chapter 15 lenses

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At the British Museum in London, England, lies

the Nimrud lens. Estimated to be 3 000 years

old, this clear rock crystal is possibly the oldest

lens found.

Historians and scientists proposed that the

ancient Assyrian race could have used the

device as a burning-glass to concentrate the

light rays from the Sun to a spot to start fires.

Lenses15Chapter


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Chapter 15: Lenses


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Lenses

There are two type of lenses,

A convex lens or converging lens bends light (by refraction)

inwards as light passes through it.

A concave lens or diverging lens bends light (by refraction)

outwards as light passes through it.

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Convex lens

Focal length (f)Optical centre (C)

Focal plane

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Chapter 15: Lenses


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Actions of both convex and concave lens on a beams of light:

Real images are formed only by convex lens. When the image of a point

object is viewed through a convex lens, the light rays coming from the object

converge to a point after passing through the lens.

A virtual image is formed when the light rays diverge from a point after

passing through a lens. Virtual images can be formed by both concave andconvex lenses.

Chapter 15: Lenses

Convex lens Concave lens


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Ray diagram

The object is represented by an arrow and is labelled with

the letter O.

The image of the object is also represented by an arrow. It is

labelled with the letter I.

The relative size between the arrows represent the

magnification of the image.

O

I

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image

C

Focal length

Principle axis

1. The base of the object, O, is positioned on the principal axis.

2. Three light rays are drawn starting from the top of the object:

(a) Ray 1 is drawn horizontally until it meets the lens. This ray will emerge fromthe lens passing through the focal point F.

(b) Ray 2 is drawn directly towards the optical centre O. This ray will pass

through without a change in direction.

(c) Ray 3 is drawn passing through the focal point F before it meets the lens.

This ray will emerge from the lens horizontally.

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Properties of the image produced are determined by the

position of the object.

There are 6 possibilities,

1) At a distance greater than 2f(two focal lengths) from the lens2) At a distance exactly 2ffrom the lens3) At a distance between 1fand 2faway from the lens4) At a distance exactly ffrom the lens5) At a distance less than ffrom the lens6) At infinity

Chapter 15: Lenses

Ray diagrams shown are forconvex lens. Can you draw the corresponding ray

diagrams for a concave lens?


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Image Formation in the Eye

1. When the human eye focuses on a point object, the lens behind the

cornea changes its curvature to converge the rays to a point on theretina.

2. The light rays from the object is refracted at the cornea and then at

the lens.

3. The image formed on the retina is converted into electrical signals to

the brain.

Chapter 15: Lenses

Lens

CorneaRetina

Near object


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There is a limit to how much the eye can change the curvature of the lens.

A normal person is able to focus on an object as close as 20 to 25 cm from

the eye.

A person with long-sightedness is unable to focus objects close to the eye

because the rays do not converge on the retina as shown.

Chapter 15: Lenses

Lens

CorneaRetina

Near object

Image does not form

on the retina


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People with long-sightedness are prescribed spectacles with converginglenses (convex lenses) so that a sharp image is able to form on the retina.

Chapter 15: Lenses

Lens

CorneaRetina

Near object

Convex lens


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Magnifying Glass


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*Note that an enlarged and upright image is formed only if the object is placedwithin the focal length of the convex lens.

A magnifying glass

makes use of a singleconvex lens to produce

an enlarged and upright

image.

The enlarged image lies

on the same side of the

object.

Convex lens

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A camera uses lenses to produce a real but diminished image of an objecton a light-sensitive film.

The camera controls the amount of light that is exposed to the film. Images

of objects of various distances are focused onto the film by adjusting the

distance between the lens and the film.

Object

Film roll

Image

formed on

film is real

but inverted

Lens Light rays from

the top of object

Light rays from

the base of object

Chapter 15: Lenses

Camera


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Condenser

lensProjection

lens

Slide

Enlarged

image of

slide

Screen

Concave

mirror

Lamp

A slide projector is designed to produce a real and enlarged image ofa slide on a screen. The projection lens rests on an adjustable mount

so that the image on the screen can be focussed by adjusting the

distance between the projection lens and the slide.

Chapter 15: Lenses

Slide Projector


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Light rays coming

from infinityObjective lensEye piece

Fig. 19.36 Ray diagram of a telescope

A simple telescope uses two lenses to magnify the image of a distant

object. The eye piece determines the degree of magnification.

Chapter 15: Lenses

Telescope

chapter 15 lenses

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