SNC2D Optics
Images in Lenses
(pp. 492, 556 – 560)
We have seen that when light rays pass through a lens, they refract in a number of different ways.
Rules for Drawing Ray Diagrams for Converging Lenses
1. Parallel Ray: An incident ray that is parallel to the principal axis is refracted such that
it passes through (or appears to have originated from) the principal focus (F).
2. Centre Ray: An incident ray passing through the optical centre of the lens continues
to travel in a straight line.
3. Focal Ray: An incident ray passing through (or heading toward) the secondary
principal focus (F') is refracted such that it travels parallel to the principal axis.
Locating Images for Converging Lens Systems
Using the ray diagram rules above for a converging lens, you can predict the images that
are produced for a lens system.
Example 1:
Step 1: Draw a Parallel Ray –
draw a ray that runs parallel to
the principal axis from the top
of the object to the lens axis;
the ray then passes through the
focal point on the opposite side
of the lens.
Step 2: Draw a Centre Ray –
draw a ray that runs from the
top of the object through the
centre of the lens. It should
intersect the Parallel Ray.
Principal axis
Lens axis
Step 3: Draw a Focal Ray – draw a ray from the top of the object through the focal point on the same side
as the object until it hits the lens axis; the ray then runs parallel to the principal axis on the other side of the
lens. It should intersect with the other 2 rays.
Step 4: Draw the real image – the top of the image is at the point where the 3 rays meet. The bottom of the
image is on the principal axis.
What do you notice about the size of the image compared to the object?
What do you notice about the attitude (upright or inverted)?
What do you notice about the location of the image?
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SNC2D Optics
Describing the Characteristics of Images (SALT)
The properties of an image can be described using the following four characteristics:
1. Size (compared to the object: same size, smaller, or larger?)
2. Attitude (is the image upright compared to the object or inverted?)
3. Location (in relation to the focus F: e.g. between F and 2F, beyond 2F, etc.)
4. Type (real or virtual; a real image is an image that can be projected onto a screen –
light rays are actually arriving at the image location)
The acronym SALT can be used to remember these image characteristics.
Example 2: Using the Ray Diagram rules, find the image produced by the lens system
below. Use SALT to describe the image.
2f
f
f
2f
Description of Image:
Size:
Attitude:
Location:
Type:
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SNC2D Optics
Homework: Ray Diagram Practice
For each of the following,
1. Use the rules for drawing ray diagrams to locate the image for each object.
2. Describe the image characteristics using SALT (Size, Attitude, Location, Type)
3. ***Caution! For (iii), draw a Parallel Ray and a Centre Ray, but trace them
backwards (on the same side as the object) until they intersect. This is a virtual
image.
For the following diagram, use the rules for drawing ray diagrams to locate PF, the
principal focus, and SF, the secondary focus.
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