September 30, 2010
Lesson objectives Teachers' notes
1)Students will discover how light interacts with certain types of surfaces
2) Students will understand the laws governing the phenomenon of reflection
3) Discover how images are formed, and their characteristics
Reflection of Light
September 30, 2010
Teachers' notesLesson objectives
Subject:
Topic:
Grade(s):
Prior knowledge:
Cross-curricular link(s):
Physics
Reflection of Light
11
Waves, Ellectromagnetic Spectrum, Basic Reflection of Light, the eye
Geometry
Lesson notes:
This lesson activity focuses on students' knowledge of the 2 types of reflection, geometry of reflection, reflection on a plane mirror: laws of reflection, reflection on spherical mirrors, images.
September 30, 2010
Types of reflection
Specular reflection
Reflection refers to the change in the direction of light after it meets a surface that returns it to its original medium.
occurs on a smooth surface such as a mirror, calm body of water, polished metal. The light rays are parrallel before and after the rays reach the surface.
September 30, 2010
Types of reflection\
Diffuse reflection
Reflection refers to the change in the direction of light after it meets a surface that returns it to its original medium.
occurs on a rough, matte, or dull surface such as rock, rough water, wood, brick, paper. The light rays are reflected in a disorderly manner (not parallel), the reflect in different directions. No discernable image is produced however it allows us to see objects around us, our eyes detect the light rays.
September 30, 2010
When the size of the surface irregularities is smaller than the light's wavelength (λ), the rays are reflected in parallel rays producing a clear image (mirror image)
Specular reflection λ ≥ irregularities on surface
September 30, 2010
When the size of the surface irregularities is greater than or approximately equal to the light's wavelength (λ), the rays are reflected in a disorderly manner, in different directions.
Diffuse reflection λ ≤ irregularities on surface
September 30, 2010
Types of reflection
Specular reflection
Diffuse reflection
Reflection refers to the change in the direction of light after it meets a surface that returns it to its original medium.
September 30, 2010
1. Explain the difference between specular reflection and diffuse reflection.
2. A man is polishing his kitchen floor. He takes a break midway through and notices that half of the floor is shiny but the other half still looks dull. Explain this phenomenon based on what you know about reflection.
September 30, 2010
3. Under bright light, why is it easier to read a book printed on porous paper with a matte finish rather than on smooth, glossy paper?
4. Why does the image of the Sun on the rough surface of a lake appear distorted?
September 30, 2010
5. Indicate whether each of the following examples involves diffuse reflection or specular reflectiona) a flashlight beam on a brick wallb)a boat's reflection on calm waters of a lakec)the blank page of a notebookd)the Sun's reflection on a car's metallic paint.e) a person's reflection in a mirror
September 30, 2010
6. Green light (λ=550 nm) is shone on a metallic surface that has surface irregularities whose average size is 50nm.a) Will the reflection be specular or diffuse? Explainb)The surface is then scored using a tool that creates numerous grooves with an average depth of 2μm. What type of reflection occurs after the surface is scored?
1 nm = 1.0 x 10 -9 1μm = 1.0 x 10-6
September 30, 2010
Geometry of reflectionNormal: imaginary line (usually draw a dotted line) perpendicular to the reflective surface (⊥ tangent)Plane of incidence: defined by the incident ray and normal.Incident ray: light ray that travels toward the reflective surfaceAngle of incidence (θi): formed by the incident ray and the normal Reflected ray: light ray travels away from the reflective surfaceAngle of reflection (θr): formed by the reflected ray and the normal
Terminology
September 30, 2010
Geometry of reflection
Terminology
Incident ray: light ray that travels toward the reflective surface
Normal: imaginary line (usually draw a dotted line) perpendicular to the reflective surfacePlane of incidence: defined by the incident ray and normal.Angle of incidence: formed by the indicent ray and the normalReflected ray: light ray travels away from the reflective surfaceAngle fo reflection: formed by the reflecten ray and the normal
Incident ray: light ray that travels toward the reflective surface
Normal: imaginary line (usually draw a dotted line) perpendicular to the reflective surfacePlane of incidence: defined by the incident ray and normal.Angle of incidence: formed by the indicent ray and the normalReflected ray: light ray travels away from the reflective surfaceAngle fo reflection: formed by the reflecten ray and the normal
September 30, 2010
Geometry of reflection
Terminology
Incident ray: light ray that travels toward the reflective surface
Normal: imaginary line (usually draw a dotted line) perpendicular to the reflective surfacePlane of incidence: defined by the incident ray and normal.Angle of incidence: formed by the indicent ray and the normalReflected ray: light ray travels away from the reflective surfaceAngle fo reflection: formed by the reflecten ray and the normal
September 30, 2010
Laws of reflection
Incident ray: light ray that travels toward the reflective surface
