What is Light?
Light is a form of electromagnetic radiation that can be seen by the eye.
What is Electromagnetic Radiation?
Electromagnetic radiation is a term used to describe waves that are created by the motion of electrically charged
particles. All forms of electromagnetic radiation travel at the same speed: 300 000 000 metres/second (3x108
m/s or 300 000 km/s). There are many different forms of electromagnetic waves including:
Figure 1: Different types of Electromagnetic Radiation
xrays
ultraviolet
Violet light
Red light
microwave
radio
wavelength
Electromagnetic radiation can be classified according to the wavelength of the waves. The wavelength of radio
waves used by radio stations can be several hundred metres long, or as short as a few metres. The wavelength
of microwaves in microwave ovens is about 12 cm. From the diagram, ____________ has the shortest
wavelength.
Furthermore, different colours of light have different wavelengths. Red has a ___________ wavelength than
violet light. Our eyes cannot see electromagnetic radiation that has a longer wavelength than red (ie
microwaves) and a shorter wavelength than violet (ie xrays).
Vision
Our eyes can see objects that either emit light or reflect light
a) Emission
There are many sources of light. The sun is the earth’s primary source of light, however many other things can
also emit light. Light is emitted from light bulbs, televisions and computer monitors, glow sticks, fireflies,
LEDs, fireworks and during stormy weather (lightning).
Match the following terms to the definitions: chemiluminescence, bioluminescence, incandescence,
fluorescence, phosphorescence, triboluminescence.
Term
Definition
the emission of light by a living
organism through a chemical reaction
Examples
the emission of light by a material
after being exposed to ultraviolet
radiation or other types of radiation
the emission of light through a
chemical reaction
the emission of light by a material
that undergoes friction and/or the
emission of light that results from the
breaking of certain crystals
the emission of light by a heated solid
the emission of light by a material
while exposed to ultraviolet radiation
b) Reflection
The paper you are currently reading, or the pencil/pen you write with do not emit light. When these objects are
placed in a dark room, they cannot be seen. Most objects that we use on a daily basis can only be seen because
they reflect light, which then enters the eye. If the light reflected by an object does not enter your eye, then the
object cannot be seen.
Properties of Light
Light travels in straight lines. To represent light, a ray is drawn that illustrates the direction light travels in.
light ray
Reflection of Light
When a flashlight is aimed at a shiny smooth
surface, such as a mirror, the light will reflect.
The four rays (figure 1) represent the light
coming from the flashlight. These are called
_____________ rays.
Figure 1: Label the mirror, incident rays and reflected rays
When light shines on a surface, we say that light
is _________ on the surface.
The four rays that are lighter represent the light
that is ____________, and are called
______________ rays.
Reflection of Light - Geometry
Angles in optics are __________ measured
with respect to the ___________. The
_________ is an imaginary line that is drawn
____________ to the surface.
Figure 2: Label the normal, the incident ray, the reflected ray,
the angle of incidence and the angle of reflection. Measure the
angle of incidence
The angle of ______________ is measured
between the normal and the incident ray.
The angle of ______________ is measured
between the normal and the reflected ray.
Exercise 1: Measure the angle of incidence and draw the reflected ray. Label the normal.
a)
b)
e)
f)
c)
d)
g)
h)
Exercise 2: Draw the normal for the following figure at points a b c d and e.
e
d
a
c
b
Refraction of Light
Light sometimes can pass through a medium or substance that it is incident on. For example, in figure 3 light
initially is incident on the glass and passes through it. Light not only reflects off of the glass but also can travel
through the glass. The ray of light that travels through the glass (and then through the water) is called the
_______________ ray.
Notice that the pencil
appears bent. This occurs
because light rays change
direction when traveling
through a different medium
or substance (from air to
glass to water).
Figure 3:
Figure 4: Label the normal, incident ray and
refracted ray. Draw the reflected ray. Measure the
angles of incidence and refraction.
air
air
glass
glass
For figure 4, light travels
from air into glass. Light
travels in straight lines, but
changes direction (refracts)
when entering the glass.
Exercise 3: Label the normal and draw the reflected ray. Measure the angles of incidence and
refraction.
a)
b)
Exercise 4: Light travels from air into glass. The angle of incidence is 40 degrees and the angle of
refraction is 29 degrees. Draw the incident ray, the refracted ray, the reflected ray and the normal.
The angle light refracts at when it enters another medium (substance) also depends on the ____________
__________ of the medium. Different mediums have different optical densities. The more optically dense a
medium is, the more light _____________ or bends. In the example below, glass is more _________
_________ than water and hence causes light to bend more.
For the water, draw a refracted ray with an angle of refraction of 34 degrees. For the glass, draw a refracted ray
with an angle of refraction of 28 degrees. Measure the angle of incidence and label it in the figure.
water
glass
Exercise 5: The two pieces of glass are similar, however one piece has a higher optical density. Draw
the reflected rays. The angles of incidence are the same. Which piece of glass is more optically
dense?
