What Is Light?

13.1 Lenses and the Formation of Images

Two important lenses that need to be known in this chapter are: Converging lens and Diverging lens

But first off lets start with the definitions of both types of lenses.

Converging lens: This type of lens is thickest in the middle which allows incident parallel light rays to converge through a single point after refraction.

Converging Lens!!!

Video on Converging Lens

Extra explanation on Converging lenses

Diverging lens: This type of lens is thinnest in the middle and that causes incident parallel light rays to diverge (spread apart) after refraction takes place.

Video on Diverging Lens

Extra Explanation on Diverging Lenses

It is important that you understand the difference between a Converging Lens and a Diverging Lens. In a converging lens after the parallel light rays pass through the converging lens they all converge (come together) in a single point after refraction takes place. Meanwhile in a Diverging Lens when the parallel light rays pass through the lens they all diverge( separate from each other) after refraction takes place. Also notice how the shapes of the lenses are different.

Simplifying the Path of Light Rays Through a Lens

A helpful tip when drawing rays being refracted in a converging lens is by first drawing a dashed vertical line through the center of the lens. Once you make a dashed line you can imagine a ray going towards the central line and then stopping there and then on the other side of the dashed line make a new line (which is continues) started from your last line and make it look refracted. You can use this helpful tip for both converging and diverging lenses.

The Terminology of Converging Lenses

Center of lens is called Optical Center(O)
The line that goes through the optical center which is perpendicular to the "dashed line" located in the center of the lens is called the Principal Axis
Light rays parallel to the principal axis converge through a single point on the principal axis called the principal focus
Secondary principal focus is labelled (F')

The Terminology of Diverging Lenses

Unlike the converging lens, when light rays that are parallel to the principal axis of a diverging lens DO NOT converge but diverge instead
If you look at the diverging rays, and you continue the lines, looking backwards it appears as though it came from the virtual focus which is now identified as the principal focus (F)
The secondary principal focus(F) is located on the other side of the lens which is where the rays diverge
Remember that the principal focus (F) and the secondary principal focus (F') are equally far apart from the optical center, this applies for both converging and diverging lenses

Key Notes:

Difference between converging and diverging are in a converging lens the parallel light rays comes together in one point after refraction, but in diverging lenses parallel light rays diverge (spread apart) after refraction
In a converging lens the principal focus is located on the OPPOSITE side of the lens as the incident rays
In a diverging lens the principal focus is located on the SAME side of the lens as the incident rays
A useful tip when drawing the path of light rays through a line, is to make a dashed line in the centre of lens, that way it is easier to make a refracted ray
Optical Center is identified as "O"
Principal focus is identified as "F"
Secondary Principal Focus is identified as "F'|
In a converging lens the principal focus is located at the opposite side from the incident rays