Reflection of Light. 1)Students will discover how light interacts with certain types of surfaces

Transcription

1 Reflection of Light 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 Lesson objectives Teachers' notes

2 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. Lesson objectives Teachers' notes

3 Types of reflection Reflection refers to the change in the direction of light after it meets a surface that returns it to its original medium. Specular reflection 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.

4 \ Types of reflection Reflection refers to the change in the direction of light after it meets a surface that returns it to its original medium. Diffuse reflection 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.

5 Specular reflection irregularities on surface When the size of the surface irregularities is smaller than the light's wavelength ( ), the rays are reflected in parallel rays producing a clear image (mirror image)

6 Diffuse reflection irregularities on surface When the size of the surface irregularities is greater than or approximately equal to the light's wavelength ( ), the rays are reflected in a disorderly manner, in different directions.

7 Reflection refers to the change in the direction of light after it meets a surface that returns it to its original medium. Specular reflection Diffuse reflection Types of reflection

8 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.

9 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?

10 5. Indicate whether each of the following examples involves diffuse reflection or specular reflection a) a flashlight beam on a brick wall b)a boat's reflection on calm waters of a lake c)the blank page of a notebook d)the Sun's reflection on a car's metallic paint. e) a person's reflection in a mirror September 30, 2010

11 6. Green light ( =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? Explain b)the surface is then scored using a tool that creates numerous grooves with an average depth of 2 m. What type of reflection occurs after the surface is scored? 1 nm = 1.0 x m = 1.0 x 10-6

12 Normal: imaginary line (usually draw a dotted line) perpendicular to the reflective surface Geometry ( tangent) of reflection Plane of incidence: defined by the incident ray and normal. Incident ray: light ray that travels toward the reflective surface Angle of incidence (θ i ): formed by the incident ray and the normal Reflected ray: light ray travels away from the reflective surface Angle of reflection (θ r ): formed by the reflected ray and the normal Terminology

13 Geometry of reflection Terminology Incident ray: light ray that travels toward the reflective surface Incident ray: light ray that travels toward the reflective surface Normal: imaginary line (usually draw a dotted line) perpendicular Normal: imaginary to the reflective line (usually surface draw a dotted line) Plane perpendicular of incidence: to the defined reflective by the surface incident ray and normal. Plane of incidence: defined by the incident ray and Angle normal. of incidence: formed by the indicent ray and the Angle normal of incidence: formed by the indicent ray and Reflected the normal ray: light ray travels away from the reflective surface Reflected ray: light ray travels away from the reflective Angle surface fo reflection: formed by the reflecten ray and the Angle normal fo reflection: formed by the reflecten ray and the normal

14 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 surface Plane of incidence: defined by the incident ray and normal. Angle of incidence: formed by the indicent ray and the normal Reflected ray: light ray travels away from the reflective surface Angle fo reflection: formed by the reflecten ray and the normal

15 Laws of reflection 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) Incident ray: light ray that travels toward the reflective surface Normal: imaginary line (usually draw a dotted line) perpendicular to the reflective surface Plane of incidence: defined by the incident ray and normal. Angle of incidence: formed by the indicent ray and the normal Reflected ray: light ray travels away from the reflective surface Angle fo reflection: formed by the reflecten ray and the normal

16 Laws of reflection Second law of relection: the angle of incidence is equal to the angle of reflection: Incident ray: light ray that travels toward the reflective surface Normal: imaginary line (usually draw a dotted line) perpendicular to the reflective surface Plane of incidence: defined by the incident ray and normal. Angle of incidence: formed by the indicent ray and the normal Reflected ray: light ray travels away from the reflective surface Angle fo reflection: formed by the reflecten ray and the normal θ i = θ r θ i θ r

17 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 surface Plane of incidence: defined by the incident ray and normal. Angle of incidence: formed by the indicent ray and the normal Reflected ray: light ray travels away from the reflective surface Angle fo reflection: formed by the reflecten ray and the normal

18 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 surface Plane of incidence: defined by the incident ray and normal. Angle of incidence: formed by the indicent ray and the normal Reflected ray: light ray travels away from the reflective surface Angle fo reflection: formed by the reflecten ray and the normal

19 Spherical mirrors The image formed by any mirror is located either where the reflected light converges, or where the reflected light appears to diverge from.. Terminology Characteristic lengths Incident Three principal ray: light points ray that travels toward the reflective surface 1. Focal length (f) 1. Centre of curvature (C) Normal: imaginary line (usually 2. draw Radius a dotted of Curvature line) (R) perpendicular 2. Focal Point to (F) the reflective surface Plane of incidence: defined by the incident ray and normal. 3. Vertex (v) Angle of incidence: formed by the indicent ray and the 4. Principal normal axis (P) Reflected ray: light ray travels away from the reflective surface Angle fo reflection: formed by the reflecten ray and the normal

