Lenses
- Created by: poppyggggg
- Created on: 11-05-16 19:22
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
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
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
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
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
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
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
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
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
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
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
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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