EM Waves, Reflection

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Jerome Lawson
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EM Waves, Reflection Level : Conceptual Physics Teacher : Kim Warm-up Question We heard that Superman can see through objects because he has something called x-ray vision.

1 EM Waves, Reflection Level : Conceptual Physics Teacher : Kim Warm-up Question We heard that Superman can see through objects because he has something called x-ray vision. In the movie Superman(1978), Lex Luthor needed to know any weakness before confronting Superman, and he correctly figured out that Superman should not be able to see through lead. How did Lex Luthor know this without encountering Superman? Review in Sound waves, Speed and Frequency Sound waves are caused by the vibration of an object and all waves travel at a certain frequency. The vibration of our voice also produce sound waves that travels through space. All sound waves travels at the same speed in air, which is approximately 340m/s. Even though we can increase the frequency of the sound produced by our voice, the speed of individual waves does not change. That is, the different sounds we hear in our surrounding all travel at the same speed. 340m/s 340m/s Electromagnetic Spectrum Just as a vibrating object, like tuning forks or voice cords charged particle! produces sound waves, vibrating charged particles produce its own kinds of waves. These waves do not need a medium to travel and behave differently from sound waves. Scientists called this type of wave, electromagnetic waves or EM waves. When these EM waves interact with matter, the frequency changes resulting in different behaviors. That is, depending on how these charged particles interact with other charge particles, these charged particles produce EM waves with different frequencies. But all the waves travel at the same speed in the same medium. Scientist decided to name these with EM waves according to their frequencies. See chart next page~! 1

Radio waves has a wavelength of 1 10-4 m to 1 10 5 m. That is, λ=0.0001m to 10000m.

2 Radio waves has a wavelength of m to m. That is, λ=0.0001m to 10000m. Since radio waves has such a long wavelength, they can jump over mountains but they have difficulties in traveling through tunnels. X-rays has a wavelength of m to m. That is, λ= m to λ= m. X-rays has such a small wavelength, so they can pass through most solid objects. However, lead is so dense, x-rays cannot penetrate it. This is why hospitals use lead to protect doctors from the radiation of x-rays. Visible light waves has a wavelength of λ= m to m. Q1) Let s say there is a tall man, a baby and a tiny ant. Their goal is to pass the wall. Who will likely succeed? TALL MAN!! BABY!! ANT!! 2

3 What is Seeing? The reason why we see things is because our eyes are adapted to use visible light to see things. When the sun, or fire or lamp emits light rays, those light rays will bounce off objects. When our eyes catch the reflected lights, we see the object. *~The reason why cannot see through walls is because light cannot penetrate walls. *~The reason why can see through glass is because glass is made so that light passes through. Q2) Why can Superman see through walls but have trouble seeing through lead? Q3) If humans were able to use x-rays to see, then using ordinary walls for privacy would not work. Why? Cloaking Device? A cloaking device is based on the concept of deflecting light off objects to make them seem invisible. That is, by controlling the light ray to not bounce off but to be deflected or bend off the object, then the object cannot be seen. eye! light ray! Reflected light ray! Fig1. The object can be seen!! eye! light ray! Fig2. The object cannot be seen!! 3

4 Reflection *~ Is there a device that enables us to see behind us? When a wave reaches a boundary between two media, some or the entire wave bounces back into the first medium. This is called Reflection Q4) Light is a wave and will reflect or transmit(=go through) when it reaches upon materials. When light reaches the following materials, light will reflect some and transmit some. (Home) Windows Mirrors Metals car windows i) Which material above will transmit most of the light? ii) Which material above will reflect most of the light? Q5) When you look at car windows, you can see inside the car but you can also see your reflection. Why? Q6) You discover a house with glass door at night. The house is bright inside with people inside. When you stand in front of the window, can you see the people better or can the people see you better? Justify your answer. (hint: when light strikes glass perpendicularly, about 4% of its energy is reflected while the rest is transmitted The Law of Reflection Incident rays and reflected rays make equal angles with a line perpendicular to the surface, called the normal Normal Line ( *~The law of reflection applies to both partially reflected and totally reflected waves ) 4

Diffuse Reflection ~Diffuse reflection is the reason why we can see objects from any direction~ - When we touch the surface of the paper in a book, it feels smooth.

5 Diffuse Reflection ~Diffuse reflection is the reason why we can see objects from any direction~ - When we touch the surface of the paper in a book, it feels smooth. - When we compare the surface with the wavelength of light, paper is considered rough - When light is incident on a surface, it is reflected in many directions. This is called diffuse reflection - Although the reflection of each single ray obeys the law of reflection, the many different angles that incident rays encounter cause reflection in many directions - The rays of light incident on a page encounter millions of tiny flat surfaces facing in all directions, so they reflect in all directions 5

Reflection and Refraction. Chapter 29

$Reflection and Refraction. Chapter 29$ Reflection and Refraction Chapter 29 Reflection When a wave reaches a boundary between two media, some or all of the wave bounces back into the first medium. Reflection The angle of incidence is equal