9/22/20
Tan TY 1
Light (Reflection)
A. Recall and use the terms for reflection, including normal, angle of incidence and angle of reflection
B. state that, for reflection, the angle of incidence is equal to the angle of reflection and use this principle in constructions, measurements and calculations
C. Recall and use the terms for refraction, including angle of incidence, angle of refraction and normal
D. Recall and apply the relationship sin i / sin r = constant to new situations or to solve related problems
E. Define refractive index of a medium in terms of the ratio of speed of light in vacuum and in the medium
F. Explain the terms critical angle and total internal reflection
Light
Some characteristics of light that you have learned.
– Light is a form of energy. – Light travels in a straight line.
– Light travel at the speed of 3x108 m/s in vacuum. – Diffused and regular reflection
–
Word search
– revision exercise
Characteristic of Images formed by mirrors
1. Image is as far behind the mirror as the object is in front of the
mirror.
2. Image is laterally inverted. 3. Image is virtual.
4. Image is upright.
3
Reflection
Question:
– If the object is 3m from the mirror. How far is the image from the
object?
– If the same object is being moved at the speed of 1m/s from the
mirror, how fast is the image moving away from the object?
Not in syllabus
– perform and describe an experiment to illustrate the laws of
reflection
– describe an experiment to find the position and characteristics of an
Reflection
Definition
– Angle of incidence: angle between the incident ray and the normal – Angle of reflection: angle between the reflected ray and the normal – Normal: an imaginary line that is perpendicular to a surface. It
meets the surface at right angles.
Laws of reflection
1. The angle of incidence is equal to the angle of reflection.
2. The incident ray, reflected ray and the normal all lie on the same plane.
5
Reflection
Ray diagram (pg. 224)
1. Locate the position of the image.
2. Draw the ray from the eye to the image. Note that broken line denotes virtual rays. 3. Locate the point of reflection
on the mirror.
4. Join the point of reflection to the object. Add the arrows to represent the direction of travel of the light ray. 5. Show that the angle of
incidence is equal to the angle of reflection
.
Exercise
– Trace out the diagram and
show how the eye sees O1
Reflection
Application
–
Periscope
Two mirrors at 45º to stem
Look over objects
Is the image viewed through the
periscope upright? Draw another ray of light to find out
–
Mirror in a meter
To check for parallax errors
Ensure no image can be seen
mirror
image appears to come from
the back object
7
interesting reflection phenomenon...
the kaleido
scope
the kaleido
scope
the world of unlimited patterns...
the simple one...
• When two mirrors are inclined at right angle we have not only the images I1 and I2 formed by a single reflection, but in addition, two extra images produced by two reflections. • Geometrically, the object and all the images
lie on a circle whose centre is at the
intersection of the mirrors as shown below.
O I3
I2 I
1
the actual one...
• The kaleidoscope consists of 2 strips of plane mirror placed at an angle of 60o inside
a tube.
• At the bottom of the tube is a glass plate to admit light, on which is scattered small pieces of brightly coloured glass.
• These pieces of coloured glass act as objects, and on looking down the tube, 5 images are seen, which together with the object form a symmetrical pattern in 6 sectors.
60o
9
The figure below shows the position of the eye of a car driver, the
wing mirror of the lorry and the front of a lorry.
(a) Draw rays to indicate the part of the front of the lorry that can be seen in the mirror by the driver (or the range of vision of the driver). [2]
(b) State how the following will affect the range of vision of the driver.
(i) The mirror is rotated clockwise by 20. [1]
(ii) A convex mirror is used. [1]
Challenge
An object O, is placed 2m in front of a mirror. A light ray from the
object is incident on the mirror at an angle of 30º from the normal.
a. State the angle of reflection.
[1]
b. When the mirror is shifted 1m away from the object. State the new
distance from the object to the image.
[1]
c. When the mirror is rotated clockwise 20º , what is the change in the
angle of reflection?
[1]
d. How much did the reflected ray turn by?
[1]
30º
•
9/22/20
Tan TY 11
Light (Refraction)
A. R
ecall and use the terms for reflection, including
normal, angle of incidence and angle of reflection
B.state that, for reflection, the angle of incidence is
equal to the angle of reflection and use this principle
in constructions, measurements and calculations
C. Recall and use the terms for refraction, including angle of incidence, angle of refraction and normal
D. Recall and apply the relationship sin i / sin r = constant to new situations or to solve related problems
E. Define refractive index of a medium in terms of the ratio of speed of light in vacuum and in the medium
F. Explain the terms critical angle and total internal reflection
G. Identify the main ideas in total internal reflection and apply them to the use of optical fibres in telecommunication and state the
Refraction
Refraction is the change in direction, or the bending of light ray when it passes
from one medium to another.
Examples of refraction that you have come across:
– A pencil in a glass of water looks bent.
– The swimming pool appears more shallow that it actually is.
Light ray bends towards the normal when it travels from a less dense to a
denser medium. Vice versa.
applet
stick
air
water
video
13
Refraction
Laws of refraction
1. The incident ray, refracted ray and the normal at the point of incidence all lie in the same plane.
2. For two given medium, the ratio sin i / sin r is a constant, where i is the angle of incidence and r is the angle of refraction. (Snell's law)
Definition
– Angle of incidence: angle between the
incident ray and the normal
– Angle of refraction: angle between the
refracted ray and the normal
– Normal: an imaginary line that is
Refractive Index
Refractive index () of a medium is the ratio of speed of light in a
vacuum and in the medium.
It is also the ratio of sin i and sin r, where i is the angle (of incidence) in
vacuum and r is the angle (of refraction) in the medium.
–
The value of
is always understood to refer to the
refractive index of a light ray
from vacuum
to a given
medium. Thus its value is always greater than 1.
