THE LAWS OF REFLECTION

The Laws of Reflection Yesterdays lab showed us the two laws of reflection: The angle of incidence equals the angle of reflection. The incident ray, the reflected ray, and the normal all lie in the same plane.

When more than one incident light ray is reflected off a surface, the laws of reflection still hold but the surface affects the way you see the reflected ray. Specular Reflection – is the reflection of light off a smooth , shiny surface. Reflection off a plane mirror is specular reflection. Diffuse Reflection – results from the reflection of light off an irregular or dull surface. Light reflecting off a crumpled piece of aluminum foil is diffuse reflection.

Images in Plane Mirrors Images, such as writing, appear backwards when looking in a plane mirror. Try the “Writing Reflectively Activity:

Activity: Writing Reflectively Place a mirror on the upper half of a sheet of paper. You can use mirror supports to hold the mirror. While looking into the mirror, carefully print your name on the piece of paper so that it appears correctly in the mirror, not on the page. You may practise a few times until you are able to do this successfully, try writing with the hand that you do not usually use. Once you are comfortable with writing using a mirror, use the mirror to carefully print a short (appropriate) message. Exchange your message with your seating partner. Try to decipher the message that you have received from your partner. Check the accuracy of your translation of your partner’s message by using a mirror.

Activity: Writing Reflectively Conclusion: Was writing looking into a mirror difficult? Why? Did you find it easier writing with one hand than with the other? If so, suggest a reason why. From this activity, what general conclusion can you make about how an object and its image in a plane mirror are related?

True Story Leonardo da Vinci was a left-handed Italian artist and scientist born in the 15th century. He used mirror writing in his notebooks when he was writing about his inventions and other ideas. Why do you think he did this?

Using Light Rays to Locate an Image Light sources radiates millions of light rays in all directions. The ones that help us “see” an image are the ones that reflect off the mirror and into our eye. When we look into a mirror, we are seeing a virtual image Virtual Image – an image formed by light coming from an apparent light source; light is not arriving at or coming from the actual image location. We perceive virtual images because our brains know that light travels in straight lines so when we look into a mirror, your brain “projects” these light rays backwards in a straight line. Take a look.

Characteristics of an Image in a Plane Mirror Size of Image (compared to the object; same size, smaller, or larger) Attitude of Image (which way the image is oriented compared to the object: upright or inverted) Location of Image (distance from mirror, closer than, farther than, or same distance as the object to the mirror) Type of Image (real or virtual). A real image is formed when light is actually arriving at the image location. You will learn more about real images in subsequent lessons. SALT

Let’s go back to our tree picture: Size of Image – same Attitude of image – upright Location of Image – same distance as trees to mirror. Type of Image – virtual (behind mirror)

Using Light Rays to Locate an Image This can be done using rays at an angle and rays perpendicular to the mirror. Choose how many points on an object you will need. Draw a incident ray from point A directly to the mirror at a 90o angle. Because this is the “normal” to the mirror, the angle of incidence is 0. Therefore, the angle of reflection is also 0. To locate the image, extend the reflected ray back behind the mirror using a dashed line. Draw another incident ray from point A at an angle to the mirror. At the point where the incident ray hits the mirror, draw a normal. Measure the angle of incidence with a protractor. Using the knowledge that the angle of reflection is equal to the angle of incidence, draw the reflected ray. Repeat this step for any other points on the object.

A1 A B B1

References Specular reflection image. http://micro.magnet.fsu.edu/optics/lightandcolor/reflection.html. Accessed October 18, 2010. Science Perspectives 10. Nelson Education Ltd. Toronto, ON. 2010.