Everything Science Grade 11

CHAPTER 13. GEOMETRICAL OPTICS 13.2

Example 4: Locating the image position for a diverging lens: I

QUESTION

An object is placed 4 cmto the left of a diverginglens which has a focal length of 6 cm.

1. What is the positionof the image?


2. Is the image real or virtual?

SOLUTION

Step 1 : Set up the problem

Draw the lens, object, principal axis and focal points.

F 1 Object O F 2

Step 2 : Draw the three light rays to locate the image

• R 1 goes from the top of theobject parallel to the principal axis. To determine the angle it
  has when it leaves the lens on the other side, wedraw the dashed line from the focus F 1
  through the point where R 1 hits the lens. (Remember: for a diverging lens, the light ray
  on the opposite side of the lens to the object hasto bend away from the principal axis.)


• R 2 goes from the top of theobject in the direction of the other focal point F 2. After it
  passes through the lens,it travels parallel to the principal axis.


• R 3 goes from the top of the lens, straight throughthe optical centre withits direction
  unchanged.


• Just like for converginglenses, the image is found at the position whereall the light rays
  intersect.

F 1 Object O F 2

R 1

R 2

R 3

Step 3 : Draw the image

Draw the image at the point where all three raysintersect.