Applications of Lenses

The Camera

If an object is beyond 2F, the image from a converging lens will be inverted, real, and smaller, and will always lie somewhere between F and 2F on the other side of the lens

As the object changes position, its image will change location, but will always be between F and 2F

Since you cannot move the film to the location of the image, you focus by moving the lenses so the image falls perfectly on the film

The Movie Projector

A movie projector works almost opposite to a camera A projector takes a small object (the film) and projects a

large, inverted, real image on a screen Because the image is larger than the object, the film

must be located between F and 2F Also, because the image is inverted, the film must be

loaded into the projector upside down so the final image appears upright

The Magnifying Glass

A magnifying glass is a converging lens The object must be located between F and the lens, so

no real image is produced – it is virtual The refracted rays diverge, but the brain extends these

rays backwards and produces an enlarged, upright, virtual image located on the same side of the lens as the object

The earliest telescopes were designed by two of history’s first astronomers◦ Galileo Galilei (1564-1642)◦ Johannes Kepler (1571-1630)

Each had their own design, using different combinations of lenses◦ Galileo’s had two lenses – one converging and one diverging◦ Kepler’s used two converging lenses, but this also inverted

the image (not a problem for astronomy, but annoying to use on the ground!)

Sir Isaac Newton later refined both designs using mirrors as well as lenses

The Telescope

The Telescope

Two lenses in a telescope: Light enters through the objective lens Observer looks through the eyepiece

F1 is the focal point of the objective lens, and F2 is the focal point of the eyepiece

The two lenses are positioned so that the focal points to the right of both lenses are at the same place

Galileo’s Design:

The Telescope

Rays (shown in red) from a distant star or planet pass through the objective lens

If an eyepiece were not present, the objective lens would focus the image to the far right of the diagram

The rays reach the eyepiece before an image is formed, refracting the rays (shown in green) and creating a virtual, upright image between the two lenses

Galileo’s Design:

The Telescope

Kepler’s telescope used two converging lenses, positioned so that their focal points are at the same point between the lenses

Light from distant stars or planets (shown in red) enters through the objective lens and forms an image between the two lenses

The image formed by the objective lens becomes the object for the eyepiece lens

Kepler’s Design:

The Telescope

Light from the first image (shown in green) then passes through the eyepiece lens and forms a virtual image that appears to come from just beyond the objective lens

The final image formed is inverted and larger than the image from the objective lens

Kepler’s Design:

The Telescope

Galileo’s and Kepler’s designs suffered from a defect known as chromatic aberration, which affected focusing around the edges of the lenses

Newton overcame this problem by using a concave mirror as the objective

The objective in a telescope does not have to be a lens; it only has to be able to collect and focus light on a point

Today we call this a reflecting telescope; ones that use only lenses are called refracting telescopes

Newton’s Design:

The Telescope

Light enters the telescope and travels to the concave mirror objective

The mirror reflects the light towards focal point F1 Before the rays reach F1, a plane mirror reflects them to

F2, the focal point of the eyepiece lens Rays from F2 pass through the eyepiece, magnifying the

image

Newton’s Design:

The Microscope

Similar to Kepler’s telescope design Rays from the eyepiece (shown in red) pass through the

objective lens, and the refracted rays form an inverted, real image between the lenses

Rays from the image (shown in green) pass through the eyepiece, which again refracts the rays, and then forms the final inverted, virtual image

The image produced by the objective lens basically becomes the object for the eyepiece