Normal: imaginary line (usually draw a dotted line) perpendicular to the reflective surfacePlane of incidence: defined by the incident ray and normal.Angle of incidence: formed by the indicent ray and the normalReflected ray: light ray travels away from the reflective surfaceAngle fo reflection: formed by the reflecten ray and the normal
First law of reflection: the incident ray, the reflected ray and the normal are all located in the same plane (on the plane of incidence)
September 30, 2010
Laws of reflection
Incident ray: light ray that travels toward the reflective surface
Normal: imaginary line (usually draw a dotted line) perpendicular to the reflective surfacePlane of incidence: defined by the incident ray and normal.Angle of incidence: formed by the indicent ray and the normalReflected ray: light ray travels away from the reflective surfaceAngle fo reflection: formed by the reflecten ray and the normal
Second law of relection: the angle of incidence is equal to the angle of reflection: θi = θr
θi θr
September 30, 2010
Reflection on spherical mirrors
Incident ray: light ray that travels toward the reflective surface
Normal: imaginary line (usually draw a dotted line) perpendicular to the reflective surfacePlane of incidence: defined by the incident ray and normal.Angle of incidence: formed by the indicent ray and the normalReflected ray: light ray travels away from the reflective surfaceAngle fo reflection: formed by the reflecten ray and the normal
September 30, 2010
Reflection on spherical mirrors
Incident ray: light ray that travels toward the reflective surface
Normal: imaginary line (usually draw a dotted line) perpendicular to the reflective surfacePlane of incidence: defined by the incident ray and normal.Angle of incidence: formed by the indicent ray and the normalReflected ray: light ray travels away from the reflective surfaceAngle fo reflection: formed by the reflecten ray and the normal
September 30, 2010
Spherical mirrors
Incident ray: light ray that travels toward the reflective surface
Normal: imaginary line (usually draw a dotted line) perpendicular to the reflective surfacePlane of incidence: defined by the incident ray and normal.Angle of incidence: formed by the indicent ray and the normalReflected ray: light ray travels away from the reflective surfaceAngle fo reflection: formed by the reflecten ray and the normal
Terminology
Three principal points
1. Centre of curvature (C)
2. Focal Point (F)
3. Vertex (v)
4. Principal axis (P)
Characteristic lengths
1. Focal length (f)
2. Radius of Curvature (R)
The image formed by any mirror is located either where the reflected light converges, or where the reflected light appears to diverge from.
.
September 30, 2010
Spherical mirrorsA spherical mirror is simply a piece cut out of a reflective sphere.
Terminology
Three principal points
1. Centre of curvature (C): is the center of the sphere it was cut from
2. Focal Point (F) (the point where parallel rays are focused) is located half the distance (midpoint) from the mirror to the center of curvature (0.5 x r)3. Vertex (v) the geometric centre of the mirror's surface
Principal Axis
The line that joins all 3 points
September 30, 2010
CPrincipal Axis VF
September 30, 2010
Spherical mirrorsA spherical mirror is simply a piece cut out of a reflective sphere.
Incident ray: light ray that travels toward the reflective surface
Normal: imaginary line (usually draw a dotted line) perpendicular to the reflective surfacePlane of incidence: defined by the incident ray and normal.Angle of incidence: formed by the indicent ray and the normalReflected ray: light ray travels away from the reflective surfaceAngle fo reflection: formed by the reflecten ray and the normal
TerminologyThree principal points
1. Centre of curvature (C): is the center of the sphere it was cut from
2. Focal Point (F) (the point where parallel rays are focused) is located half the distance (midpoint) from the mirror to the center of curvature (0.5 x r)3. Vertex (v) the geometric centre of the mirror's surface
0 1 2 3 4 5
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 150°
0°
0
September 30, 2010
Spherical mirrorsA spherical mirror is simply a piece cut out of a reflective sphere.
Incident ray: light ray that travels toward the reflective surface
Normal: imaginary line (usually draw a dotted line) perpendicular to the reflective surfacePlane of incidence: defined by the incident ray and normal.Angle of incidence: formed by the indicent ray and the normalReflected ray: light ray travels away from the reflective surfaceAngle fo reflection: formed by the reflecten ray and the normal
Terminology
Characteristic lengths
1. Focal length (f) f = R / 22. Radius of Curvature (R) radius of the sphere the mirror was cut from
R
I used this compass todraw the mirror, thereforethe radius is 7.1 cm
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
0 1 2 3 4 50°
0°
208
September 30, 2010
Spherical mirrorsGeometric considerations
Incident ray: light ray that travels toward the reflective surface
Normal: imaginary line (usually draw a dotted line) perpendicular to the reflective surfacePlane of incidence: defined by the incident ray and normal.Angle of incidence: formed by the indicent ray and the normalReflected ray: light ray travels away from the reflective surfaceAngle fo reflection: formed by the reflecten ray and the normal
Certain relationships can be deduced from the symmetry of spherical mirrors
point ofincidence
1. Normal:
in a sphere all radii are perpendicular to the surface
Important:
i.e. the radius that touches the point of incidence is the normal.
