Behavior of Light
Reflection occurs with all types of waves: electromagnetic waves, sound waves, Science Words
and water waves. Look at the light beam striking a mirror in Figure 19-20B. The reflection
refraction beam striking the mirror is called the incident beam. The beam that bounces off the
diffraction mirror is called the reflected beam. diffraction grating
Objectives Law of Reflection
Notice that in Figure 19-20B, a line is drawn perpendicular to the surface of the • State and give an mirror. This line is called the normal. The angle formed by the incident beam and example of the law of the normal is the angle of incidence, labeled i. The angle formed by the reflected reflection. beam and the normal is the angle of reflection, labeled r. The law of reflection
states that the angle of incidence is eaual to the angle of reflection. Any reflected • Explain how refraction
light, whether it is reflected from a mirror, a piece of foil, or the moon, follows the is used to separate
white light into the law of reflection. colors of the spectrum,
If you took a smooth piece of aluminum foil and looked into it, you would see a • Describe how slightly distorted image of yourself. What if you crumpled up the foil and then diffraction and unfolded it again? What would you see? The creases in the foil create an uneven interference patterns
demonstrate the wave surface. You couldn't see an image of yourself because the reflected light is
behavior of light. scattered in different directions. Smooth surfaces reflect light in one direction, but
rough surfaces scatter light in many directions.
Seeing yourself in a
mirror (A) is an
illustration of the i = angle of incidence
reflection of light,
illustrated in B. How Normal many times is light
reflected when you angle of reflection use a mirror?
The handle of the net looks bent in (A)
because the light waves are refracted as
they change speed when they pass from the
water to the air. Light slows down and
refracts toward the normal as it passes into a
more dense medium (B). Light speeds up
and bends away from the normal as it
passes into a less dense medium (C).
Place your finger at an angle halfway into a glass of water and look at it through the side of the glass. Do you notice anything unusual? Your finger appears to be bent or even split into two pieces at the point where it enters the water. This happens because of refraction.
Refraction is the bending of waves caused by a change in their speed when they move from one medium to another. The amount of bending that occurs depends on the speed of light in both materials. The greater the difference between the speeds of light in two media/
the more the light is bent as it passes at an angle from one medium to another. The light-refracting properties of certain materials can be especially pleasing to the human eye. When we admire the sparkle in a diamond or other gems, we are seeing refraction in action. Perhaps you have a decorative glass prism hung in a window where you live. When the sun is shining, the prism refracts the sunlight. Have you
observed the colorful refraction pattern that the prism projects in the room? Figure 19-21A shows an example of refraction.
When light passes into a material that slows it down. Figure 19-21B
shows that the light is refracted (bent) toward an imaginary line, called the normal, drawn perpendicularly through the surface of the medium at the point where the wave strikes the surface. The angle between the normal and the ray entering the medium is called the angle of incidence. The angle
between the normal and the ray after it has entered the medium is called the angle of refraction. As
Figure 19-21C shows, when light passes into a material in which light travels faster, it will be
refracted (bent) away from the normal. The amount of refraction also depends on the wavelength of
the light. In the visible spectrum, the wavelengths of light vary from the longer red waves to the shorter
Figure 19-22 White light is refracted into the colors of the visible spectrum as it passes through a prism.
Figure 19-22 shows what happens when
white light passes through a prism. The
triangular prism refracts the light twice:
once when it enters the prism and again
when it leaves the prism and reenters the air.
Because the shorter wavelengths of light are
refracted more than the longer wavelengths,
violet light is bent the most. Which color of
light would you expect to bend the least?
As a result of this varied refraction/ the
different colors are separated when they
emerge from the prism.
Does the light leaving a prism remind you
of a rainbow? Like prisms, rain droplets
also refract light. The refraction of the
different wavelengths can cause white light
from the sun to separate into the individual colors of the visible spectrum. Isaac Newton recognized that white light actually includes all of the
seven colors of the rainbow. In order of decreasing wavelength, the colors you should see in the rain-bow are red, orange, yellow, green, blue, indigo and violet— the same order as the electromagnetic spectrum.
Diffraction and Interference MiniLAB
One of the sounds of summer is the bells or music played How does water bend light? by vendors selling ice cream as they drive their trucks through your neighborhood. Maybe you have heard cars Procedure 1. Place a penny at the bottom of a drive by your home with blasting stereos. You can hear short,opaque cup. Set it on a table these sounds even when buildings separate you from the in front of you. source. Sound waves can bend around corners to reach you. 2. Have a partner slowly slide the
cup away from you just until you Diffraction is the bending of waves around a barrier. In the can no longer see the penny. 1600s, an Italian physicist, Francesco Grimaldi, observed 3. Without disturbing the penny, light and dark areas on the edge of a shadow. If you have your partner slowly pour water
into the cup until you can see the observe the shadow formed when light passes through an penny. open door, you'll see no clearly defined boundary between 4. Reverse roles and repeat the light and dark. Instead, you will see a gradual transition. experiment.
Grimaldi explained this phenomenon by suggesting that Analysis light could be bent around the edges of barriers. 1. What did you observe? Explain Electromagnetic waves, sound waves, and water waves can how this is possible.
2. Sketch the light path from the all be diffracted. penny to your eye after the water