When light beams really cross at the picture, they get inverted, or flipped upside down. When light beams do not truly intersect at the picture, the result is a "virtual" image. Images are either real or virtual.
|Real Image||Virtual Image|
|An image formed when the light rays converge at a particular point after refraction and reflection is known as a real image.||An image formed when rays appear to meet a particular point after the reflection from the mirror is known as a virtual image.|
A genuine picture is formed when rays converge, but a virtual image is formed when rays appear to diverge. Concave mirrors and converging lenses may create actual pictures only if the object is situated farther away from the mirror/lens than the focal point, and this real image is inverted. If you look at your reflection in a mirror or through a lens, the image is reversed compared to what is seen by normal sight.
Inverted images are used for visual entertainment in mirages and sandstorms. They can also help scientists understand how objects as far away as the moon affect the Earth's atmosphere. Images of astronomical bodies such as stars, planets, and galaxies captured by telescopes are always reversed relative to what is seen with normal eyes. This has important consequences for astronomers who use these images to study distant objects.
Inverted images can be created artificially by using convex glass (or plastic) plates and light sources behind the plate. The image is inverted because light travels faster than sound, so waves from the source reach the back of the plate before those coming from the front. Since particles on the surface of the plate reflect light in different directions depending on their position relative to the wavefront, they cause the reflected light to seem scattered rather than concentrated as it would be from a flat surface.
Inverted images can also be created with purely optical devices. One example is the lenticular lens, which has two opposite surfaces that converge towards one central point.
(If you think about it correctly, you'll recall that a true picture has to be where the light is, which implies in front of a mirror or behind a lens.) Diverging lenses or positioning an item inside the focal length of a converging lens produce virtual pictures. Mirrors reflect images so they too are virtual.
You may have heard that telescopes make views into space, but this is only true for optical telescopes. Radio telescopes can see objects that are invisible to us, such as black holes and distant galaxies. But even radio telescopes need objects outside our own galaxy to see them because of the effect of cosmic expansion on their signals. The farther away an object is, the more rapidly it moves away from us as measured by a telescope on Earth. This means that the image of that object will be stretched out like rubber when seen through a telescope.
Finally, note that all these images are just ideas that travel down threads from your brain to your eyes. There are no physical images being projected onto some sort of screen for you to look at.
A real picture is a collection of focus points formed by converging rays, whereas a virtual image is a collection of focus points formed by diverging ray extensions. For example, if you look into a mirror, you see your own image, which is a virtual image.
Real images are always visible unless obscured by something (such as the case with the mirror example). Virtual images can be seen only with special equipment such as microscopes or telescopes.
For example, consider the case where there is a convex lens between you and a wall clock. You cannot see the clock because it is too far away; however, its second hand moves slowly, indicating that it is still running. The space behind the clock is hollow; thus, there is no physical object obstructing the flow of light from it to your eye. However, since no light reaches the back of the clock, you would not be able to see it. This is an example of a virtual image: you can see evidence of its existence without actually seeing it. Real images are always visible evidence that objects exist while virtual images are invisible objects that reveal information about how things are positioned in relation to one another.
In conclusion, virtual images are images that appear to be far away but are really closer than they seem.