Embedded Files
Competency 4: Describe what happens when light is reflected, refracted, transmitted, and absorbed
Reflection of Light
Reflection of Light
Reflection is when light bounces off an object. If the surface is smooth and shiny, like glass, water, or polished metal, the light will reflect at the same angle as it hit the surface. This is called specular reflection.
Types of reflection
Types of reflection
Light reflects from a smooth surface at the same angle as it hits the surface. For a smooth surface, reflected light rays travel in the same direction. This is called specular reflection.
Specular reflection is the angle at which light hits a reflecting surface is called the angle of incidence, and the angle at which light bounces off a reflecting surface is called the angle of reflection.
For a rough surface, reflected light rays scatter in all directions. This is called diffuse reflection.
Diffuse reflection is when light hits an object and reflects in lots of different directions. This happens when the surface is rough. Most of the things we see are because light from a source has reflected off it.
For example, if you look at a bird, light has reflected off that bird and traveled in nearly all directions. If some of that light enters your eyes, it hits the retina at the back of your eyes. An electrical signal is passed to your brain, and your brain interprets the signals as an image.
Refraction of Light
Refraction of Light
Refraction is the bending of light (it also happens with sound, water, and other waves) as it passes from one transparent substance into another.
This bending by refraction makes it possible for us to have lenses, magnifying glasses, prisms, and rainbows. Even our eyes depend upon this bending of light. Without refraction, we wouldn’t be able to focus light onto our retina.
Change of speed causes a change of direction. Light refracts whenever it travels at an angle into a substance with a different refractive index (optical density). This change of direction is caused by a change in speed. For example, when light travels from air into water, it slows down, causing it to continue to travel at a different angle or direction. How much does light bend?
Refraction of light in water
When light travels from air into water, it slows down, causing it to change direction slightly. This change of direction is called refraction. When light enters a more dense substance (higher refractive index), it ‘bends’ more towards the normal line.
The amount of bending depends on two things:
● Change in speed – if a substance causes the light to speed up or slow down more, it will refract (bend) more.
● The angle of the incident ray – if the light is entering the substance at a greater angle, the amount of refraction will also be more noticeable. On the other hand, if the light is entering the new substance from straight on (at 90° to the surface), the light will still slow down, but it won’t change direction at all.
If light enters any substance with a higher refractive index (such as from air into glass) it slows down. The light bends towards the normal line.
If light travels into a substance with a lower refractive index (such as from water into the air) it speeds up. The light bends away from the normal line. A higher refractive index shows that light will slow down and change direction more as it enters the substance.
Lenses
Lenses
A lens is simply a curved block of glass or plastic. There are two kinds of lenses. A biconvex lens is thicker in the middle than it is at the edges. This is the kind of lens used for a magnifying glass. Parallel rays of light can be focused on a focal point. A biconvex lens is called a converging lens.
Converging lens
Converging lens
Each light ray entering a converging (convex) lens refracts inwards as it enters the lens and inwards again as it leaves. These refractions cause parallel light rays to spread out, traveling directly away from an imaginary focal point.
A biconcave lens curve is thinner in the middle than it is at the edges. Light rays refract outwards (spread apart) as they enter the lens and again as they leave.
Concave lens
Concave lens
Each light ray entering a diverging (concave) lens refracts outwards as it enters the lens and outwards again as it leaves. These refractions cause parallel light rays to spread out, traveling directly away from an imaginary focal point.
Light is Transmitted
Light is Transmitted
Reflection and transmission of light waves occur because the frequencies of the light waves do not match the natural frequencies of vibration of the objects. When light waves of these frequencies strike an object, the electrons in the atoms of the object begin vibrating. But instead of vibrating in resonance at large amplitude, the electrons vibrate for brief periods with small amplitudes of vibration; then the energy is reemitted as a light wave. If the object is transparent, then the vibrations of the electrons are passed on to neighboring atoms through the bulk of the material and reemitted on the opposite side of the object. Such frequencies of light waves are said to be transmitted. If the object is opaque, then the vibrations of the electrons are not passed from atom to atom through the bulk of the material. Rather the electrons of atoms on the material's surface vibrate for short periods and then reemit the energy as a reflected light wave. Such frequencies of light are said to be reflected.
Where Does Color Come From?
Where Does Color Come From?
The color of the objects that we see is largely due to the way those objects interact with light and ultimately reflect or transmit them to our eyes. The color of an object is not actually within the object itself. Rather, the color is in the light that shines upon it and is ultimately reflected or transmitted to our eyes. We know that the visible light spectrum consists of a range of frequencies, each of which corresponds to a specific color. When visible light strikes an object and a specific frequency becomes absorbed, that frequency of light will never make it to our eyes. Any visible light that strikes the object and becomes reflected or transmitted to our eyes will contribute to the color appearance of that object. So the color is not in the object itself, but in the light that strikes the object and ultimately reaches our eye. The only role that the object plays is that it might contain atoms capable of selectively absorbing one or more frequencies of the visible light that shine upon it. So if an object absorbs all of the frequencies of visible light except for the frequency associated with green light, then the object will appear green in the presence of ROYGBIV. And if an object absorbs all of the frequencies of visible light except for the frequency associated with blue light, then the object will appear blue in the presence of ROYGBIV.
Light is Absorbed
Light is Absorbed
Visible light waves consist of a continuous range of wavelengths or frequencies. When a light wave with a single frequency strikes an object, several things could happen. The light wave could be absorbed by the object, in which case its energy is converted to heat. The light wave could be reflected by the object. And the light wave could be transmitted by the object. Rarely however does just a single frequency of light strike an object. While it does happen, it is more usual that visible light of many frequencies or even all frequencies is incident towards the surface of objects. When this occurs, objects tend to selectively absorb, reflect, or transmit light at certain frequencies. That is, one object might reflect green light while absorbing all other frequencies of visible light. Another object might selectively transmit blue light while absorbing all other frequencies of visible light. How visible light interacts with an object is dependent upon the frequency of the light and the nature of the atoms of the object.
Visible Light Absorption
Visible Light Absorption
Atoms and molecules contain electrons. It is often useful to think of these electrons as being attached to the atoms by springs. The electrons and their attached springs tend to vibrate at specific frequencies. Similar to a tuning fork or even a musical instrument, the electrons of atoms have a natural frequency at which they tend to vibrate. When a light wave with that same natural frequency impinges upon an atom, then the electrons of that atom will be set into vibrational motion. (This is merely another example of the resonance principle introduced in Unit 11 of The Physics Classroom Tutorial.) If a light wave of a given frequency strikes a material with electrons having the same vibrational frequencies, then those electrons will absorb the energy of the light wave and transform it into vibrational motion. During its vibration, the electrons interact with neighboring atoms in such a manner as to convert its vibrational energy into thermal energy. Subsequently, the light wave with that given frequency is absorbed by the object, never again to be released in the form of light. So the selective absorption of light by a particular material occurs because the selected frequency of the light wave matches the frequency at which electrons in the atoms of that material vibrate. Since different atoms and molecules have different natural frequencies of vibration, they will selectively absorb different frequencies of visible light.
Page updated
Report abuse