The electromagnetic spectrum consists of the range of all possible wave frequencies of electromagnetic radiation. Electromagnetic radiation with low frequency and long wavelengths are found in the microwave, radio, and infrared portions of the electromagnetic spectrum.
1. 4/21/2020 Teacher Background | STEMscopes Properties of Visible Light Teacher Background Properties of Visible Light The electromagnetic spectrum consists of the range of all possible wave frequencies of electromagnetic radiation. Electromagnetic radiation with low frequency and long wavelengths is found in the microwave, radio, and infrared portions of the electromagnetic spectrum. High-frequency, short wavelengths are found in the ultraviolet, x-rays, and gamma rays portion of the electromagnetic spectrum. Visible light occurs in the middle of those two extremes. Visible light is the small part of the electromagnetic spectrum that we can see. Colors exist at different wavelengths from lowest energy to highest energy: red, orange, yellow, green, blue, indigo, and violet. Light waves can be re ected, absorbed, or transmitted through various materials. Re ection is the ability of a wave to bounce back but not be absorbed. When a wave strikes a boundary, it makes an abrupt change of direction, depending upon the angle the wave strikes; however, the speed of the wave is not changed. One characteristic of waves is the law of re ection. This law states that the angle at which the wave approaches a at, re ective surface is equal to the angle at which the wave leaves the surface. Some surfaces are shiny and re ect a mirror-like image called specular re ection. When solid surfaces are rough, light can be scattered resulting in diffuse re ection. Scattering or diffusing occurs when light bounces off an object in a variety of directions. The amount of scattering that takes place depends on the wavelength of the light and the size and structure of the object. This phenomenon causes the sky to be blue. Incoming light rays from the Sun encounter the oxygen and nitrogen molecules in the atmosphere which cause the shortest wavelengths of visible light (blue and violet) to be scattered in every direction. The longer wavelengths of red and yellow are not affected by these molecules and pass right through. Re ection also depends on the shape of the surface. If the re ective surface is convex (curves away from the incoming light), the light rays bounce off and spread out (diverge), like looking at the backside of a spoon. The image in a convex mirror appears to be smaller, upright, and covers a wider eld of view. Security mirrors on the sides of halls or building walls and side mirrors on trucks and cars take advantage of the wide eld of view created from a convex mirror re ection. If the re ective surface is concave (curves toward the incoming light), the light rays bounce off and come to a point (converge), like looking at the bowl of a spoon. The image is reversed but larger. Telescopes, a dentist’s mirror tool, and cosmetic mirrors take advantage of the magni ed image created from a concave mirror https://app.acceleratelearning.com/scopes/15628/elements/692926 1/3
2. 4/21/2020 Teacher Background | STEMscopes re ection. When the object is between the center of the curvature and the focal point, the image is enlarged. The parabolic-shaped concave mirror focuses the parallel lights to a single point. The behavior of re ected light follows the law of re ection, i.e., the angle of incidence equals the angle of re ection. This means that the angle of the incoming ray on shiny surfaces will equal the angle of the outgoing ray. The speed and direction of a wave changes when a wave crosses from one medium to another. This change causes the wave to bend toward the denser medium if it is moving at a non-perpendicular angle to the boundary. This phenomenon, known as wave refraction, causes objects in water to appear to be bent when light passes from air through water. The combination of the refraction of sunlight as it enters a water droplet and then is re ected off the back of the droplet and refracted back out the droplet causes the sunlight to split into its various wavelengths, which produces a rainbow or spectrum of color. The principle of refraction is used, for example, in making lenses for cameras, eyeglasses, and refracting telescopes. Different mediums have different refraction indexes. The higher the index number, the more light is slowed down and therefore bent. Absorption of light occurs when a ray of light strikes a surface and is not re ected or refracted. The best example of this is wearing the color black outside. We see the black color because all the wavelengths of light are absorbed by it, and we feel heat because of the energy being carried by them. To compare, wearing a white shirt in the Sun is much cooler because all the wavelengths are re ected, allowing us to see white. Since different atoms and molecules have different natural frequencies of vibration, they will selectively absorb different frequencies of visible light. So most objects will selectively absorb light while also transmitting and/or re ecting some of it. When absorption occurs, heat energy is generated. For example, a red apple absorbs all frequencies of white light but re ects shades of red to the observer’s eye. Visible light rays from light sources such as the Sun or lamps fall onto objects and then re ect off and enter the eye. The rays are refracted rst by the transparent cornea and then by the lens which focuses the rays on the retina at the back of the eye. The retina converts the light to electrical nerve impulses, which are sent to the brain to be interpreted for shape, dimension, color, etc. The eye can only detect wavelengths of visible light between approximately 750 to 400 nanometers in wavelength in the electromagnetic spectrum, which correspond to the colors and shades of red, yellow, green, blue, indigo, and violet. The eye cannot see high-frequency wavelengths such as ultraviolet, x-rays, and gamma rays nor low-frequency wavelengths https://app.acceleratelearning.com/scopes/15628/elements/692926 2/3
