Real Images and Virtual Images

Lenses and mirrors make images. Some images can be caught on a screen (like a projector). Others can be seen only by looking into the lens or mirror (like a bathroom mirror). These are called real and virtual images.

Key terms

• Principal axis: uploaded_notes_styles A straight line through the centre of the lens.
• Principal focus (focal point): The point where rays parallel to the principal axis meet (or appear to come from) after passing through the lens.
• Focal length (f): The distance from the centre of the lens to the principal focus.

Real images

A real image forms when light rays actually meet after refraction. It can be projected onto a screen.

• Formed by a thin converging lens when the object is outside the focal length (object distance > f).
• Usually inverted (upside down).
• Size can be enlarged, same size, or diminished, depending on object distance.
• Examples: projector, camera, the eye’s image on the retina.

Virtual images

A virtual image forms when light rays spread out (diverge) but seem to come from a point behind the lens or mirror. The rays do not actually meet there; we trace them backwards to find the image. A virtual image cannot be formed on a screen.

• Converging lens makes a virtual image when the object is inside the focal length (object distance < f). The image is upright and enlarged (magnifying glass).
• A thin diverging lens always gives a virtual, upright, diminished image.
• Plane mirrors give a virtual, upright image that is the same size and appears the same distance behind the mirror as the object is in front.

Ray diagram rules (lenses)

• Converging lens: a ray parallel to the principal axis goes through the far focus; a ray through the centre of the lens goes straight. Where the rays meet = real image. If they do not meet, extend them backwards with dashed lines to find the virtual image.
• Diverging lens: a ray parallel to the axis emerges as if from the near focus; a ray through the centre goes straight. The backward extensions meet at a virtual image.

Real-world connections

• Magnifying glass: virtual, upright, enlarged image when the object is closer than f.
• Projector: real, inverted image formed on a screen.
• Bathroom mirror: virtual image, same size, upright, appears behind the mirror.

Common misconceptions

• Virtual does not mean “not visible.” You can see a virtual image; it just cannot be projected.
• Real images are not always larger; they can be smaller or the same size.
• Inverted means upside down, not left-right reversed (plane mirrors swap left and right).