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Reference: Physics for every day phenomena, McGraw-hill.

We can use the wave model to explain some of the light phenomena like interference or diffraction. The model explains that light is an electromagnetic wave carrying energy. Like light, sound is a wave too.  In the case of the sound, the air is waving. In the case of the light , electric and magnetic fields are waving. We will study this model in a coming chapter. We can also explain light using the particle model. This model explain other phenomena like the photoelectric effect (quantum mechanics). The light is made of photons, tiny mass less particles. (They are really particle-wave). When studying the formation of images it is convenient to use a third model. The ray model. Light can be visualized as consisting of rays, emerging from a source (like a lamp), going in a straight line unless deflected in some way. Light can be defected at a boundary (it will bounce back if it encounters a mirror, it will bend a the boundary of 2 different materials). MORE about the nature of light. (U don’t have too, for your culture)

In empty space, light travels in a straight line. The path changes only if the light interacts with a material object.

 

 

 

To visualize the reflection of light, we can use the wave model (green lines) or the ray model (just the orange rays). It is easier to pretend that light are made of rays when studying geometric optics.

Please note: the coming ray is called the incident ray, the ray leaving the surface is called the reflected ray. The imaginary line perpendicular to the surface is called the normal. We have also the angle of incidence Θi and the angle of refection Θr.

Source: physics of everyday phenomena, McGraw-hill.             

When striking an object, 3 things can happen:

A) light can go through. You can see the building across the street through the window because the light from the building is able to cross the glass and to reach your eye. In the case, the light is transmitted.

 

B) Light can bounce back off the surface. A surface can reflect light as shown in one plane. This is called specular reflection. (mirror type surface. Or light can be reflected in all direction, diffusely (most objects). This is called diffuse reflection (from diffuse reflector)

 

C) Light can also be absorbed by a material. (black objects). A perfectly black objects will absorb all the light that comes to it.

(called black body)

 

INTRODUCTION

Usually an object will absorb some of the light and reflect the other part (B and C). (a red object absorb all the colors but red). Or an object will reflect some of the light and transmit the other part (piece of glass, you can see yourself in the window but you can also see the building across)

II Reflection, seeing things

 

The law of reflection states that the angle of reflection is equal to the angle of incidence. We suppose the reflection to be specular (beam of light striking a mirror). The ray of incidence and the ray of reflection both lie in the same plane. Observe the figure. The imaginary line perpendicular to the surface is called the normal. Using this idea, you can explain the formation of an image in a mirror/ See figure below.

(source: physics of every day phenomena, Mcgraw hill).

 

 

We can see the candle in the mirror because our brain extend the reflected rays. The image is called virtual. No rays reach that kind of image. The virtual image is the symmetric of the object about the mirror.

1) Can you draw a boy looking at his feet in the mirror ? Can you see that if the mirror is too short, he can’t see his feet ?

 

animation

2) A beam of light strikes at an angle of 40 degrees. What is the angle between the incident and reflected beam (draw the situation) ?

 

3) If you are to look in a vertical mirror and see your feet, a ray must reflect from your feet, and then specularly from the mirror to your eye. What is the smallest vertical mirror in which you can see your feet if you are 5ft tall ?

 

4) you are taking a picture of your self in a mirror while standing 12ft from the mirror. At what distance should you focus your camera ?

 

5) What is meant by the phrase “normal to the surface ?

6) Complete the figure A by tracing the reflected beam and the normal. The surface is a mirror. Using a protractor, find the angle of incidence.

 

 

 

7) draw the virtual image of the smiley in the mirror. Trace a few rays going from the object (the smiley) to your eye. Don’t forget to trace the virtual rays

(in the mirror)

 

YOU NEED A RULER !!!

AND A SHARP PENCIL and COLOR PENCILS

Virtual

image

object

8) A dry road is a diffuse reflector, while a wet road is not. Sketch a car with its headlights illuminating the road ahead. Show why the wet road appear darker to the driver than the dry road would.

 

9) Why is it desirable that the pages of a book be rough rather then smooth and glossy?

 

10) A ray of light incident upon a mirror makes an angle of 36 degrees with the mirror. What is the angle between the incident ray and the reflected ray.

Fig A

III Bending rays

 

11) imagine an roman army, marching in close row in a field. They all march at the same speed. They arrive, at an angle, to an area covered with mud. (see figure). What would happen to the troop ?

(hint: In a same row, the soldiers in the mud will move slower than those still on the grass)

 

12) When crossing a boundary between 2 materials (like going from air to glass or air to water) , light bends. We say the light is refracted. It bends toward the normal if the second medium is denser then the first one. (air to glass) . It bends away from the normal if the second medium is less dense. (water to air). In the 2 following figures, can you trace the refraction ray? Don’t forget to place θr