4. Absorbed Properties of Visible Light Picture Vocabulary To soak up or take something in Frequency Light The number of wave cycles that pass a given A form of energy that exhibits wave-like behavior point per unit of time as it travels through space; part of the electromagnetic spectrum Light Path Reflected Path followed by a beam of light when passing through an optical device or a particular part of one Energy waves bouncing off the surface of an object (i.e., mirrors or echoes return energy back to the source)
5. Transmitted Transparent Allowing light to pass through so that objects Passed from one place to another behind can be distinctly seen
6. Properties of Visible LIght A rainbow is a perfect example of how visible light is composed of a spectrum of colors. To see a rainbow, your back must be to the Sun as you look up at about a 40-degree angle to where the air has suspended droplets of water. Each droplet of water acts as a tiny prism that transmits, refracts, and reflects light to your visible light – the range of wavelengths of electromagnetic radiation that our eyes can detect spectrum – the band of colors transmit – to pass through a medium produced when light is separated into its refract – to bend or change direction of a wave component wavelengths reflect – to bounce back Electromagnetic Waves Light is made of electromagnetic waves that do not need a medium in which to travel. Actually, light waves are a combination of electric and magnetic energy that travel as particles called photons. These photons have different wavelengths along the electromagnetic spectrum. In order of decreasing wavelengths, they are the following: radio waves (lowest-energy photos), microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays (highest-energy photons). Notice that visible light is only a small portion of the entire electromagnetic spectrum. In a vacuum, all wavelengths travel at the same speed of 300,000,000 kilometers/second, which is 186,000 miles/second. Frequency refers to the number of waves that pass a given point in a given period of time (usually 1 second). Electromagnetic waves with longer wavelengths have lower frequencies; in other words, fewer waves pass a given point each second. Electromagnetic waves with shorter wavelengths have higher frequencies, which means more waves pass a given point each second. 1
7. Properties of Visible LIght Visible Light and Color Visible light is the only part of the electromagnetic spectrum that human eyes can detect. Visible light is made up of photons of different wavelengths ranging from about 400 to 700 nanometers (nm). A nanometer is a billionth of a meter. Each of these different wavelengths corresponds to a different color. We see the longest wavelengths of visible light as red light. We see the shortest wavelengths as violet light. White light can be separated into different colors using a special lens called a prism. As visible light passes from air into the glass, the change in medium causes a change in direction and speed. As a result, violet wavelengths are refracted, or bent, at a greater angle than red wavelengths. Refraction causing the separation of white light into its various color wavelengths is called dispersion. Wave Behavior When electromagnetic waves encounter surfaces of different media, they behave differently in the form of reflection (bouncing), scattering (incoming photons reflecting in all directions), refraction (bending), dispersion (when refracting white light can be separated into different wavelengths), absorption (light energy transferring to another medium), and diffraction (spreading or bending around the edges of an obstacle). Transmission of waves occurs when light waves pass through a medium. Sometimes light waves can pass through a medium without being refracted, such as with clear glass. 2
8. Properties of Visible LIght The most commonly observed wave behavior is reflection. You observe this phenomenon whenever you see your image in a mirror. Reflection of light waves occurs whenever a light wave strikes a surface and then bounces back. The angle of incidence equals the angle of reflection. Scattering is different from reflection in that scattered light goes back out in all directions. White surfaces, such as walls, scatter light in all the colors that strike them. Absorption Not all light is reflected or scattered. Black absorbs all light. Colored surfaces absorb all wavelengths except for the color reflected. Absorption occurs when all or some of the light energy from light waves is transferred from one medium to another. Certain pigments reflect or transmit the wavelengths they cannot absorb, making them appear in the corresponding color. 3
