# Lenses - General Physics - Lecture Notes, Study notes for Physics. Birla Institute of Technology and Science

## Physics

Description: This is the Lecture Notes of General Physics which includes Potential Difference and Capacitance, Charge of Coulomb, Unit of Potential Difference, Work, Charge and Voltage, Positive Charge, Symbol for Capacitance etc. Key important points are: Lenses, Types of Spherical Lens, Convex Lenses, Ray Diagrams, Centre of Curvature, Parallel to Principle Axis, Focal Length, Object Distance, Image Distance, Two Lenses in Contact
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Chapter 5: Lenses
Please remember to photocopy 4 pages onto one sheet by going A3→A4 and using back to back on the photocopier
Two types of spherical lens: convex (which is also called ‘converging’ because it causes rays which are arrive parallel
to the principle axis to converge) and concave (caves in as you look at italso called ‘diverging’).
Convex (converging) lensesray diagrams
You should be able to draw a ray diagram showing how an image is formed by a convex lens when the object is
placed
(i) outside the focus resulting in a real image
(ii) inside the focus – resulting in a virtual image
Three Rules
From top of object to lens (parallel to principle axis) and after passing through the lens then passes through the focal
point on the other side.
From top of object through focal point and after passing through the lens continues on the other side parallel to the
principle axis.
Centre of Curvature: From top of object to centre of curvature and continues straight through.
For each of the following label the focal point, the object and the image.
Put arrows on all rays, and state whether the image is real or virtual, upright or inverted, magnified or diminished
Object outside f Object inside f
Notice that when the object is inside the focal point the light rays never intersect, but from the viewer’s perspective
they appear to do so behind the mirror (the viewer is to the right of the lens in the diagrams above).
Note
A real image is always on the other side of the lens (to the object) and is inverted.
A virtual image is always on the same side of the lens is upright.
Concave (diverging) lenses ray diagrams
Here only one diagram is needed; the image is always diminished, upright and virtual.
Two Rules
(i) From top of object to the lens parallel to principle axis and up as if coming from the focal point.
(ii) From top of object to the lens as if passing through centre of curvature.
Notice that in this situation (similar to the convex mirror when the object is inside the focus) light rays never intersect,
but from the viewer’s perspective they appear to do so at the same side of the lens as the mirror.
The image is therefore always virtual, regardless of where the object is placed.
2
Maths Problems
Relationship between focal length (f), object distance (u) and image distance (v)
Convention:
For a convex lens f is positive
For a concave lens f is negative
For a real image v is positive
For a virtual image v is negative
The last two lines are what is referred to as the ‘Real is Positive’ convention (RiP).
Remember that for a convex lens the image is only virtual if the object is inside the focus.
For a concave lens the image is always virtual.
u is always positive for both types of lens
Magnification
If you are told that v is virtual, or if it is obvious from the question (because the lens is diverging or because the object
is inside the focal length if the lens is converging) then you should make the value for v negative at the beginning of
the question.
Note: If you are told that v is virtual, or if it is obvious from the question (because the lens is concave, or because the
object is inside the focal length if the lens is convex) then you should make the value for v negative.
vuf
111 +=
3
Power of a Lens*
Power of a Lens = 1/focal length
The unit of power is m-1.
Convention
The power of a converging (convex) lens is taken as positive (+) {because f is positive}.
The power of a diverging (concave) lens is taken as negative (-) {because f is negative}.
Two Lenses in Contact
If two lenses of power P1 and P2 are placed in contact, the power P of the combination is given by
Remember to use the correct sign notation.
It follows from this that if two lenses of focal length f 1 and f2 are placed in contact, the focal length f of the
combination is given by
Remember to use the correct sign notation.
Now look over problems 6 8, page 52, then try questions 1 10, page 53.
The Eye*
Retina - light sensitive screen at the back of the eye,
Optic Nerve carries the information in electrical form to the
brain
Cornea – together with The Lens, form part of the focusing
system.
Iris – acts like a shutter to control the amount of light entering the
eye
Power of Accommodation (page 54)
The Power of Accommodation of the eye is its ability to focus a
real image of an object on the retina, whether the object is near to
or far away from your eye.
PTotal = P1 + P2
4
Defects of Vision; short and long-sightedness
Short Sight
A Short-sighted person can see nearby objects clearly but cannot bring distant objects into focus.
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Long Sight
A Long-Sighted person can see distant objects clearly but cannot bring nearby objects into focus.
Leaving Cert Physics syllabus: Lenses
Content
Depth of Treatment
Activities
STS
Lenses
Images formed by single
thin lenses.
Knowledge that
1/f = 1/v + 1/u
Simple exercises on lenses
by ray tracing or use of
formula.
Use of lenses
m = v/u
Power of lens: P = 1/f
Two lenses in contact:
P = P1 + P2
The eye: Optical structure,
short sight, long sight, and
corrections.
Spectacles
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