Converging

Lens

LENSESLenses are mainly made of glass clear

plastics. They are widely used in spectacles, cameras, telescopes and many other optical instruments.

Our human eye has two crystalline lenses which enable us to form images.

Lenses

There are two main types of lenses:• convex lenses—these curve outwards and

are fatter in the middle• concave lenses—these curve inwards (a

little like a cave) and are thinner in the middle.

Convex lenses Concave Lenses

Lenses – An application of refractionThere are 2 basic types of lenses

A converging lens (Convex)takes light rays and bringthem to a point.

A diverging lens (concave) takes light rays and spreads them outward.

Convex Mirrors

Curves outwardReduces imagesVirtual images

Use: Rear view mirrors, store security…

CAUTION! Objects are closer than they appear!

Convex Lenses

Thicker in the center than edges. Lens that converges

(brings together) light rays.

Forms real images and virtual images depending on position of the object

The Magnifier

focus (f)focus (f) 2 f2 fx x x x

Parallel rayPasses thruThe focus

Focal ray

Refracts parallelTo principal axis

A ray thru theCenter of the

lenses

Remains unbent

MOVEMENT OF LIGHT THROUGH LENSES

CONVEX LENS In a convex lens, an incoming ray parallel to the

principal axis is refracted through the principal focus (F).

Movement of Light through a Lens

The distance from the centre line (plane) of the lens to the principal focus is called the focal length of the lens.

A ray passing through the centre of either type of lens is unaffected.

As with all images, rays of light that come from a part of the object come together again at that same part of the image.

Finding the focal lengthRays coming into a lens from a distant

object are almost parallel and form an image very close to the focus.

We can then measure the distance from lens to image to determine the focal length of the lens.

The greater the curvature of a lens, the more it bends light and hence the shorter the focal length.

Focal Length

Convex lenses produce two different types of images, depending on where the object is located.

If the object is at a distance greater than the focal length of the lens, a real image is formed.

Image type and Location

If the object is at a distance less than the focal length of the lens, a virtual image is formed.

Convex Lenses

REAL AND VIRTUAL IMAGESReal Image –

Image is made from “real” light rays that converge at a real focal point so the image is REAL

Can be projected onto a screen because light actually passes through the point where the image appears

Always invertedVirtual Image–

“Not Real” because it cannot be projected

Image only seems to be there!

Parallel ray

focus (f)focus (f) 2 f2 fx x x x

Focal ray

Image is:RealInvertedReducedAppears between f and 2f

Object beyond 2f

focus (f)focus (f) 2 f2 fx x x xFocal ray

Parallel ray

Image is:RealInvertedSame sizeAppears between f and 2f

Object at 2f

focus (f)focus (f) 2 f2 fx x x xFocal ray

Parallel ray

Image is:RealInvertedEnlargedAppears beyond 2f

Object betweenf and 2f

focus (f)focus (f) 2 f2 f

x x x x

Image is:VirtualErectEnlargedAppears on sameSide as Object

ApparentConvergence

Of rays

ObjectInsidefocus

APPLICATIOSNS

OPTICAL INSTRUMENTS Telescopes Telescopes make small, far objects appear larger. Two lenses are used. The objective lens produces a real, inverted image

just inside the focus of a second lens, called the eyepiece lens. The image produced by the first lens now acts as the object for the second lens. Because the first image is inside the focus of the second lens, the second image (the one seen by the telescope user) is virtual and enlarged compared to the first one

CAMERAS, IN BRIEF

In a pinhole camera, the hole is so small that light hitting any particular pointon the film plane must have come from a particular direction outside the camera

In a camera with a lens, the same applies: that a point on the film planemore-or-less corresponds to a direction outside the camera. Lenses havethe important advantage of collecting more light than the pinhole admits

pinholeimage atfilm plane

object

image atfilm plane

object

lens

JAZAKALLAH!

PREPARED BY:

CLASS:XI-OM. FAHD-UN-NABI KHAN