A
B
Refraction of Light and Index of Refraction
The index of refraction is a quantity that is used to measure the ___________ ___________. As the index of
refraction for a medium increases, the optical density ____________. The index of refraction is defined by the
following ratio
speed of light in a vacuum
Index of refraction of a medium =
speed of light in the medium
n=
c
v
The speed of light in a vacuum is ____________ m/s. This speed is given the special symbol c
c = 3x108 m/s
As light travels from the sun through the vacuum of space, it travels at the speed of light. The index of
refraction for a vacuum is 1 (n = 1). As sunlight enters the atmosphere, it ________ ________ very slightly, but
the index of refraction for air is approximately equal to 1 (n = 1). As sunlight enters water, it slows down
significantly. The index of refraction for water is 1.33 (n = 1.33). For other substances, such as ___________
__________ or __________, light slows down by an even greater factor (n = 1.5 and n = 2.42 respectively).
Example a: Calculate the speed of light in
diamond if its index of refraction is 2.42.
Example b: Calculate the index of refraction if the
speed of light in a material is 1.5 x 108 m/s.
For diamond, an index of refraction of 2.42 means that
light travels 2.42 times slower in diamond than in
space.
As light travels from air into this particular material,
the speed of light decreases by a factor of _________.
Refraction of Light and Index of Refraction: Case 1
Light travels from a medium with low optical density into a medium with high optical density
In this example, light travels from air (n = 1) into glass (n = 1.6). Light travels into a medium that is more
optically dense (from a low value for n to a higher value for n). In general, when light travels into a medium
that is more optically dense, the angle of incidence is always greater than the angle of refraction. For these
situations, we say that light "bends" ____________ the normal.
Figure 5: Draw the normal and the reflected ray. Measure the angle of incidence and the angle of refraction.
Air, n = 1
Glass, n = 1.6
How would the figure change if the index of refraction for glass were 2.2? Draw the refracted ray.
Air, n = 1
Glass, n = ___
For which angle of incidence does light not change directions (does not bend)?
Refraction of Light and Index of Refraction: Case 2
Light travels from a medium with high optical density into a medium with low optical density
In this example, light travels from glass (n = 1.4, a medium with high optical density) into air (n = 1, a medium
that is less optically dense). When light travels into a medium that is less optically dense (from a high value for
n to a lower value for n), the angle of incidence is always less than the angle of refraction. For these situations,
we say that light "bends" _________ from the normal. An everyday example of this situation is found in
____________________
Figure 6: Draw the normal. Measure the angle of incidence. Measure the angle of refraction. Label the
normal, incident ray and refracted ray.
Refraction of Light and Index of Refraction: Case 2 - The Critical Angle
Sometimes if light travels into a medium with a lower index of refraction (less optically dense), there exists an
incident angle for which light does not refract. The smallest incident angle for which light does not refract is
called the _____________ ___________. If the incident angle is greater than the critical angle, light is 100%
reflected. This is called _______ ____________ ______________.
Total internal reflection occurs for angles of incidence that are greater than the critical angle for the mediums.
The critical angle depends on the optical density of the mediums.
Figure 7: Light incident at an incident angle of 46°, the critical angle.
Exercise 7: Draw the normal for each example and measure the angles of incidence.
a
b
c
Glass, n = 1.6
Glass, n = 1.6
Glass, n = 1.6
Air, n = 1
Air, n = 1
Air, n = 1
angle of incidence < critical angle
angle of incidence = critical angle
total internal reflection
angle of incidence > citical angle
total internal reflection
For glass that has an optical density of 1.6, the critical angle is ________.
Exercise 8: As light travels from a mysterious substance (n is about 1.31) into air (n = 1), the critical
angle is about 50 degrees. Which of the following diagrams is not possible based on the critical angle
being 50 degrees? (for each question, select A or B and write a brief statement explaining your choice)
1.
A
B
78º 78°
78° 78°
2.
A
B
22°22°
22°22°
3.
A
B
22°22°
22°22°
The Semi-Circle Glass
Figure 8: Draw the reflected ray. Measure the angle of incidence and the angle of refraction. Why does light
not bend at point qq as it enters the glass?
qq
q
For glass that has an optical density of 1.5, the critical angle is _________.
Optical Density, Index of Refraction and Air
Air has an index of refraction that is approximately 1. However, the index of refraction of air varies with
temperature, hence different temperatures of air have different optical densities.
This can be seen on a hot summer day. If you view the hood of a car or pavement, it
appears that the air above it shimmers. Due to convection, air of different temperatures
flows above the pavement or the hood, constantly changing the optical density. Hence,
light no longer travels in a straight line and refracts. The refraction ultimately causes
shimmering to occur.