Module 5

Module 5—Wave Theory of Light

 

Read

 

Read “The Law of Reflection” on pages 653–654 in your textbook.

 

Try This

 

TR 5. Why is the angle of incidence and reflection measured from the normal instead of from the mirror’s surface? (Hint: Look at the specular reflection and diffuse reflection diagrams.)

 

Image Formation in Plane Mirrors

 

Ray diagrams, similar to those that have been used in the previous activities, can illustrate the process of image formation. Consider two light rays originating from the top of a tree by a smooth lake. Constructing a ray diagram that illustrates the law of reflection predicts that an image of the tree will form upside down. The dotted lines “appear” to your eyes to be rays of light from the image, but they don’t actually come from the image. Since the two light rays only appear to have originated from a single point, the image formed in this ray diagram is said to be a “virtual image.”

 

 

Any image formed by a plane mirror is a virtual image. Its orientation and size may be predicted by constructing a simple ray diagram based on the law of reflection.

 

Try This

 

TR 6. Complete "QuickLab" on page 655 of your textbook. Answer questions 1, 2, 3, and 4.

 

TR 7. Using page 656 of your textbook, complete the following table by defining each image characteristic for plane mirrors. The definitions are found in the text and in “Table 13.2” in the textbook. “Table 13.2” gives you vocabulary you should use when discussing these characteristics.

 

Image Characteristic	Definition
magnification
attitude
position
real image
virtual image

 

Image Formation for Curved Mirrors

 

Images formed by curved mirrors look different than those from flat mirrors. The process of image formation, however, is easily demonstrated graphically by ray diagrams, or mathematically using equation, both of which obey the law of reflection.

 

Similar to flat mirrors, the real or virtual image produced by a curved surface can be described by magnification, attitude (erect or inverted), and position. A curved mirror can be formed by cutting off a section of a spherical mirror. The inside surface would be a converging mirror. It has a concave surface that reflects rays to a central focal point. The outside surface would be a diverging mirror. It has a convex surface that causes the reflected light to spread out. See "Figure 13.38" on page 659 in your textbook.

 

Read

 

Read “Image Formation in a Curved Mirror” on page 657 of your textbook.

 

TR 8. Using “Figure 13.36” on page 657 of your textbook as a guide, complete the following table:

 

Term	Definition
centre of curvature (C)
radius of curvature (r)
vertex (V)
principal axis (PA)
principal focal point (F)
focal length (f)

 

Ray diagrams can be used to explain and predict how an image forms in a curved mirror. Similar to that of a flat mirror ray diagram, several rays are sketched to determine the location, size, orientation, and type of image. Generally, three rays are used to follow the path of the light. Note that there are many rays that make the image, but only a few rays are required to identify its location and characteristics.

 

In the ray diagram for a concave mirror on the right, ray 1 travels parallel to the principal axis and is reflected through the mirror’s focal point. Ray 2 is incident to the mirror’s vertex. At the vertex, the surface of the mirror is perpendicular to the principal axis, so the angles of incidence and reflection are equal. Ray 3 travels through the focal point and is reflected parallel to the principal axis. These guidelines for drawing ray diagrams are summarized in the following table.

 

Ray Number	Incident Ray	Reflected Ray
1	parallel to principal axis	through F
2	to vertex	Θr = Θi
3	through F	parallel to principal axis

 

A real image is formed where the rays meet, or converge. A virtual image is formed if the rays “appear” to have converged at some point.

 

The image in this figure is real (the light rays converge at the image), inverted (upside down relative to the object), diminished in size, and located beyond the focal point.

 

A simulation will be used to explore the image characteristics for both concave and convex mirrors.

 

Module 5: Lesson 3 Assignment

 

Remember to submit your answers to LAB 3 and LAB 4 to your teacher as part of your Module 5: Lesson 3 Assignment. 

 

LAB 3. Open the Thin Lens simulation and select “Mirror” from the drop-down menu.

 

 

Click the object arrow and move it to various locations on the principal axis. Using the simulation as a guide, draw ray diagrams and describe the image characteristics when the object is located at the positions listed in Table 1.

 

Table 1: Using Ray Diagrams to Predict Image Characteristics: Converging Mirror

 

Object Position	Ray Diagram	Image Characteristics
very far away (very left side)
outside C
at C
between C and F
at F
inside F

© F. Hwang, NTNU JAVA Virtual Physics Laboratory. Used with permission.

 

LAB 4. Switch the mirror in the simulation to a diverging, convex mirror by making the focal length negative . Complete Table 2. For image characteristics, record real or virtual, attitude, magnification, and position relative to mirror surface.

 

Table 2: Using Ray Diagrams to Predict Image Characteristics: Diverging Mirror

 

Object Position	Ray Diagram	Image Characteristics
very far away (very left side)
close to the mirror

© F. Hwang, NTNU JAVA Virtual Physics Laboratory. Used with permission.

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