Grade 10 Academic Science – Optics (Physics)
Section 13.1 Pages 551-553
Lenses have two basic shapes
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Converging Lens – Parallel light rays converge through a single point of refraction. They are thick in the middle and thin at the edges.
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Diverging Lens – Parallel light rays spread apart after refraction.
This lens is thin in the middle and thick at the edges.
Terminology of Converging Lenses (See Diagram (a))
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Optical Centre – Point of the EXACT CENTRE of the lens

Principal Focus (F) – Point on the Principal Axis of a lens where light rays parallel to the Principal Axis converge after refraction.
 There is also a SECONDARY PRINCIPAL FOCUS (F’). It is on the Principal Axis at the EXACT SAME
DISTANCE back through from the Optical Centre the lens as the Principal Focus (F)
Terminology of Diverging Lenses (see Diagram (b))

Light rays parallel to the Principal Axis of a diverging lens do NOT converge. Rather, the refracted rays spread apart

Project the diverging rays BACKWARDS through the lens to locate the VIRTUAL FOCUS. This is the
Principal Focus (F)
 There is also a SECONDARY PRINCIPAL FOCUS (F’). It is on the Principal Axis at the EXACT SAME
DISTANCE as the Optical Centre through the lens as the Principal Focus (F)

Lens – Transparent object that refracts light (i.e., changes the direction that light rays travel)
CONVERGING LENS

A converging lens is thick in the middle and thin at the edges

When parallel light rays pass through it, the rays converge (i.e., come together) at the FOCUS
(…labeled Focal Point in the illustration). The FOCUS a converging lens is the point where the light rays, which were parallel when entering the lens, cross after of leaving the lens (see illustration).

The lens is transparent so light rays can pass through both sides of the lens

As you know, the PRINCIPAL AXIS is a line that runs through the centre of the lens (…called OPTICAL
CENTRE) and the Focus
DIVERGING LENS

A diverging lens is thin at the middle and thick at the edges.

When parallel light rays pass through a diverging lens, the light rays spread out or move apart as they emerge on the other side of the lens.
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In a RAY DIAGRAM for a diverging lens (see illustration below and opposite), the refracted rays move apart

Draw a line through the middle of the lens. That is NORMAL

Draw a dotted line backwards from the point of refraction. The
FOCUS (F) is where the line converge

Since rays pass through the transparent lens on either there must be a FOCUS on both sides of the
The figure shows a RAY DIAGRAM illustrating light side, lens. how the light rays move from right to left through a diverging lens.
QUESTIONS
1. What happens when parallel light rays pass through (a) a converging lens and (b) a diverging lens?
2. Describe one similarity and one difference between the two types of lenses.
3. Why is the Focus on the diverging lens on the same side as the light entering the lens?

Drawing a Ray Diagram for a Converging Lens
Adapted from PhysicsLAB (online) http://dev.physicslab.org/Document.aspx?doctype=3&filename=GeometricOptics_ConvergingLenses.xml
A lens that is ________________________ at the centre than at its edges is a CONVERGING LENS.
When you use a magnifying glass to burn a hole in a dried leaf, you instinctively learned to place the leaf at the Focal Point
(…also called the Principal Focus…) of the magnifying glass' converging lens. WHY?
A lens is transparent. Thus, light travels through it. As such, there is a SECONDARY FOCAL POINT IN FRONT
OF THE LENS. The Secondary Focal Point is located along the Principal Axis at a point equal to the distance that the primary Focal Point is behind the lens. DRAW a Ray Diagram that illustrates the Secondary Focal Point.
You need to know THREE light rays to locate the images formed by converging lenses. Each ray starts from the top of the object.

RAY #1 runs parallel to the Principal Axis until it reaches the lens. Then, it refracts through the lens and leaves along a path that passes through the lens' principal focus

RAY #2 runs straight through the centre of the lens. It does not bend.


RAY #3 passes through the Secondary Focus until it reaches the lens. At the lens, it refracts through the lens and leaves parallel to the Principal Axis on the other side of the lens
Where the three rays meet is the image (see below).
There is also a second focus on either side of the lens (2F). It is at the same spot as the Centre of Curvature would be found – 2X away from the centre of the lens.
PRACTICE
Draw the RAY DIAGRAM if the object is

2X from the lens

At 2X


Between 2X and F
At F

In front of F