Real and virtual images
The images formed by a lens can be:
- upright or inverted (upside down compared to the object)
- magnified or diminished (smaller than the object)
- real or virtual
A An image that is formed where the rays of light are focused. is an image that can be projected onto a screen. A An image from which rays of light appear to come but do not do so in reality. appears to come from behind the lens.
To draw a Diagram that represents the direction and angle of travel of light.:
- Draw a ray from the object to the lens that is parallel to the principal axis. Once through the lens, the ray should pass through the principal focus.
- Draw a ray which passes from the object through the centre of the lens.
Some ray diagrams may also show a third ray.
Convex lenses
The type of image formed by a An object or shape that curves or bulges outwards, like a circle or sphere. lens depends on the lens used and the distance from the object to the lens.
A camera or human eye
Cameras and eyes contain convex lenses. For a distant object that is placed more than twice the The distance between the centre of the lens and the focal point. from the lens, the image is:
- inverted
- diminished
- real
Projectors
Projectors contain convex lenses. For an object placed between one and two focal lengths from the lens, the image is:
- inverted
- magnified
- real
In a film or data projector, this image is formed on a screen. Film must be loaded into the projector upside down so the projected image is the right way up.
Magnifying glasses
A magnifying glass is a convex lens used to make an object appear much larger than it actually is. This works when the object is placed at a distance less than the focal length. The image is:
- upright
- magnified
- virtual
Only the person using the magnifying glass can see the image. The image cannot be projected onto a screen because it is a virtual image.
Concave lenses
Concave lenses always produce images that are:
- upright
- diminished
- virtual
Peep hole lenses
Peep holes are set into doors so the occupant can identify a visitor before opening the door.
For an object viewed through a concave lens, light rays from the top of the object will be Deflected from a straight path. Bent. and will When a light ray splits up or spreads out. on the other side of the lens. These rays will appear:
- from the same side of the principal axis meaning the image will be upright
- further from the principal axis, so the image will be smaller than the object