Question

how do you determine if a ray is real or virtual

Answer 1

Typically you can identify the virtual rays in geometrical optical diagrams as they are the ones that are not real. That seems like a bit of a naff answer doesn't it, but bear with me. Look at the attached image . What we have here is an object located between the lens and the primary focal point. If one traces the rays of light from this object, passing through the lens, we see that the lines diverge and no image is created upon the other side of the lens. These rays are the real rays.

No image can be formed on the other side of the lens (that is the lens cannot be used to project an image), but we can extrapolate backwards using virtual rays (dashed lines) to form a virtual image. This means effectively that the object "appears" to be in this position.

In general a ray will be virtual if it is on the wrong side of the lens. In the example above the virtual rays are on the same side of the lens as the object, and pointing in the opposite direction.

A similar phenomena occurs in mirrors, except we are dealing with reflection, as opposed to refraction in this case. The image in the mirror appears the same distance behind the mirror as the object is in front of it. However, in this case, the rays are not passing through the mirror but reflecting from its surface back towards the object. these are the real rays. However, it looks as if there is an object behind the mirror projecting its image through the mirror towards us. The rays from the apparent object to the mirror plane in this case are the virtual rays, as the object does not exist behind the mirror. Effectively the image formed in mirrors are virtual images.

Answer 2

The 'real' rays are where the light 'really' goes. Imagine you have a small piece of tissue paper, then hold it where the ray will hit it. If the paper lights up then the ray is real. Real rays are continuous (although they may bend) from the light source to the image, and sometimes beyond. Real rays form real images, and are drawn with solid lines.

Virtual rays are 'pretend' rays, showing where light 'seems' to have come from, and are usually drawn by pretending that a ray of light did not bend (e.g. at a lens or a mirror) when you know well that it did. Virtual rays are drawn with dotted lines, and form virtual images (the ones that would not light up a sheet of tissue paper placed there).

Most rays you will work with are real rays; virtual rays are the ones that seem to have gone straight through a mirror or a lens.

Answer 3

ok thanks guys!