–
For most practical purposes, the value of
is obtained
with air, in place of a vacuum.
–
For a light ray travelling from a given medium to air, the
principle of reversibility of light can be used in our
calculations.
15
Refractive Index
Examples
If light is incident upon a piece of glass of refractive index 1.52, at an angle of 45°, what is the angle of refraction?
= sin i / sin r 1.52 = sin 45°/ sin r Sin r = sin 45° / 1.52
= 0.4652
r = 27.7° (3sf)
Look at eg. 12.8 (pg. 232)
Try Qn. 4b, pg. 259. The same light ray is incident on a diamond block
( = 2.4), calculate the angle of refraction.
Given c = 3 x 108m/s, calculate the speed of light in the two mediums.
Find out the relationship between the refractive index, the real depth
Refractive Index
Example
A ray of light travels from water into the air. The ray is incident upon the boundary at an angle of 30°. What is the angle of refraction in the air if the refractive index of water is 1.33?
= sin r / sin i
1.33 = sin r / sin 30°
Sin r = 1.33 x 0.5
= 0.665
r = 41.7° ( 1 d.p.)
More examples on pg. 234, 235
17
Exercise
A ray of light is incident onto a rectangular glass block. The light ray is incident at an angle of 35° from the surface of the glass block. The refracted ray of light in the glass prism was found to be 33.1°. Find:
A. The refractive index of the glass prism. B. The angle of the emergent ray.
Refraction Through Glass Block
Parallel-sided block
The angle i and r are the angles which the
light ray is incident on the glass block and the ray is deviated in the glass block.
The angle at which the ray emerges from
the block is the angle of emergence, e.
The refractive index:
= sin i / sin r
= ________
The angle of incidence is equal to the angle of emergence.
The incident ray is parallel to the emergent ray. Thus the ray is
Challenge
7 Diagram below shows a light ray moving from medium A to medium B.
Medium A has a refractive index of 1.52.
(a) Would you expect the refractive index of medium B to be greater than
or less than 1.52. Explain your deduction. [3]
(b) State the numerical value of . [1]
(c) Medium A is replaced with medium C, of refractive index 1.62. State
the change (if any) in the value of and . [2]
Medium A
20º
Medium B
9/22/20
Tan TY
Light
21(Total Internal Reflection)
A. Recall and use the terms for reflection, including normal, angle of incidence and angle of reflection
B. state that, for reflection, the angle of incidence is equal to the angle of reflection and use this principle in constructions, measurements and calculations
C. Recall and use the terms for refraction, including angle of incidence, angle of refraction and normal
D. Recall and apply the relationship sin i / sin r = constant to new situations or to solve related problems
E. Define refractive index of a medium in terms of the ratio of speed of light in vacuum and in the medium
F. Explain the terms critical angle and total internal reflection
Total Internal Reflection
Incident ray moving from a denser to a less
dense medium
– When a ray of light is incident on the
boundary of two mediums, the deviation of the light ray depends on the
mediums involved.
– When the ray of light moves from a
denser to a less denser medium, it will bend away from the normal.
– As the incident ray moves towards the
boundary, what happens to the refracted ray?
23
Total Internal Reflection
Incident ray moving from a denser to a less
dense medium
– At the point where the refracted ray is
along the boundary, the angle of
incident is known as the critical angle, c.
– The critical angle, c is defined as:
The angle of incidence for which the angle of refraction in the optically less dense medium is 90°.
– When the angle of incidence is greater
Total Internal Reflection
25
Total Internal Reflection
Total internal reflection
–
The internal reflection is the situation where all the light
rays is reflected within the medium, when it moves from
a optically denser to a less dense medium with the angle
of incident greater than critical angle.
For total internal reflection to occur:
–
The light ray must travel from a optically denser medium
to a less dense medium.
–
The angle of incidence must be greater than the critical
angle.
Calculation of critical angle
Total Internal Reflection
Eg. Given that the refractive index of water is 1.3, calculate the critical
angle of water.
= 1 / sin c
1.3 = 1 / sin c
sin c = 1/ 1.3
c = 50.3°
= sin r / sin c
1.3 = sin 90°/ sin c
sin c = 1 / 1.3
c = 50.3°
Go through on your own, E.g. 12.13 (pg. 240)
27
Total Internal Reflection
Applications
–
Prism
Total Internal Reflection
In optical fibres, the inner glass fibre has a higher
refractive index compared to the external glass coating.
Total internal reflection occurs when the angle
of incidence is greater than the critical angle.
Uses include optical transmission and endoscope.
Advantages:
– thinner and lighter
– cheaper than metals such as copper
29
Application of
Total Internal Reflection
Rainbow
A rainbow is formed by light from the sun hitting a raindrop.
How is rainbow formed in the sky? Water droplet
What happens when the light ray hits A?
A
B
C
What happens when the refracted light ray hits B?
What happens when the reflected light ray hits C?
It is refracted (bent) and then strikes the back wall of the drop at B.
It is reflected off the back wall of the drop towards C (Total Internal Reflection)
Challenge
1. Three rays of light are incident perpendicular to a semi-circular glass block. The center of the glass block is labelled O. Given that the refractive index of the glass block is 1.5, calculate the critical angle of the glass block. Trace the path of the rays of light before and after they hit the circular face of the glass block.
31
Challenge
A light ray is incident on a triangular quartz prism as shown. The angle of incidence is given as 35º.
Given that the refractive index of quartz is 1.54, calculate the angle of refraction. [2]
Complete the diagram to show the ray of light until it exits from the prism. Calculate and label all the
required angles. [4]
Light passing through the prism is partial reflected at two places. Draw on the diagram, the reflected rays.
[2]
60 º
60 º
