Real images form where light rays meet
Definition
Real image
An image formed where light rays actually converge that can be projected onto a screen.
- A converging lens produces a real image when the object is farther from the lens than the focal length.
- The image forms on the opposite side of the lens from the object.
- A real image produced by a single converging lens is inverted.
- Its size depends on object position: it can be diminished, the same size or magnified.
- A camera sensor, cinema screen and the retina receive real images because light rays reach those surfaces.
Virtual images form where rays appear to meet
Definition
Virtual image
An image formed where diverging light rays appear to come from that cannot be projected onto a screen.
- A converging lens produces a virtual image when the object is between the lens and its principal focus.
- That image is upright, magnified and on the same side of the lens as the object.
- A diverging lens produces a virtual, upright and diminished image for every position of a real object.
- In a ray diagram, dashed backward extensions meet at the virtual image position, but no light travels along those extensions.
- A magnifying glass works by placing the object inside the focal length of a converging lens, so the eye receives diverging rays that appear to come from a larger upright image.
Example
Classifying an image
- A lens forms an image on a white card placed behind the lens.
- Light rays must actually reach the card, so the image is real.
- When the object is moved inside the focal length, no sharp image can be caught on the card.
- Looking through the lens shows an upright enlarged image, so the new image is virtual.
Image descriptions use four properties
- State whether the image is real or virtual.
- State whether it is upright or inverted.
- Compare its size with the object using magnified, same size or diminished.
- State its position relative to the lens, object and principal focus where the diagram provides that information.
- A screen test distinguishes image type: a sharp image on a screen is real, while an image visible only by looking into the optical system is virtual.
Common Mistake
- Do not define a virtual image as an image that cannot be seen; virtual images are visible to the eye but cannot be projected onto a screen.
- Do not assume every magnified image is virtual; a converging lens can form a magnified real image when the object lies between FFF and 2F2F2F.
- Do not draw dashed extensions as real rays.
Exam technique
Describing lens images
- Give all four properties requested by the diagram: type, orientation, relative size and position.
- Use the screen test when experimental evidence is described.
- Base the description on the ray intersection rather than on a memorised label alone.
Self review
- Define a real image.
- Define a virtual image.
- State the screen test that distinguishes real and virtual images.
- Describe the image formed by a converging lens when the object lies inside the focal length.
- Describe the image formed by a diverging lens.
