# Light Physics Notes

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```					Contents
1.   The Nature & Propagation of Light    2
2.   Reflection                           4
3.   Refraction                           9
4.   Thin Converging Lenses              17

Concept Map

Physics                             Light - 1
1. The Nature & Propagation of Light
Light is the part of the electromagnetic spectrum that our eyes can detect.

1. White Light: made up of _________ colours.
Namely:

2. Light travels in a _____________ ____________.
This is known as the Rectilinear Propagation of Light

Experiment to Prove Light Travels in a Straight Line

3. A Ray of Light is

4. A Beam of Light is

There are 3 types of beam:

The shadow formed by a light source will depend on whether the light source is a
point source or an extended source.

Complete the following ray diagrams to show how the umbra and penumbra are
formed.

Point Source:

Extended Source:

Physics                                Light - 2

Note:
The differences in the shadows formed by a point source and an extended source. (i.e.
the umbra and the penumbra - label them on the above pictures.)

Eclipses (A real life application of shadows)
Eclipses are shadows formed by the sun.
Q.     Is the sun a point source or an extended source?

Solar Eclipse:
Also known as an Eclipse of the Sun.

Lunar Eclipse:
Also known as an Eclipse of the Moon.

Note:
Umbra, Penumbra: main causes for partial and total eclipse.

Physics                                 Light - 3
2. Reflection
The following experiment would help us study reflection.

i is known as the angle of ________________ .
r is known as the angle of ________________ .
normal is a line _____________________ to the mirror.

1ST LAW OF REFLECTION

2ND LAW OF REFLECTION

Note:
1. Normal is always 90° to the surface of the mirror where the incident ray and the
reflected ray are found.

2. The angle of incidence is always the angle between the incident ray and the
normal and the angle of reflection is the angle between the normal and the
reflected ray.

Types of Reflection
Regular Reflection:

Diffused Reflection:

Physics                                  Light - 4
Properties of Images Formed by Plane Mirrors
What is meant by a ‘plane mirror’?

An image in a plane mirror will have the following properties:

1.

2.

3.

4.

5.

Ray Diagrams for Plane Mirrors.
Look at the diagram on the right. Where does the
image appear and what is the path taken by the
light rays?

We can answer all of these questions by applying
the above properties and tracing the rays.

3 step method:
Step 1
Locate image behind mirror.

Physics                                Light - 5
- Equidistant and perpendicular
- Image size must be the same as the object

Step 2
Join the image to the eye.
- Virtual rays
- Real rays

Step 3
Draw the incident rays by means of the reflected rays.
- Do not forget to draw the arrowheads on the solid lines (real rays).

Physics                                   Light - 6
Examples:
Now it’s your turn find the path taken by the rays in moving from the object to the
eye. (Follow steps 1 to 3 above.)

OK, how often do we look at
point objects? Never. So let us
look at some real objects which
have a length and width. How
could you draw the reflections
of these arrows?

Physics                               Light - 7
To help it is often easier to think of the arrow as a line between the two ends of the
arrow. Thus reflect the end points first then fill in the missing parts. Try this larger
example.

Then draw the paths of the light as it passes from the object to your eye.

Question:
1 The diagram below shows a plane mirror placed at a distance 300 cm in front of a
patient. If the optician’s test card is fixed 80 cm behind the eyes of the patient,
what is the distance from his eyes to the image of the card?

Test                                                      Mirror
card

80 cm                   300 cm

Physics                                  Light - 8
3. Refraction
Note that REFRACTION and REFLECTION are NOT THE SAME. So take care
when using the words - it is VERY EASY TO CONFUSE THEM!!!!!

Definition:

Note:
Optical medium:
a transparent material, one that light can pass through. e.g. air, water, glass.

Optically less dense medium:
the distance between the particles in the medium are further away from each
other. e.g. air.

Optically denser medium:
the distance between the particles are nearer. e.g. glass, water.

On the diagram below label the normal, incident ray and refracted ray.

Causes of Refraction
Refraction occurs because light travels at different speeds in different media.

1. Light is refracted towards the normal if it passes from less dense to denser
medium,

2. It is refracted away from the normal if it passes from denser to less dense
medium.

Light Passing into Optically                   Light Passing into Optically

Physics                                  Light - 9
Denser Material                            Less Dense Material

Laws of Refraction

1ST LAW OF REFRACTION

2ND LAW OF REFRACTION

This law is also known as _________________________.

Refractive Index
Experiment:

From the experiment, we find out that

Physics                                 Light - 10
sin i
 constant                    - This is Snell’s Law
sin r

If the less dense medium is a vacuum (or air), the constant is called the refractive
index, n.
sin i
n =                            - This has no units, just a number.
sin r

Alternatively,
speed of light in a vacuum
Refractive index of a medium = n =
speed of light in the medium

The speed of light in a vacuum is the most important constant
in physics it is given a special symbol, c. This is the constant
in Einstein’s famous equation E=mc².

Speed of light in a vacuum, c = _______________ m/s

So we can write the equation as
c
n =
v
Note:
1. Because the speed of light in air is (nearly) the same as
that in a vacuum we can find the refractive index of a medium as light travels from
air into the medium.

2. The larger the value of the refractive index, the more the light will bend as it enters
that medium. Also the greater the refractive index the slower the speed of light will
be in that medium.

3. Where one of the media is not a vacuum (or air) we must use the equation:

Examples:
1. Given that the speed of light in vacuum is 3.0  108 m/s, calculate the refractive
index of crown glass if the speed of light in it is 2.0  108 m/s.

