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14-2 Flat mirrors 1.Distinguish between specular and diffuse reflection of light. 2.Apply the law of reflection for flat mirrors 3.Describe the nature.

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Presentation on theme: "14-2 Flat mirrors 1.Distinguish between specular and diffuse reflection of light. 2.Apply the law of reflection for flat mirrors 3.Describe the nature."— Presentation transcript:

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2 14-2 Flat mirrors 1.Distinguish between specular and diffuse reflection of light. 2.Apply the law of reflection for flat mirrors 3.Describe the nature of image formed by flat mirrors.

3 θ i = θ r Reflection of Light Waves – the law of reflection The ray of light approaching the mirror is known as the incident ray (I). The ray of light which leaves the mirror is known as the reflected ray (R). At the point of incidence where the ray strikes the mirror, a line can be drawn perpendicular to the surface of the mirror. This line is known as a normal line (N). The angle between the incident ray and the normal is known as the angle of incidence (θ i ). The angle between the reflected ray and the normal is known as the angle of reflection (θ r ). The law of reflection states that when a ray of light reflects off a surface, the angle of incidence is equal to the angle of reflection.

4 Example #1 The diagram above shows two rays of light striking a plane mirror. Which diagram below best represents the reflected rays?

5 Example #2 A ray of light strikes a mirror at an angle of incidence of 60°. What is the angle of reflection? 1.0° 2.30° 3.60° 4.90°

6 Example #3 The diagram represents a light ray being reflected from a plane mirror. The angle between the incident ray and the reflected ray is 70.°. What is the angle of incidence for this ray? 1.20.° 2.35° 3.55° 4.70.°

7 Once a normal to the surface at the point of incidence is drawn, the angle of incidence can then be determined. The light ray will then reflect in such a manner that the angle of incidence is equal to the angle of reflection. The Law of Reflection is Always Observed (regardless of the orientation of the surface)

8 light reflects off surfaces in accordance with the law of reflection. The Law of Reflection is Always Observed (regardless of the orientation of the surface)law of reflection Specular reflection: Reflection off of SMOOTH SURFACES such as mirrors or a calm body of water. Diffuse reflection: Reflection off of ROUGH SURFACES such as clothing, paper, and the asphalt roadway. Each individual ray obeys the laws of reflection. Specular vs. Diffuse Reflection

9 Why Does a Rough Surface Diffuses A Beam of Light? For each type of reflection, each individual ray follows the law of reflection. However, the roughness of the material means that each individual ray meets a surface which has a different orientation. The normal line at the point of incidence is different for different rays. Subsequently, when the individual rays reflect off the rough surface according to the law of reflection, they SCATTER in different directions. The result is that the rays of light are incident upon the surface in a concentrated bundle and are diffused upon reflection.

10 Class work – today’s date 1.Identify which angle is angle of incidence and which angle is angle of reflection. A BC D Incident angle is ___ Reflected angle is _____

11 2.A ray of light is incident towards a plane mirror at an angle of 30-degrees with the mirror surface. What will be the angle of reflection? 3.A ray of light is approaching a set of three mirrors as shown in the diagram. The light ray is approaching the first mirror at an angle of 45- degrees with the mirror surface. Trace the path of the light ray as it bounces off the mirror. Continue tracing the ray until it finally exits from the mirror system. How many times will the ray reflect before it finally exits?

12 4.A ray is incident on a plane mirror at 25 degrees (incident angle). Draw a diagram to indicate the a.The surface of the mirror b.The line of normal c.The incident ray d.The reflected ray

13 Image Formation in Plane Mirrors An image is formed in a plane mirror because light gives off from an object in a variety of directions. Some of this light (rays) reaches the mirror and reflects off the mirror according to the law of reflection. Each one of these rays of light can be extended backwards behind the mirror where they will all intersect at a point (the image point). Any person who is positioned along the line of one of these reflected rays can sight along the line and view the image - a representation of the object.