September 30, 2010
Spherical mirrorsGeometric considerations
2. Mathematical relationship between the radius of curvature (R) and the focal length
When a light ray hits a spherical concave mirror, it is reflected according to the laws of reflection (θi = θr)
The reflected ray intersects the principal axis (P) at a specific point called the focal point (F) which is separated from the vertex (V) by the focal length (f).
f = R/2
September 30, 2010
Spherical mirrorsGeometric considerations
2. Mathematical relationship between the radius of curvature (R) and the focal length
f VP
Place the letters in the correct spots
A
N
September 30, 2010
Spherical mirrorsGeometric considerations
Indicate the normal of the point of incident A (draw a line)
A
N
And θi and θr θrθi
September 30, 2010
Principal rays of a Concave Mirror
1. First Principal rayIncident ray is parallel to PReflection is to F
How to draw a ray diagram for concave mirrors
September 30, 2010
CPrincipal Axis VF
September 30, 2010
Principal rays of a Concave Mirror
1. First Principal rayIncident ray is parallel to PReflection is toward F
A
P
indicate N, θi, θr of point of incidence A using the markers
September 30, 2010
Principal rays of a Concave Mirror
2. Second Principal RayIf the incident ray travels through F it is reflected parallel to P
A
P
Indicate which way the rays are travelling....
indicate N, θi, θr of point of incidence A using the markers
September 30, 2010
CPrincipal Axis VF
September 30, 2010
Principal rays of a Concave Mirror
3. Third principal rayincident ray passes through Creflects back on itself
AP
indicate N, θi, θr of point of incidence A using the markers
Indicate which way the rays are travelling....
September 30, 2010
CPrincipal Axis VF
September 30, 2010
Principal rays of a Convex Mirror
The surface of convex mirrors cause the light rays that are parallel to its principal axis (P) to diverge (spread out). These mirrors can also be calleddiverging mirrors.
September 30, 2010
Convex Mirrors
The surface of convex mirrors cause the light rays that are parallel to its principal axis (P) to diverge (spread out). These mirrors can also be calleddiverging mirrors.
What do you notice?
September 30, 2010
Convex mirrors
V P
in this case, there is a negative focal point (it is behind the mirror)
September 30, 2010
Principal rays of a Convex Mirror
1. First principal ray.When the incident ray is parallel to Pthe reflection ray is directed away along a line "originating" from F.
How to draw a ray diagram with convex mirrors
September 30, 2010
CPrincipal Axis V F
September 30, 2010
Principal rays of a Convex Mirror
1. First principal ray.When the incident ray is parallel to Pthe reflection ray is directed away along a line originating from F
September 30, 2010
Principal rays of a Convex Mirror
2. Second Principal Ray
incident ray directed toward F (if it could pass throught the mirror) is reflected back parallel to P
Using the markers, indicate the direction of the arrows, the normal (N), and θi, θr of A.
< <
September 30, 2010
CPrincipal Axis V F
September 30, 2010
Principal rays of a Convex Mirror
Third Principal Ray
Incident ray directed at C (as if it was to pass through the mirror it would go through C)
it is reflected back on itself.
September 30, 2010
CPrincipal Axis V F
September 30, 2010
Principal rays of a Convex Mirror
Third Principal Ray
Using the markers, indicate the direction of the arrows, the normal (N), and θi, θr of A.
ACreate and Label the diagram and indicate the third principal ray.
0°
144
September 30, 2010
A0°
81
Principals of Concave and Convex mirrorsReview
Draw a concave mirror and show the three principal rays of concave mirrors
A0°
77
September 30, 2010
CPrincipal Axis VF
September 30, 2010
A0°
81
Principals of Concave and Convex mirrorsReview
Draw a concave mirror and show the three principal rays of concave mirrors
A0°
77
September 30, 2010
A0°
81
Principals of Concave and Convex mirrorsReview
Draw a convex mirror and show the three principal rays of convex mirrors
A
0°
77
September 30, 2010
CPrincipal Axis V F
September 30, 2010
A0°
81
Principals of Concave and Convex mirrorsReview
Draw a convex mirror and show the three principal rays of convex mirrors
A
0°
77
September 30, 2010
September 30, 2010
September 30, 2010
Spherical mirrors: review of reflective propertiesvideo: concave and convex mirrors
Incident ray: light ray that travels toward the reflective surface
Normal: imaginary line (usually draw a dotted line) perpendicular to the reflective surfacePlane of incidence: defined by the incident ray and normal.Angle of incidence: formed by the indicent ray and the normalReflected ray: light ray travels away from the reflective surfaceAngle fo reflection: formed by the reflecten ray and the normal
classroom demo: focal point in car lights,etc
September 30, 2010
CPrincipal Axis VF
September 30, 2010
Spherical Aberration (concave mirrors)
Spherical mirrors will not reflect on the focal point when the angles are large (when the length of the mirror is greater than the radius)
Parabolic mirrors are ideal, and all rays will reflect to the focal point
