Lenses and the Formation of Images

Basic Lens ShapesConverging Lens-thickest in the middle (thinnest at edge)-causes parallel light rays to converge (come

together) through a single point after refraction

Diverging Lens-thinnest in the middle (thickest at edge)-causes incident parallel rays to spread apart

after refraction

• We will only study the incident ray coming into the lens and the ray leaving the lens

(in actuality there are two refractions in a lens; air to glass, glass to air)

Converging Lenses Words

2F’ 2F

Converging Lenses WordsOptical Centre (O)- Centre of the LensPrincipal Axis (PA)- The line through the optical centre that

is perpendicular to the central dashed line of the lensPrincipal Focus (F)-the point on the PA of a lens where light

rays parallel to the PA converge after refractionSecondary Principal Focus (F’)- the focus that is on the same

side of the lens relative to the incident rays2F- twice the distance from the principal focus2F’-twice the distance from the secondary principal focus

Diverging Lens Words

2F2F’

-if you project the diverging rays backward off of a diverging lens, it looks as if they come from a virtual focus, this point now becomes the principal focus

Secondary Principal Focus (F’)-now on the other side of the lens where the rays actually diverge

Converging vs. Diverging Lenses

Converging Lens Diverging Lens-brings parallel light rays together through a focus after refraction

-spreads parallel light rays apart after refraction so that it looks as though they have come from a virtual focus

-the principal focus is on the OPPOSITE side of the lens as the incident rays

-the principal focus is on the same side of the lens as the incident rays

What affects what an image will look like?

• The type of lens (converging or diverging)• The location of the object

With lenses, we will use ray diagrams to draw incident rays and refracted rays.

Emergent rays-the ray that leaves the lens, refracted as it goes from the lens back to the air

• In a thin prism, the emergent ray is almost unaffected by the presence of the prism

• We will work with thin prisms. Thick Prism Thin Prism

How to locate the image in a converging Lens

A ray parallel to the principal axis is refracted through the principal focus.

• A ray through the F’ is refracted parallel to the principal axis.

• A ray through O continues straight through without being refracted

• Where the two refracted lines cross, the image is produced.

Object beyond 2F’

• Smaller• Inverted• Between F and 2F• Real

Object at 2F’

• Same Size• Inverted• At 2F• Real

Object between F’ and 2F’

• Larger• Inverted• Beyond 2F• Real

Objects beyond 2F’

-the image is smaller than the object and between 2F and F

-as you move slowly toward the lens, the image gets larger and larger

-eventually the image and the object are the same size when the object is located at 2F’ the image would be at 2F

Object at F

• No clear image formed

Object between F’ and the Lens

• Larger• Upright• Behind the Lens• Virtual