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LIGHT
Everything written in black has to go into your notebook
Everything written in blue should already be in there
WHAT IS LIGHT?
Light is a form of energy that travels away from the source producing it at a speed of 3 x 108 m s-1
Transparent: allows light to pass through it, and can see clearly through it e.g. glass
Translucent: allows light to pass through it, but cannot see clearly through it e.g. frosted glass
Opaque: does not allow light to pass through it e.g. aluminium
Light Travels in Straight Lines
Light travels in straight lines. This can be seen in the following examples
LaserBeam of light from a searchlight
It can also be shown using pieces of cardboard with a small hole in the middle and a length of thread
A plane mirror is a flat mirror
i r
Normal
Angle ofincidenc
e
Angle ofreflectio
n
Plane Mirror
Reflected rayIncident ray
Plane Mirror (diagram on page 1)
LAWS OF REFLECTION OF LIGHT
1. The incident ray, the normal and the reflected ray all lie in the same plane
2. The angle of incidence is equal to the angle of reflection (i = r)
HOW IS AN IMAGE FORMED IN A PLANE MIRROR (diagram page 2)
Properties of an image in a plane mirror
The image is:Laterally inverted
E.g. your right hand appears as a left hand
The “ambulance” signErectVirtualSame size as object
Uses of Plane Mirrors
Make up mirrorThe periscope
Diagram page 3
A virtual image cannot be formed on a screen
A real image can be formed on a screen
Experiment to prove the angle of incidence equals the angle of reflection (page 26)
i r
Plane mirror
Pins
Sheet of paper
Diagram on page 26
Reflection is the bouncing of light off an object
Experiment to prove the angle of incidence equals the angle of reflection (written up in homework copy)
Object pin
Plane mirror
Finder pin
O M I
Diagram (in homework copy)
The following goes in your homework copy
Method
1. Set up the apparatus as in the diagram
2. Move the finder pin in and out behind the mirror until there is no parallax between the object and its image in the mirror
3. Measure the distance from the object to the mirror (OM), and the distance from the mirror to the image pin (MI)
Result
OM and MI are equal
Conclusion
The image is as far behind the mirror as the object is in front of it
Spherical Mirrors (page 4)
CONVEX CONCAVE
The line from the centre of curvature to the pole is called the principal axis
Rules for Ray Diagrams for Concave Mirror
1. A ray travelling parallel to the principal axis is reflected through the focus
2. A ray travelling through the focus is reflected parallel to the principal axis
3. For a ray which strikes the pole, angle i will be equal to angle r
Top of page 5
“In parallel, out through the focus”
“In through the focus, out parallel”
Uses of concave mirrors
SpotlightsReflectors in car headlightsShaving and make-up mirrors
Uses of convex mirrors
Shops (to deter shoplifters)BusesDangerous bends in roads
They give a wide field of view
The Mirror Formulae
fvu
111
fvu
111
fvu
111
u = distance from object to mirrorv = distance from image to mirrorf = focal length
Example 2
When an object is placed 16 cm in front of a concave mirror of focal length 8 cm, an image is formed. Find the distance of the image from the mirror and say whether it is real or virtual.
8
11
16
1v
16
1
8
11
v
16
121 vfvu
111
16
11v
v = 16 cm
It is a real image since the object is outside f
Magnification
m =
m =
u
v
object ofheight
image ofheight
Example 3 (HL)
An object is placed 20 cm from a concave mirror of focal length 25 cm. Find the position, magnification and nature of the image.
25
11
20
1v
20
1
25
11
v
100
541 vfvu
111
100
11 v
v = 100 cm
It is a virtual image since the object is inside f
m =
m =
m = 5
u
v
u
v
Example 4 (HL)
A concave mirror of focal length 10 cm forms an erect image four times the size of the object. Calculate the object distance and its nature.