20 Spherical mirrors A 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

21 Principal Axis C F V

22 Spherical mirrors A 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 Incident ray: light ray that travels toward surface 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 0 Normal: imaginary line (usually draw a perpendicular to the reflective surface Plane of incidence: defined by the inci normal. Angle of incidence: formed by the indic the normal Reflected ray: light ray travels away fro surface Angle fo reflection: formed by the refle the normal

23 Spherical mirrors A spherical mirror is simply a piece cut out of a reflective sphere. Terminology Characteristic lengths 1. Focal length (f) f = R / 2 ncident 2. Radius ray: of light Curvature ray that (R) travels radius toward of the the sphere reflective the surface mirror was cut from I used this compass to Normal: imaginary line (usually draw a dotted line) perpendicular to the reflective surface draw the mirror, Plane of incidence: defined by therefore incident ray and normal. Angle of incidence: R formed by the indicent ray and he normal Reflected 0 ray: light ray travels away from the reflective surface Angle fo reflection: formed by the reflecten ray and he normal the radius is 7.1 cm

24 Spherical mirrors Geometric considerations Certain relationships can be deduced from the symmetry of spherical mirrors 1. Normal: in a Incident sphere all ray: radii light are ray perpendicular that travels toward the reflective the surface Important: Normal: imaginary line (usually draw a dotted line) perpendicular to the reflective surface Plane of incidence: defined by the point incident ofray and normal. Angle of incidence: formed by the incidence indicent ray and the normal Reflected ray: light ray travels away from the reflective surface Angle fo reflection: formed by the reflecten ray and the normal i.e. the radius that touches the point of incidence is the normal.

25 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 (θ i = θ r ) 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 Spherical mirrors Geometric considerations

26 Spherical mirrors Geometric considerations 2. Mathematical relationship between the radius of curvature (R) and the focal length A N f V P Place the letters in the correct spots

27 Spherical mirrors Geometric considerations Indicate the normal of the point of incident A (draw a line) A N And θ i and θ r θ i θ r

28 Principal rays of a Concave Mirror How to draw a ray diagram for concave mirrors 1. First Principal ray Incident ray is parallel to P Reflection is to F

29 Principal Axis C F V

30 Principal rays of a Concave Mirror 1. First Principal ray Incident ray is parallel to P Reflection is toward F indicate N, θ i, θ r of point of incidence A using the markers P A

31 Principal rays of a Concave Mirror 2. Second Principal Ray If the incident ray travels through F it is reflected parallel to P Indicate which way the rays are travelling... P indicate N, θ i, θ r of point of incidence A using the markers A

32 Principal Axis C F V

33 Principal rays of a Concave Mirror 3. Third principal ray incident ray passes through C reflects back on itself Indicate which way the rays are travelling... P A indicate N, θ i, θ r of point of incidence A using the markers

34 Principal Axis C F V

35 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 called diverging mirrors.

36 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 called diverging mirrors. What do you notice?

37 Convex mirrors V P in this case, there is a negative focal point (it is behind the mirror)

38 Principal rays of a Convex Mirror How to draw a ray diagram with convex mirrors 1. First principal ray. When the incident ray is parallel to P the reflection ray is directed away along a line "originating" from F.

39 Principal Axis V F C

40 Principal rays of a Convex Mirror 1. First principal ray. When the incident ray is parallel to P the reflection ray is directed away along a line originating from F

41 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 θ i, θ r of A.

42 Principal Axis V F C

43 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.

44 Principal Axis V F C

45 Principal rays of a Convex Mirror Third Principal Ray Create and Label the diagram and indicate the third principal ray A Using the markers, indicate the direction of the arrows, the normal (N), and θ i, θ r of A.

46 Principals of Concave and Convex mirrors Review Draw a concave mirror and show the three principal rays of concave mirrors 0 81 A A 0 77

47 Principal Axis C F V

48 Principals of Concave and Convex mirrors Review Draw a concave mirror and show the three principal rays of concave mirrors 0 81 A A 0 77

49 Principals of Concave and Convex mirrors Review Draw a convex mirror and show the three principal rays of convex mirrors 0 81 A 0 77 A

50 Principal Axis V F C

51 Principals of Concave and Convex mirrors Review Draw a convex mirror and show the three principal rays of convex mirrors 0 81 A 0 77 A

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54 Spherical mirrors: review of reflective properties video: 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 surface Plane of incidence: defined by the incident ray and normal. Angle of incidence: formed by the indicent ray and the normal Reflected ray: light ray travels away from the reflective surface Angle fo reflection: formed by the reflecten ray and the normal classroom demo: focal point in car lights,etc

55 Principal Axis C F V

56 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