and half the object height.
Third diagram: virtual, upright and magnified
Backward extensions meet 8 cm on the object's side, 4 cm above the axis. The actual emergent rays diverge. The image is upright and twice the object height.

These descriptions come from the completed ray paths and labelled sizes. The same converging lens can produce different image characteristics; it does not always magnify.

Find an image and describe what you observe

For a real-image investigation, align an illuminated arrow, the lens and a screen along the principal axis. Move the screen until the image is sharp. Measure the image distance from the lens centre, then compare image height and orientation with the object. Use the same length units for a size comparison.

Move the object to another suitable position and refocus the screen. Keep the alignment and distinguish the lens-to-object distance from the lens-to-screen distance. A millimetre ruler can measure positions and heights, but a blurred edge limits how precisely the image height can be read.

For a supplied arrangement known to produce a virtual image, look through the lens at the object and describe the apparent image. It can be seen even though a screen at the apparent position does not show it. Its image position may be inferred from backward ray extensions.

A blurred screen is not proof of a virtual image. The screen may simply be in the wrong place or the apparatus misaligned. Use the ray behaviour and the stated setup as evidence.

Optional check An upright object is above the principal axis. In a completed lens diagram, actual refracted rays meet below the axis and form an image half the object height. Which description follows?
An upright object is above the principal axis. In a completed lens diagram, actual refracted rays meet below the axis and form an image half the object height. Which description follows?