In books, we use ray diagrams to represent images formed by thin lenses. Ray diagrams are straight lines with arrows that shows direction of light rays.
Points to note when drawing ray diagrams
- Real rays and real images are drawn using solid lines
- virtual rays and virtual images are drawn in broken lines
- To locate the image, two of the three important rays are drawn from the tip of the object towards the lens. The first ray parallel to principal axis and through principal focus, the second ray from the tip of the object through the optical center or the third that passes through the principal focus before moving parallel to the principal axis.
- Where two or more rays intersect after refraction by the lenses is the tip of the image.
- if the object stands and is perpendicular to the principal axis, the image is also perpendicular to the principal axis.
- To complete the image, a line is drawn perpendicular to the principal axis from the tip of the image
- If the foot of the object crosses the principal axis, two of the three rays used to locate image should be drawn for both the tip and the foot of the object. A point object for the image tip should be joined with point image of the foot to get the desired image.
- converging lenses are represented by the following diagram in drawings:
concave lenses is usually represented in the diagrams by the picture below.
Example problem
An object 15cm tall has been placed 32cm from a concave lens of focal length 20 cm. By scale drawing, determine:
(a) The position of the image formed
(b) Magnification of the image
(c) The height of the image
solution
We use the scale of 1 cm to represent 5 cm and using two rays to form an image, the resultant image after reflection is as shown
(a) From the diagram, one can see that the image is formed 52cm from the lense.
(b) From the diagram, the height of the image from the principal axis is 24 cm.
substituting for the values of hi and ho, we have:
The same result could be obtained by finding ration of image distance to object distance with some slight variation that comes with measurement errors:
(c ) From the scale diagram, the height of the image is about 24.5cm
Remarks:
The diagram can aslo be used to give further insights about the image formed. For example we can see it is upside down, it is formed by two rays actually meeting, hence the image is real.
Also the image is taller than the object, hence it is magnified just by looking
Related Topics
- Introduction to Thin lenses
- Terms used in thin lenses
- Focal length by non-parallax method
- Estimating focal length
- focusing a distance object
- The lens formula
- Linear magnification
- Ray diagrams
- Image formation by thin lenses
- Introduction to thin lenses
- Finding focal length by displacement