Download correcting human eye defects ppt File

Magnification
 Upon completion of the topic of Magnification,
you will be able to:
 Calculate the magnification of an object
 Describe how a converging lens is used in a
magnifying glass.
Calculating Magnification
 Lenses are often used to magnify tiny objects.
 As the light passes through the lens, it is
refracted in such a way as to produce a larger
image.
 Formula:
𝑖𝑚𝑎𝑔𝑒 ℎ𝑒𝑖𝑔ℎ𝑡
𝑀𝑎𝑔𝑛𝑖𝑓𝑖𝑐𝑎𝑡𝑖𝑜𝑛 =
𝑜𝑏𝑗𝑒𝑐𝑡 ℎ𝑒𝑖𝑔ℎ𝑡
Worked Example
 If the magnification is 1, the image is the same
height as the object.
 If the magnification is less than one then the
image is smaller than the object.
 If light from an object of height 40 cm passes
through a lens with magnification 0.5, what
height is the image?
0.5 = x/0.4
X=(0.5)(0.4) = 0.2 m = 20 cm.
 An image 2 cm high is focussed onto a film. The
object is 10 cm high. Calculate magnification.
 Light from a 2 cm high object produces an image 56
cm high. Calculate magnification.
 An image 4.8 cm high is focussed on a CCD by a
lens. The object being photographed is 19.2 cm tall.
Calculate the magnification.
Basic Questions
 A magnifying glass uses a single converging lens to magnify an
object.
 For the lens to act as a magnifying glass, the object must be
close to the lens.
 It needs to be nearer to the lens than the principal focus
(distance from lens must be less than the focal length)
 The image produced by the magnifying glass is on the same
side as the object.
Magnified, Upright, Virtual image which is further away from the
lens.
The Human Eye
 You should be able to
 Describe the Structure of the Human Eye, and state the
functions of each part of the eye.
 State the range of vision of the human eye.
The Human Eye:
Main Parts
DON’T
TAKE
DOWN
lens
The eye contains a ________.
This is held in
suspensory ligaments
place by __________
________. It is unlike most
shape
others in that it can change ________.
Once light
refracted and focussed
passes through this, it is _________
onto the back of the eye onto a layer called the
retina
____________.
This detects the light, and sends a
optic
signal to the brain through the _________
nerve
________.
The Human Eye:
Other Parts
DON’T
TAKE
DOWN
- The cornea is a hard,
 The eye needs to be protected transparent layer.
from two things!
- The Iris is the coloured
 Physical Damage
part of the eye. It
 Too much light which would
controls how much light
desensitise the retina.
enters the eye through
the pupil.
- The pupil is the gap in
the centre of the iris.
Complete the table
Part
Iris
Pupil
Lens
Retina
Optic Nerve
Cornea
Suspensory Ligaments
Only one sentence is needed!
Factual & Concise
Function
Changing Focus
 As the lens is able to change the shape of its lens, the
focal point can change.
 This occurs due to the ciliary muscles, which can
contract and expand.
 When they contract, the lens is stretched, and far
away objects can be seen.
 When they expand, the lens is compressed, and we can
see nearby objects.
The range of vision
 The range of vision describes the distance from the
nearest point of to the furthest point that the eye can
focus on.
 The range of human vision is
 25 cm to infinity
 The eye can see objects so far away as the light from
these is travelling parallel to the principal axis.
AQA Questions
 How does the lens change shape? Mention the two kep
parts in your answer.
 Suggest a reason as to why it becomes difficult for the
eye to focus on nearby objects.
Problems with Eyesight
 After studying this topic, you should be able to
 Understand what is meant by Long- and shortsightedness.
 Know the reasons for each condition.
 Describe how corrective lenses can be used.
What causes problems with eyesight??
 The lens
 The cornea
 The overall shape of the eye
 Lazy ligaments
 Crazy Ciliarys
 Each of these can result in light not being focussed on
the retina.
Long-Sightedness
 The eye can not focus on objects that are close to the
eye.
1. Eye may be too short
2. May not be able to make the lens fat enough to focus
the light.
3. Often happens as people get older, as the lens loses
elasticity.
Get me my
reading glasses!!
Short Sightedness
 Cant focus on objects that are far away.
Eye may be slightly long
2. Can’t make the lens thin enough to focus
the light on the retina.
1.
 People naturally squint to try and see things
far away.
Using Glasses
 Corrective Lenses are used to help form a clear
image.
 These refract the light passing through them so as to
form a clear image on the retina.
Correcting Long-Sightedness
 A convex lens is used.
 Converges light from nearby objects
onto the retina.
 This would be used in reading glasses.
 Hypermetropia is the medical term
for longsightedness.
Yay….
Correcting Short Sightedness
 We use a diverging lens to correct this.
 Allows the light to be focussed on the
retina instead of in front of it.
 Myopia is the medical term for short-
sightedness.