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Virtual image

Virtual image is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Virtual image rather than just read about it. In short: In optics, the image of an object is defined as the collection of focus points of light rays coming from the object. A real image is the collection of focus points made by real converging rays, while a virtual image is the collection of focus points made by backward extensions of real diverging rays.

Virtual image — main illustration
Virtual image — illustration

Key takeaways

  • Virtual image belongs to physics; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Virtual image to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Virtual image from memory before moving on to harder problems.

Reference excerpt

In optics, the image of an object is defined as the collection of focus points of light rays coming from the object. A real image is the collection of focus points made by real converging rays, while a virtual image is the collection of focus points made by backward extensions of real diverging rays. A virtual image is found by tracing real rays, that emerge from an optical device (lens, mirror, or some combination), backward to perceived or apparent origins of real ray divergences. In other words, a virtual image point is from which the real rays appear (not actually) to be emitted (while a real image point is to which real rays converge). There is a concept virtual object that is similarly defined; an object is virtual when forward extensions of real rays converge toward it. In other words, a virtual object point is to which the real rays appear (not actually) to converge (while a real object point is from which real rays emit). This is observed in ray tracing for a multi-lenses system or a diverging lens. For a diverging lens, forward extension of real converging rays toward it will meet the converging point, so the point is a virtual object.

For a (refracting) lens, the real image of an object is formed on the opposite side of the lens while the virtual image is formed on the same side as the object. For a (reflecting) mirror, the real image is on the same side as the object while the virtual image is on the opposite side of, or "behind", the mirror. In diagrams of optical systems, virtual rays (forming virtual images) are conventionally represented by dotted lines, to contrast with the solid lines of real rays. Because the rays are not actually present at the apparent location, and thus never really converge at any point, a virtual image cannot be projected onto a screen by putting it at the location of the virtual image. In contrast, a real image can be projected on the screen as it is formed by rays that converge on a real location. A real image can be projected onto a diffusely reflecting screen so people can see the image (the image on the screen plays as an object to be imaged by human eyes).

A plane mirror forms a virtual image positioned behind the mirror. Although the rays of light seem to come from behind the mirror, light from the source only exists in front of the mirror. The image in a plane mirror is not magnified (that is, the image is the same size as the object) and appears to be as far behind the mirror as the object is in front of the mirror. A diverging lens (one that is thicker at the edges than the middle) or a concave mirror forms a virtual image. Such an image is reduced in size when compared to the original object. A converging lens (one that is thicker in the middle than at the edges) or a convex mirror is also capable of producing a virtual image if the object is within the focal length. Such an image will be magnified. In contrast, an object placed in front of a converging lens or concave mirror at a position beyond the focal length produces a real image. Such an image will be magnified if the position of the object is within twice the focal length, or else the image will be reduced if the object is further than this distance.

See also Focal plane Image plane Lens Erect image

Notes

References

Illustrations

Virtual image: The formation of the virtual image A' of the object A via a plane mirror. For people looking at the mirror, the object A is apparently located at the position of A' although it does not physically exist there. The magnification of the virtual image formed by the plane mirror is 1.
The formation of the virtual image A' of the object A via a plane mirror. For people looking at the mirror, the object A is apparently located at the position of A' although it does not physically exist there. The magnification of the virtual image formed by the plane mirror is 1.
Virtual image: A diverging lens with converging rays on it. The back focal point of this lens is also the virtual object for these rays.
A diverging lens with converging rays on it. The back focal point of this lens is also the virtual object for these rays.
Virtual image: Top: The formation of a virtual image using a diverging lens. Bottom: The formation of a virtual image using a convex mirror. In both diagrams, f is the focal point, O is the object, and I is the virtual image, shown in grey. Solid blue lines indicate (real) light rays and dashed blue lines indicate backward extension of the real rays.
Top: The formation of a virtual image using a diverging lens. Bottom: The formation of a virtual image using a convex mirror. In both diagrams, f is the focal point, O is the object, and I is the virtual image, shown in grey. Solid blue lines indicate (real) light rays and dashed blue lines indicate backward extension of the real rays.

Worked examples

Example 1 — a first encounter with Virtual image

Start with the simplest possible case. Write down what Virtual image claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Virtual image before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Virtual image ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Virtual image

In research
Virtual image appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Virtual image in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Virtual image is common in secondary-school and first-year university syllabi. It links to neighbouring topics Geometrical optics, so understanding it makes those chapters shorter.
In everyday life
Look for Virtual image outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.
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How to study Virtual image in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Virtual image means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Virtual image out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Virtual image in simple terms?

In optics, the image of an object is defined as the collection of focus points of light rays coming from the object. A real image is the collection of focus points made by real converging rays, while a virtual image is the collection of focus points made by backward extensions of real diverging ray…

Why does Virtual image matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Virtual image?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Virtual image.

Tags

  • Geometrical optics

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