SNC2D Optics

Images in Lenses

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.

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?

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.

Description of Image:

Size:

Attitude:

Location:

Type:

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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