A real image is formed where rays actually meet, so it can be shown on a screen. A virtual image is formed where rays only appear to meet when traced backwards, so it cannot be caught on a screen.
This lesson completes drawing a simple lens image, and it links back to the plane mirror in constructing a reflected ray. Both belong to light and imaging.
How can I tell the difference in a diagram?
Look at the rays after the lens or mirror. If the real rays cross each other, the crossing point is a real image. If the real rays are still spreading apart and you need to extend them backwards to find a meeting point, that point is a virtual image.
In your drawing, real rays are solid lines with arrows. The backward extensions are dashed lines without arrows. The virtual image arrow is drawn where the dashed lines meet.
| Feature | Real image | Virtual image |
|---|---|---|
| Rays | Actually meet | Only appear to meet, traced back |
| Screen | Can be shown on a screen | Cannot be shown on a screen |
| Lines in diagram | Solid | Dashed extensions |
| Example | Converging lens, object beyond f | Plane mirror; converging lens, object inside f |
| Orientation from a converging lens | Inverted | Upright |
When does a converging lens make each kind?
If the object is beyond the principal focus (distance greater than f), the image is real and inverted. It can be smaller, the same size or larger, depending on whether the object is beyond 2f, at 2f, or between f and 2f.
If the object is inside the focal length (distance less than f), the image is virtual, upright, larger than the object, and on the same side of the lens as the object. A magnifying glass is a converging lens used this way, with the object held closer than f.
How do I construct a virtual image, step by step?
- Draw the axis, lens and F on both sides, then the object inside f.
- Ray 1: from the tip of the object, draw a ray parallel to the axis to the lens, then bending through the far F.
- Ray 2: from the tip of the object, draw a ray through the optical centre, going straight on.
- Check: the two rays diverge after the lens. They do not cross.
- Extend both rays backwards with dashed lines, on the object side, until they meet.
- Draw the image arrow at that meeting point, and describe it as virtual, upright and magnified.
Worked example
A converging lens has focal length 6.0 cm. An object 2.0 cm tall is 4.0 cm from the lens. (Invented example data.) Construct the image and describe it.
Step 1, set up: the object distance 4.0 cm is less than f = 6.0 cm, so the object is inside the focal length.
Step 2, ray 1: from the tip of the object, the ray parallel to the axis meets the lens 2.0 cm above the axis. It bends to pass through the far F, 6.0 cm beyond the lens, and slopes down to the right.
Step 3, ray 2: the ray through the optical centre goes straight on, also sloping down to the right.
Step 4, trace backwards: after the lens the two rays spread apart. Extending them back with dashed lines to the object side, they meet about 12 cm from the lens, 6.0 cm above the axis.
Step 5, describe: the image is 12 cm from the lens on the same side as the object, 6.0 cm tall (3 times the object height), upright and virtual. The magnification is 6.0 ÷ 2.0 = 3.
Step 6, check: for an object inside f, the image should be upright, magnified and on the same side, which matches. Only dashed lines meet at the image point, so it is virtual and could not be caught on a screen.
The mistake to watch for
A frequent error is to judge real or virtual by size or by whether the image can be seen.
Mistaken answer: “The image is bigger than the object, so it is virtual.” or “I can see the image in the mirror, so it is real.”
Size does not decide it. A real image can be magnified, and a virtual image can be seen by the eye even though it cannot be put on a screen.
The correction is to test the rays: do the real rays actually cross (real) or only their backward extensions (virtual)? Also state the test in your answer: “the rays appear to come from the image, but do not actually pass through it”.
Check yourself
Try these, then open each answer.
1. An object is 8.0 cm from a converging lens of focal length 5.0 cm. Is the image real or virtual, upright or inverted, larger or smaller?
Show answer
The object is beyond f and between f and 2f (5 cm to 10 cm). The image forms about 13 cm away on the other side. It is real, inverted and larger than the object.
2. Is the image in a plane mirror real or virtual? Give a reason.
Show answer
Virtual. The reflected rays only appear to come from a point behind the mirror, and no screen placed there would show the image.
3. A student draws a ray diagram for an object inside the focal length of a converging lens using solid lines for the backward extensions. What should be changed?
Show answer
The backward extensions must be dashed lines (without arrows), because they are not real rays. The image arrow should also be drawn dashed if the question asks for a virtual image to be shown, and described as virtual and upright.
Where this leads next
A useful way to consolidate is to mix real and virtual cases in the light and imaging practice set. If reflection or refraction are still weak, return to constructing a reflected ray or to using a refraction relationship.
Some students draw the rays correctly yet describe the image with the wrong word because the two cases look alike. Our teachers turn that into a quick spoken test in online one-to-one Physics tuition.