Physics                                 Light - 11
2. Given that the refractive index of diamond is 2.4, if speed of light in vacuum is 3.0
 108 m/s calculate the speed of light in the diamond.

3. Complete the following diagram to show the paths of the two rays through the
glass block.

4. A beam of light passes from air into water and then into glass. The angle of the
light from the surface of the water is 30°. The refractive index of water and glass
are 1.3 and 1.7 respectively. The three boundaries are parallel to each other.

Air                Water             Glass      Air

30°

a)        Calculate the angle of refraction for the light entering the water.

b)        Show the approximate path of the light ray as it passes through the water and
glass and then back into the air.

c)        Calculate the angle which light travelling through the glass makes with the
normal to the glass-water boundary.

Physics                                     Light - 12
Some Daily Phenomenon of Refraction
1. Swimming pool (how things appear to be nearer to the surface)
Have you ever noticed that a swimming pool appears to be shallower than it really
is! This is due to refraction, it is also why a fish swimming in the water appears to
be nearer to the surface than it really is.

Fact:
The refractive index of a
medium can be found from

Real depth
n =
Apparent depth

Questions:
1. What is the refractive index of the water in the example above?

2. The following scaled diagram shows a girl looking down on a glass block placed
on top of a mirror. It also shows a light ray passing into the top of the glass block.

eye
140°

Image of mirror
as seen by girl

Mirror

Physics                                 Light - 13
a)        By taking any necessary measurements, find the refractive index of the glass
block.

b)        Calculate the critical angle for light passing through the glass block.

c)        Given that the speed of light in air is 3.00 × 108 m/s, what is the speed of light
inside the glass block?

d)        Calculate the angle of refraction of the ray entering the glass block.

e)        Complete the diagram to show clearly the path of the light ray until it leaves
the glass block.

2. ‘Bent’ object in liquids

Physics                                    Light - 14
3. Dispersion of White Light
White light is made up of 7 colours. (Remember - ROY G. BIV)
Splitting white light into these colours is known as the dispersion of white light.

Causes of dispersion:
a) Different colours travel at different speeds in glass

 Red deviates the least

 Violet deviates the most

(In a vacuum, all colours travel at the same speed.   c=                  m/s)

b) Each colour of light gets refracted by a different angle as it passes between
the air and glass.

Total Internal Reflection & the Critical Angle
When light passes from a denser medium to a less dense medium the emergent ray is
refracted away from the normal.

As the incident angle increases eventually the refracted ray will have an angle of 90.
This special angle is known as the critical angle.

The critical angle can be found from the equation

1
sin c =
n

When the angle of incidence is increased beyond the critical angle the light is reflected
from the boundary. This is known as total internal reflection.

Physics                                   Light - 15
Example:
a) The following glass prism has a refractive index of 1.5. Calculate the critical angle
for light passing through the prism.

b) On the diagram to the right complete the
light ray passing through the prism.

Uses of Total Internal Reflection
1. Optical Instruments

Question:
Why do you think prisms are used instead of mirrors?

2. Light Pipes
Light enters at one end and repeatedly reflects
from the walls of the light pipe until it emerges
from the other end.

Question:
What are light pipes used for?

Physics                                 Light - 16
4. Thin Converging Lenses
Types of lenses
1. Converging lenses (Convex):

2. Diverging Lenses (Concave):

Question:
How can you tell if a lens is a diverging or converging lens?

Action of Converging Lens on Rays

OR

figure 1                                          figure 2

Notes:
 Parallel rays will converge after passing through the convex lens. (figure 1)

 Due to the reversibility of light, diverging rays will be parallel after passing through
the convex lens. (figure 2)

Terms Used in a Lens Ray Diagram
You should be familiar with the 5 following terms associated with
1. Principal axis
2. Optical centre, C
3. Principal focus, F
4. Focal length, f
5. Focal plane

Physics                                 Light - 17
Note:
Thin converging lenses have 2 Principal Foci. (One on each side of the lens.)

Ray Diagrams for Converging Lenses
There are 3 rays that we must consider:

1. Incident ray through optical centre, C, without bending.

2. Incident ray parallel to principal axis, refracted to pass through F.

3. Incident ray passing through F is refracted parallel to the principal axis.

Note:
All that is needed to draw a ray diagram are only 2 rays:
Ray (1) and either ray (2) or ray (3). (Depending on the situation.)

How to Draw A Ray Diagram
1. Draw a line from the arrow tip, y, through the optical centre C.

Physics                                   Light - 18
2. Draw a line from the arrow tip, y, to the lens parallel to the principal axis.
Continue this line through the principal focus on the other side of the lens.
3. The image is located where the two rays meet.

y

F                    C                F
x

Questions:
1. Is the image formed on the same side of the lens as the object?

2. Is the image formed enlarged, diminished or the same size as the object?

3. Is the image upright or inverted?

4. Is the image real or virtual?

Now try to draw the ray diagram for the following case.

F                    C         F

Question:
How is the image different from the image in the first diagram?

Physics                                    Light - 19
Ray Diagram                         Notes
Image:

Uses:

Image:

Uses:

Image:

Uses:

Image:

Uses:

Image:

Uses:

Image:

Uses:

Physics                 Light - 20
Further Example:
A magnifying glass is a lens that makes
things appear to be larger than they really
are, like the example shown on the right of a
person looking at a fly.

The diagram shows an object, O, placed near
to a thin converging lens, L, which is being used as a magnifying glass to produce a
magnified upright image. F marks the focal point of the lens.

Complete the diagram to show how the image is formed and show clearly where the
eye should be placed in order to view an image.

O
F

L

Physics                                Light - 21

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Description: Light Physics Notes for GSCE O Level
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