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Light
By Neil Bronks
Light is a form of energy
Crooke’s Radiometer proves light has energy
Turns in sunlight as the light heats the black side
Light travels in straight lines
Reflection-Light bouncing off object
Incident ray
Normal
Reflected ray
Angle of incidence
Angle of reflection
Mirror
Angle of incidence = Angle of reflection
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.
Virtual Image
An image that is formed by the eyeCan not appear on a screen
d d
Real ImageRays really meetCan be formed on a screen
F2F
FPrincipal Axis
Pole
Concave Mirror
Object
All ray diagrams in curved mirrors and lens are drawn using the same set of rays.
F
F
You can draw any ray diagram by combining 2 of these rays
The only difference is where the object is based.
Ray Diagrams- Object outside 2F
1/. Inverted
2/. Smaller
3/. RealF
The images can be formed on a screen so they are real.
2F
Object at 2F1/. Inverted
2/. Same Size
3/. Real
The image is at 2F
F2F
Object between 2F and F1/. Inverted
2/. Magnified
3/. Real
The image is outside 2F
F2F
Object at F
The image is at infinity
F2F
Object inside F
1/. Upright
2/. Magnified
3/. Virtual The image is behind the mirror
F
Convex Mirror
1/. Upright
2/. Smaller
3/. Virtual
The image is behind the mirror
F
Convex Mirror – only one ray diagram
The image is behind the mirror
F
Uses of curved mirrors
Concave Mirrors Dentists MirrorsMake –up mirrors
• Convex MirrorSecurity Mirrors
Rear view mirrors
Calculations
Use the formula
F
vuf
111
u
v
f=focal length
u=object distance
v=image distance
ExampleAn object is placed 20cm from a concave
mirror of focal length 10cm find the position of the image formed. What is the nature of the image?
Collect info f=10 and u=20
20
1
10
11
v
Using the formula
vuf
111
vuf
111
10 20V=20cm real20
11v
u
vm
20
20
Magnification
What is the magnification in the last question?
Well u=20 and v=20As
u
vm
u
vm
2
2
• m=1• Image is same
size
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
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
Questions
An object 2cm high is placed 40cm in front of a concave mirror of focal length 10cm find the image position and height.
An image in a concave mirror focal length 25cm is 10cm high if the object is 2cm high find the distance the object is from the mirror.
MEASUREMENT OF THE FOCAL LENGTH OF A CONCAVE MIRROR
u
v
Lamp-box
Crosswire
Screen
Concave mirror
Approximate focal length by focusing image of window onto sheet of paper.
Place the lamp-box well outside the approximate focal length
Move the screen until a clear inverted image of the crosswire is obtained.
Measure the distance u from the crosswire to the mirror, using the metre stick.
Measure the distance v from the screen to the mirror. Repeat this procedure for different values of u. Calculate f each time and then find an average value.
Precautions The largest errors are in measuring with
the meter rule and finding the exact position of the sharpest image.
Refraction
Fisherman use a trident as light is bent at the surface
The fisherman sees the fish and tries to spear it
Refraction into glass or water
Light bends towards the normal due to entering a
more dense mediumAIR
WATER
Refraction out of glass or water
Light bends away from the normal due to entering a less
dense medium
Refraction through a glass block
Light bends towards the normal due to entering a
more dense medium
Light bends away from the normal due to entering a less
dense medium
Light slows down but is not bent, due to
entering along the normal
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 = 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.
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
H/W
LC Ord 2006 Q2
Refractive Index
Ratio of speeds
5.1/200000000
/300000000sm
sm
c
cn
water
air
Real and Apparent Depth
A pool appears shallower
Apparent
aln
Re
Cork
Pin
MirrorApparent depth
Pin
Image
WaterReal depth
MEASUREMENT OF THE REFRACTIVE INDEX OF A LIQUID
Finding No Parallax – Looking Down
Pin atbottom
Pin reflectionin mirror
Parallax No Parallax
Set up the apparatus as shown.
Adjust the height of the pin in the cork above the mirror until there is no parallax between its image in the mirror and the image of the pin in the water.