10
111vu
4u
vM
fvu
111
u = 7.5 cm
1
4
u
v
10
1
4
11
uu
10
1
4
14
u
10
1
4
3
uuv 4
304 u
It is a virtual image since the object is inside f
Experiment to Measure the Focal Length of a Concave Mirror (page 30)
RAY BOX
CONCAVE MIRROR
SCREEN
CROSS THREADS
Diagram page 30
Light (2) Refraction and Lenses
Refraction of light is the bending of light as it goes from one optical medium to another
A medium is a substance; e.g. glass, air etc.
Incident ray
Refracted ray
Glass block
i
r
(Page 12, under diagram)
Less dense to more dense: bends towards normal
More dense to less dense: bends away from normal
The Laws of Refraction of Light
1. The incident ray, the normal and the refracted ray all lie in the same plane
2. where n is a constant
This is called Snell’s Law
nrsin
isin
Experiment to Verify Snell’s Law and determine the refractive index of glass (diagram page 27)
Sheet of paper
Glass Block
Pins
Enter the following results at the top of page 28, and draw the corresponding graph underneath
Sin i
Sin r
Your graph in page 28 should look like this
Real and Apparent Depth (page 12)
A swimming pool appears to be less deep than it actually is, due to refraction at the surface of the water
We can calculate the refractive index of a liquid by using
n = depthApparent
depth Real
Critical angle
The critical angle is the angle of incidence in the denser medium when the angle of refraction is 90˚
Total Internal Reflection
This occurs when the angle of incidence in the denser medium exceed the critical angle
The ray of light is refracted away from the normal As i is increased so is r Eventually r = 90˚ At this point i has reached the ‘critical angle’ If i is increased beyond the critical angle, the ray does not enter
the second medium It is reflected back into the first medium
We can also find the refractive index of a material using n =
CSin
1
C = critical angle
Example
The critical angle of glass is 41.81˚Find the refractive index of glassn =
n = 1/0.666
n = 1.5
CSin
1
n =
rsin
isin
depthApparent
depth Real
CSin
1
mediumin light of speed
airin light of speed
Applications of Total Internal Reflection
Periscopes (using a prism)Diamonds and bicycle reflectorsOptical fibres – in telecommunications
and by doctors
Total internal reflection in a prism
Total internal reflection in a prism
Total internal reflection in a prism
Remember that rays are path-reversible
AIR
GLASS
A
B
Example
The refractive index of glass is 1.5This value is for a ray of light
travelling from air into glass
So = = 1.5 =
Or = =
Asin
B sin
5.1
1
B sin
Asin
gan
ag n
Mirages
Mirages are caused by the refraction of light in air due to temperature variations
LENSES
Convex lens (converging)
Concave lens (diverging)
Ray diagrams for lenses
1. Ray incident parallel to principal axis is refracted out through focus
2. Ray incident through focus is reflected out parallel to axis
3. Ray incident through optic centre continues in straight line
Lens formulae
fvu
111
fvu
111
fvu
111
u = distance from object to lens
v = distance from image to lens
f = focal length
Magnification
m =
Or m =
u
v
object ofheight
image ofheight
Experiment to measure the Focal Length of a Concave Lens (page 29)
RAY BOX
CONVEX LENS
SCREEN
CROSS THREADS
(Diagram page 29)
Two Lenses in Contact
21
111
ffF
Where F = focal length of combinationf1 and f2 are the focal lengths of the two lenses
Dispersion is the breaking up of white light into its constituent colours
Spectrum of Visible Light
R
O
Y
G
B
I
V
Red is deviated the least and has the longest wavelength
Violet is deviated the most and has the shortest wavelength
Uses of lenses
Magnifying glass
Spectacles
Binoculars
Compound microscope
Astronomical telescope
Magnifying glass/Simple Microscope
Is simply a convex lens, with the object placed inside the focus point
Image is magnified, erect and virtual
F F
Objective lens
Eyepiece
Fo Fe
The Compound Microscope
The compound microscope
Consists of 2 convex lenses
The first image is formed at the focal point of the eyepiece
The final image is formed at infinity so we view it with a relaxed eye
This is called ‘normal adjustment’
The image formed is inverted
Objective lens
Eyepiece
FoFe
The Astronomical Telescope
Same description as for compound microscope