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15 Image characteristics in a plane mirror 1.An image has the same size as the object. 2.The image is as far behind the mirror as the object is in front of the mirror. 3.The image has the same orientation as the object. 4.The image is laterally inverted. (left and right reversal) 5.The image is virtual, no actual light meet at the image position. Virtual image can not be captured on a screen.

16 Ray diagrams 1. Draw the image of the object. 2. Pick one extreme on the image of the object and draw the reflected ray that will travel to the eye as it sights at this point. 3. Draw the incident ray for light traveling from the corresponding extreme on the object to the mirror. 4. Repeat steps 2 and 3 for another extreme on the object.

17 Practice – draw ray diagram

18 What Portion of a Mirror is Required? Ray diagrams can be used to determine what portion of a plane mirror must be used in order to view an image. In order to view his image, the man must look as low as his feet, and as high as the tip of his head. The man only needs the portion of mirror extending between points X and Y in order to view his entire image. All other portions of the mirror are useless to the task of this man viewing his own image. to view an image of yourself in a plane mirror, you will need an amount of mirror equal to one-half of your height.

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21 Class work – today’s date 1.If Suzie stands 3 feet in front of a plane mirror, how far from the person will her image be located? 2.If a toddler crawls towards a mirror at a rate of 0.25 m/s, then at what speed will the toddler and the toddler's image approach each other? 3.A ray is incident on a plane mirror at 60 degrees (incident angle). Draw a diagram to indicate the a.The surface of the mirror b.The line of normal c.The incident ray with direction d.The reflected ray with direction

22 4.In the diagram, a light ray leaves a light source and reflects from a plane mirror. At which point does the image of the source appear to be located? a.A b.B c.C d.D 5.When a ray of light strikes a mirror perpendicular to its surface, the angle of reflection is a.0° b.45° c.60° d.90°

23 6.Parallel light rays are incident on the surface of a plane mirror. Upon reflection from the mirror, the light rays will a.converge b.diverge c.be parallel d.be scattered 7.A plane mirror produces an image of an object. Compared to the object, the image appears a.inverted and the same size b.reversed and the same size c.inverted and larger d.reversed and larger

24 8.When a student looks into a plane mirror, she sees a virtual image of herself. However, when she looks into a sheet of paper, no such image forms. Which light phenomenon occurs at the surface of the paper? a.regular reflection b.diffuse reflection c.polarization d.resonance

25 9.In the diagram, a light ray, R, strikes the boundary of air and water. Look at the graph and estimate the approximate degree of the angle of incidence. a.31° b.61° c.119° d.149° 10.A ray of light strikes a plane mirror at an angle of incidence equal to 45°. What is the angle between the incident ray and the reflected ray?

26 11.Use a protractor and ruler to measure and draw the reflected rays in each of the following pictures. a. b.

27 Lab 35 – law of reflection Purpose: Verify the law of reflection: the angle of incidence and the angle of reflection are equal. Material: Plane mirror, Pins, Plane paper, Cardboard, Straightedge, protractor, pencil Procedur: – Briefly describe how the lab is going to be done. Someone who was not present during the lab should be able to understand how the experiment was perforem and be able to reporduce the results by reading your procedure. Data section: – The Data Section should include a data table with labeled column headings (and units) Conclusion/discussion of results: 1.What is the relationship between the incident and reflected angles? 2.Does the light reflect off the front surface or back surface of the mirror? Is there any evidence of this from your experiment?

28 Procedure: 1.Place the mirror on a piece of paper. Draw a line to indicate the front of the mirror. 2.Stick two pins to indicate the path of the ray going from the pin to the mirror; stick two more pins – make sure they line up with the images of previous two pins – this line indicate the reflected rays. Now you should have all four pins in front of the mirror. 3.Remove the pins and carefully trace the path of the incident beam and the reflected beam on the paper. 4.Using a protractor, draw a normal line for the place where the incident ray from the pins hit the mirror. 5.Now measure both the incident and reflected angles. Each person in your lab team must do this for themselves. Combine your data on your data chart.

29 Data table nameangle of incidence ( o ) angle of reflection ( o )


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