Measure the distance from the pin in the cork to the back of the mirror – this is the apparent depth.
Measure the depth of the container – this is the real depth.
Calculate the refractive index n= Real/Apparent
Repeat using different size containers and get an average value for n.
Refraction out of glass or water
Light stays in denser medium
Reflected like a mirror
Angle i = angle r
Snell’s Window
Finding the Critical Angle…1) Ray gets refracted
4) Total Internal Reflection3) Ray still gets refracted (just!)
2) Ray still gets refracted
THE CRITICAL ANGLE
Semi-Circular Block Expt and on the internet click here
Mirages
Critical Angle
Varies according to refractive index n
C1
sin
n
145sin
n
17071.0
7071.0
1n 41.1n
Uses of Total Internal Reflection
Optical fibres:
An optical fibre is a long, thin, transparent rod made of glass or plastic. Light is internally reflected from one end to the other, making it possible to send large chunks of information
Optical fibres can be used for communications by sending e-m signals through the cable. The main advantage of this is a reduced signal loss. Also no magnetic interference.
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.
1) Endoscopes (a medical device used to see inside the body):
2) Binoculars and periscopes (using “reflecting prisms”)
Now is a good time to get out the light demo kit
H/W
LC Ord 2003 Q7
Focal Point
Focal Point
Lenses Two types of lenses
Converging LensDiverging Lens
Focal Length=f
Focal Length=f
2FF F
Optical Centre
Ray Diagrams
2FF F
2FF F2F
Converging Lens- Object outside 2F Image is
1/. Real
2/. Inverted
3/. Smaller
2FF F2F
Object at 2F Image is
1/. Real
2/. Inverted
3/. Same size
2FF F2F
Object between 2F and F
Image is
1/. Real
2/. Inverted
3/. Magnified
FF
Object at F
Image is at infinity
FF
Object inside F Image is
1/. Virtual
2/. Erect
3/. Magnified
H/W
Draw the 5 ray diagrams for the converging lens and the diagram for the diverging lens .
Write 3 characteristics of each image.
Calculations
Use the formula
vuf
111
u
v
f=focal length
u=object distance
v=image distance
2FF F2F
vuf
111 =
-120
ExampleAn object is placed 30cm from a
converging lens of focal length 40cm find the position of the image formed. What is the nature of the image?
Collect info f=40 and u=30
Using the formula
vuf
111
vuf
111
40 30 vuf
111
v 3040- V=120cm
virtual
u
vm
120
30
Magnification
What is the magnification in the last question?
Well u=30 and v=120As
u
vm
u
vm
4
1• Image is larger
u v
Lamp-box with crosswire Lens Screen
MEASUREMENT OF THE FOCAL LENGTH OF A CONVERGING LENS
Show on OPTICAL BENCH
1. Place the lamp-box well outside the approximate focal length 2. Move the screen until a clear inverted image of the crosswire is obtained.3. Measure the distance u from the crosswire to the lens, using the metre stick.4. Measure the distance v from the screen to the lens. 5. Calculate the focal length of the lens using
6. Repeat this procedure for different values of u. 7. Calculate f each time and then find the average value.
vuf
111
H/W
LC Ord 2002 Q3
Accommodation
The width of the lens is controlled by the ciliary muscles.
For distant objects the lens is stretched.
For close up objects the muscles relax.
Diverging Lens
FF
Image is
1/. Virtual
2/. Upright
3/. 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
vuf
111 =
-20
ExampleAn object is placed 30cm from a diverging
lens of focal length 60cm find the position of the image formed. What is the nature of the image? (Remember f must be negative)
Collect info f=-60 and u=30Using the formula
vuf
111
vuf
111
-60 30 vuf
111
v 30-60- V=20cm
virtual
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
Myopia (Short Sighted)
Image is formed in front of the retina.
Correct with diverging lens.
Hyperopia (Long-Sighted)
Image is formed behind the retina.
Correct with a converging lens
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
Lens in Contact
Most camera lens are made up of two lens joined to prevent dispersion of the light.
The power of the total lens is Ptotal=P1+ P2
H/W
LC Higher 2002 Q12 (b)LC Higher 2003 Q3
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