There are a few things you need to know in order to draw images with lenses. As with mirrors, there is an incident ray, however, the ray that comes out through the lens is termed the emergent ray.
There is a trick that can be used to simplify the drawing of refraction in a converging lens. Draw the incident ray to the center of the lens, where it will refract through the principal focus.
The same trick can be used for diverging lenses, with one small alteration. Draw the incident ray to the center of the lens, as you did with converging lenses. However, instead of continuing it through the principal focus, draw a dotted (virtual) line from the principal focus so that it meets up with the incident ray, then continue it from that point onwards.
Three Kinds of Lines You Need to Draw to Find Images in Converging Lenses:
- An incident ray that is drawn from the object parallel to the principal axis. When it reaches the centre of the lens, the line is drawn through the principal focus.
- The incident ray drawn from the object through the optical centre continues on without refracting at all.
- The incident ray drawn from the object through the secondary principal focus will refract parallel to the principal axis.
Three Kinds of Lines You Need to Draw to Find Images in Diverging Lenses:
- As with converging lenses, one line is drawn from the object, parallel to the principal axis, to the centre of the lens. Connect this line by drawing a dotted (virtual) line from the principal focal point. From there, draw the emergent ray away from that point along the same line as the principal focal point.
- The line through the optical centre. It does not refract.
- This line is drawn as though it is going through the secondary focus. It refract parallel to the principal axis. Project this ray backwards using a dotted line.
Only two lines are necessary to determine the position of an image. Where the lines intersect will give you its position.
Diverging lenses always produce images that are:
On the same side as the object