Light By Neil Bronks
Dec 18, 2015
Laws of Reflection
The angle of incidence ,i, is always equal to the angle of reflection, r.
The incident ray, reflected ray and the normal all lie on the same plane.
Ray Diagrams- Object outside 2F
1/. Inverted
2/. Smaller
3/. RealF
The images can be formed on a screen so they are real.
2F
Uses of curved mirrors
Concave Mirrors Dentists MirrorsMake –up mirrors
•Convex MirrorSecurity Mirrors
Rear view mirrors
60
1
ExampleAn object is placed 20cm from a concave
mirror of focal length 30cm find the position of the image formed. What is the nature of the image?
Collect info f=30 and u=20
Using the formula
vuf
111
v
1
20
1
30
1
20
1
30
11
vV=60cm Virtual
u
vm
20
20
Magnification
What is the magnification in the last question?
Well u=20 and v=60As
20
60
u
vm
u
vm
6
2
• m=3• Image is magnified
60
5
ExampleAn object is placed 30cm from a convex
mirror of focal length 20cm find the position of the image formed. What is the nature of the image?
Collect info f=-20 and u=30
Using the formula
vuf
111
v
1
30
1
20
1
20
1
30
11
v
V=60/5cm =12cm VirtualThe minus is
Because theMirror is convex
Laws of REFRACTION
The incident ray, refracted ray and normal all lie on the same plane
SNELLS LAW the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for 2 given media.
sin i =constant =n (Refractive Index)
sin r
Proving Snell’s Law
i
r
Sin i
Sin r
A straight line though the origin proves Snell’s law.
The slope is the refractive index.
Laser
Glass Block
Protractor
Cork
Pin
MirrorApparent depth
Pin
Image
Water
Real depth
MEASUREMENT OF THE REFRACTIVE INDEX OF A LIQUID
Finding the Critical Angle…1) Ray gets refracted
4) Ray gets internally reflected3) Ray still gets refracted (just!)
2) Ray still gets refracted
THE CRITICAL ANGLE
Practical Fibre Optics
It is important to coat the strand in a material of low n.
This increases Total Internal Reflection
The light can not leak into the next strand.
u
vm
12
30
Magnification
What is the magnification in the last question?
Well u=30 and v=12As
u
vm
u
vm
2
5• Image is smaller
60
5
ExampleAn object is placed 30cm from a diverging
lens of focal length 20cm find the position of the image formed. What is the nature of the image?
Collect info f=-20 and u=30
Using the formula
vuf
111
v
1
30
1
20
1
20
1
30
11
v
V=60/5cm =12cm VirtualThe minus is
Because theDiverging lens
u
vm
20
30
Magnification
What is the magnification in the last question?
Well u=30 and v=20As
u
vm
u
vm
2
3• Image is smaller
Sign Convention
f Positive
Veither
f Positive
Veither
f negative
Vnegative
f negative
Vnegative
vuf
111
Power of LensOpticians use power to describe lenses.
P=
So a focal length of 10cm= 0.1m is written as P=10m-1
A diverging lens with a negative focal
length f=-40cm=-0.4mHas a power of P = -2.5m-1
f
1