9. Properties of Visible LIght Law of Reflection The reflection of waves from a boundary is similar to the way a billiard ball strikes and bounces away from a wall. If a ball strikes a wall head-on (meaning it is traveling perpendicular to the wall), the ball will bounce back in exactly the same direction from which it traveled. However, if a ball strikes a wall at an angle to the perpendicular (called the angle of incidence), it will bounce away from the wall at the same angle to the perpendicular (the angle of reflection). Waves The reflection of hot-air balloons is behave in the same way, as shown below. In fact, this seen on the surface of a still lake. is called the law of reflection. While a wave’s direction Some of the light that has been changes during reflection, its speed does not. reflected from the hot-air balloon travels through the air to the lake. When this light strikes this boundary, some of it reflects back into the air. This causes an image of the hot-air balloon to appear on the lake’s surface. A wave (red arrow) strikes a barrier (blue line) at an angle θi to the perpendicular line. This is the angle of incidence. The wave bounces away at an angle θr. This is the angle of reflection. The law of reflection states that the angle of incidence is equal to the angle of reflection. Not all materials transmit light clearly. For example, transparent objects allow all light to pass. What do you think are good uses of transparent materials? Translucent objects transmit only some light, resulting in objects on the opposite side appearing fuzzy, such as some office windows or shower doors. Opaque surfaces do not allow light to pass through them. Light striking an opaque surface is either absorbed by the surface or reflected from it. 4
10. Properties of Visible LIght A wave can also be transmitted through a boundary, meaning that it passes through the boundary from one medium to another. In fact, at most boundaries, part of the wave is reflected and part of it is transmitted. Most waves move at different speeds through different media. For example, light moves quickly through air but more slowly through water. Waves change speeds as they transmit from one medium to another. This change in speed usually causes a wave to refract, or bend, as it passes through the boundary. Refraction can be thought of as the black dots in this diagram, which represents a group of students marching across a room. The students represent a light wave. The left side of the room represents air (a relatively fast medium), and the right side represents water (a relatively slow medium). When the students pass from left to right, one student crosses the boundary first. This student begins moving slowly, but the rest of the group still travels quickly, causing the students to change direction, or bend, as they move into the new medium. This is known as refraction. Refraction causes light from the Sun to spread out into different colors as it passes from air into new media, such as water or glass. For example, the different wavelengths of sunlight all refract at different angles when they pass through a raindrop. This causes sunlight to spread out into a rainbow as it refracts through raindrops after a storm. Refraction through concave or convex lenses allows correction for poor vision or magnification of objects. Refraction through water bends the image. Refracting telescopes use lenses to gather light from distant sources, such as planets, stars, or galaxies, and focus that light through an eyepiece. . 5
11. Properties of Visible LIght Have you ever watched water waves pass through a narrow slit in a barrier? When the waves pass through, they spread out radially. This is known as diffraction. For example, when a light bulb is turned on in a dark room, light waves spread out radially from the light bulb. However, once the waves have traveled a certain distance, they can be thought of as plane waves that move in Plane waves (left) spread out radially parallel sheets, as shown in the image on the right. When a (right) when they pass through plane wave passes through a narrow slit or a barrier, the a slit in a barrier. wave will act as though it is originating from a point source again. Thus, the wave spreads out, or diffracts. This can also happen when a wave approaches a solid barrier. In these cases, when the wave passes along any edge of the barrier, it will spread out radially around the barrier. In this way, waves will appear to bend around barriers. This is the reason you can hear sounds that are emitted from behind a wall or large building. Everyday Life: Examples of Diffraction You can see an example of diffraction when light passes through tiny grooves on the surface of a CD and the light is separated into a rainbow of colors. This effect is also seen when light is diffracted off the surface of oil. Diffraction is also used in producing special holographic images. Special photographic techniques can diffract light and record it in 3-D. Diffraction images can be used as holograms as a security measure on identification cards or credit cards. These images can only be read by an optical scanner. Supermarket checkout scanners use holographic optical elements (HOEs) that can read a universal product code (UPC) from any 6
12. Properties of Visible LIght In this exercise, you will show what you know about reflection and refraction. In the diagram on the right, the double lines represent a pane of glass and the single line represents a mirror, both shown edgewise. The arrow is a ray of light. Draw the path of the light ray until it reaches point X. Show how the change in the speed of the light ray affects its direction as it passes from one medium to another. Remember, light travels more slowly in glass than it does in air. Hint: The light will bend both when it enters and when it exits the glass! Explain how a rainbow is produced. Why do you see something as white, black, or another specific color? What are electromagnetic waves, and how can they be seen? 7
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