# Lenses

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## Converging Lenses a.k.a Convex Lenses

Lenses refract light, changing it's direction. A lens acts like lots of little prisms all causing it to converge

Converging lens= A lens that makes light rays parallel to the principal axis converge to a point    it is curved on both sides

The point where parallel rays meet is called the principal focal point

Focal Point = the distance from the centre of the lens to the point where light rays parallel to the principal axis are focussed (in diverging lens appear to diverge from)

Focal length= distance between the centre of the lens and the pricipal focus.                           The closer the object is to the lens, the further it's focal point will be

-Cameras contain converging lenses that form a real image of an object

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## Diverging Lenses a.k.a Concave Lenses

-Makes parallel rays diverge and the point where rays appear to come from is the principal focus point

-Virtual images are formed (cannot be projected onto a screen)

-Image is smaller than the object

-Image is upright

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## Describing Images

Virtual Images cannot be projected onto a screen and are on the same sides of the lens (very often a diverging lens but occasionally converging)

Virtual images are formed in convex lenses when the object is closer to the lens than the principal focus point

Upright= right way up

They can be bigger or smaller than the object

The light rays never meet they only appear to

Real images can be projected onto a screen and image and object are on opposite sides of the lens

Inverted=wrong way up

Smaller than the object

Formed by a convex lens when object is at or further than the principal focus

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## Magnification of a Lens

Equation for magnification= IMAGE HEIGHT                                                                                                                                                                .                                           OBJECT HEIGHT

Doesn't matter what unit is used as long as they are both the same because the answer at the end is always just a number

A magnification above 1 shows that the image is larger than the object (magnified)

A magnification lower than 1 shows that the image is smaller than the object (diminished)

A magnification of 1 shows that the image and the object are the same size

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## The Lens Formula

u= object distance (object to lens)                                                                                                     v=image distance (lens to image)                                                                                                             f=focal length

Real images are always given a positive sign

Virtual images are always given a negative sign

Focal length is positive for a converging lens and negative for a diverging lens

TIP: When 1/f has been calculated make sure you find the reciprocal to get f (-1)

eg: 1/f = +41 (^-1)       f= 1/41

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## The Structure of the Eye

Cornea= transparent layer that eye and helps focus light onto the retina

Iris = coloured ring of muscle that controls the amount of light entering the eye

Pupil = the central hole formed by the iris. Light enters the eye through the pupil

Lens = focuses light onto the retina

Ciliary Muscles = attached to the lens by suspensory ligaments. The muscle changes the thickness of the eye lens

Retina= the light sensitive cells around the inside of the eye

Optic Nerve = carries nerve impluses from the retina to the brain

The normal range of vision is from 25cm to infinity

Blind spot is an area where the retina is not sensitive to light

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## Thickness of the lens

-When looking at an object far away lens muscles are relaxed and in the thinnest and least powerful state

Rays of light coming from an object very far away are virtually parallel

-When looking at an object nearer, the lens muscles pull the lens into a thicker and more powerful state

NEARER THE OBJECT, THE THICKER THE LENS

This is done so rays can be focussed onto the retina and a normal working eye can do this

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## Short Sightedness a.k.a myopia

Someone who is short sighted can focus in objects near to them but cannot focussed on objects far away. This is because their eyeball is too long for the lens system

Images of objects far away will be refracted too much and the image will be formed before the retina and appear blurry

A concave/diverging spectacle lens can be placed in front of the eye to make rays diverge slightly before entering the eye so that they focus on the retina

If you wear somebody else's glasses it will be uncomfortable because you eye has to change accomodation

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## Long sightedness a.k.a Hypermetropia

Somebody who is long sighted can see distant object clearly but cannot focus on objects near to them because the eyeball is too short for the lens system

When looking at an object near to them, the image is focussed behind the retina and the lens muscles squeeze the lens to is thickest position but this is still not enough to make rays focus

A convex spectacle will refract more and therefore help to focus the image onto the retine

By adding the lenses together, you will add the strength and shorten the path

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## Power of the lens

Power is measured in (D) Dioptres and focal length in metres

-A positive value of Dioptres is given for a converging lens (Long sighted)

-A negative value for of Dioptres is given for a diverging lens (Short sighted)

eg: F= -0.25   P= 1/f = 1/-0.25  =-4D

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## Laser Treatment

A laser produces a narrow concentrated beam of light

Lasers can be used to fix sight defects

For short sightedness and laser can be used to make part of the cornea slightly thinner and this has the same effect as a spectacle diverging lens

This laser is called an excimer

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## Focal length dependent on refractive index

Focal length depends on the refractive index of a lens

1) The larger the refractive index/the greater the curvature in the lens, the greater the power of the lens

2)The greater the refractive index of a lens, the flatter and thinner the lens can be manufactured because the lens surface will be less curved

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