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Light and Optics

Science, Grade 6


Table Of Contents: Light and Optics

1. Light

2.1. Electromagnetic Waves
What surrounds you and bombards you constantly? Most of it is invisible but you can’t imagine living without it. It is electromagnetic radiation, a type of energy commonly known as light. This energy is produced by the vibration of charged particles. As charged particles move back and forth, the electric field around it vibrates creating a vibrating magnetic field. The two vibrating fields, which are at right angles to each other, produce electromagnetic waves. These waves can travel through materials as well as a vacuum. All electromagnetic waves travel at the incredible speed of about 300,000 km/s in a vacuum, often called the speed of light. This speed is equal to the wavelength of light times its frequency and is represented by the equation c = wavelength x frequency.
2.2. Light: Wave or Particle
Most of us think of light as a wave. Waves easily explain interactions such as reflection. However, early in the 20th century, some scientists noticed that light hitting a metal surface can sometimes eject electrons. How can light waves do this? Albert Einstein showed this can only happen if light is made up of tiny particles called photons. Einstein revolutionized physics by describing light as photons. Scientists now believe light exhibits both wave and particle properties.
2.3. Electromagnetic Spectrum
Although every electromagnetic wave travels at the same speed, each can have a different wavelength and frequency. The electromagnetic spectrum organizes the types of light by decreasing wavelength and increasing frequency, from left to right. It includes radio waves, microwaves, infrared light, visible light, ultraviolet light, x-rays, and gamma rays. The energy of the electromagnetic wave is also related to wavelength and frequency. Energy is directly proportional to frequency and inversely proportional to wavelength. Higher frequency, shorter wavelength waves have higher energy. The wave energy increases from left to right across the spectrum. On the spectrum, radio waves have the lowest energy while gamma rays have the highest energy.
2.4. Radio Waves and Microwaves
Radio waves and microwaves have the longest wavelength but the lowest energy on the electromagnetic spectrum. Radio waves can range in wavelength from thousands of meters to about 30 cm. Heinrich Hertz discovered radio waves in 1888. Today, in addition to using radio waves to broadcast radio and TV signals, astronomers use radio signals from distant parts of the universe, to study the composition of stars and planets. Microwaves have wavelengths from about 30 cm to 1 mm. In addition to cooking our food, microwaves are used by cell phones and GPS devices.
2.5. Infrared
Infrared light has slightly longer wavelengths then visible light in the range of 700 nm and 1mm. Your TV remote uses infrared light to send a signal to change the channel. You can’t see infrared light, but you can sometimes sense it as heat. Infrared cameras and night vision goggles allow you to see infrared light. Warm objects will appear as bright colors. A limited amount of infrared light from the Sun penetrates Earth’s atmosphere and warms the Earth. Carbon dioxide traps this infrared light and causes warmer than normal temperatures creating the greenhouse effect.
2.6. Visible Light
Your eyes are tuned to see a very small portion of the electromagnetic spectrum which scientists call visible light. Visible light waves have wavelengths in the range of 400 nm and 700 nm. These wavelengths are seen by humans as different colors. The longest wavelength, 700 nm, is seen as red light, while the shortest wavelength of 400 nm is seen as violet light. This range of colors which humans can see is called the visible spectrum. Some of the energy from the Sun that reaches Earth is the visible light part of the spectrum known as white light. White light is a combination of all of the visible light wavelengths or colors. These colors can be observed by passing light through a triangular prism which separates the light into its component colors - red, orange, yellow, green, blue and violet. The separation of visible light into its different colors is known as dispersion.
2.7. Ultraviolet Light
Ultraviolet light has a slightly shorter wavelength than visible light. The wavelength of ultraviolet light ranges between 60 nm and 400 nm. A limited amount of ultraviolet light from the Sun reaches Earth. While too much ultraviolet light can cause sunburn and even potentially skin cancer, our skin cells need ultraviolet light to produce vitamin D.
2.8. X-rays and Gamma Rays
X-rays have wavelengths ranging from 0.001 nm to 60 nm and can penetrate many types of materials, including your body. Gamma rays have wavelengths shorter than 0.1 nm but the highest frequency of all electromagnetic waves. Gamma rays can easily pass through most materials. Fortunately, the Earth’s atmosphere blocks out most of this type of electromagnetic radiation coming from outer space. Doctors use both of these electromagnetic waves in small amounts as either an x ray to look inside the human body, or as gamma rays to kill cancer cells. Astronomers use satellites in outer space to collect x ray and gamma ray information to study the universe.

2. Pause and Interact

3.1. Review
Use the whiteboard tools to complete the concept map.
3.2. Electromagnetic Spectrum
Click on the Terms button, then click and drag each term to the correct box. Use the reset button to clear the terms and start over. Use the gear button to customize the draggable terms.

3. Wave Interactions

4.1. Transmission of Light
Why can you see through glass but not wood? Why do you see a blurry image through frosted glass? When light strikes the surface of any material, it can be reflected, transmitted, or absorbed. Glass allows most of the light to be transmitted and allows us to see through the glass. These kinds of materials are called transparent. Frosted glass allows light to be transmitted, however the rays are bent in many directions, causing a blurry image. Materials like this are called translucent. Opaque materials such as wood absorb the light so you cannot see through it. Opaque materials absorb or reflect light but do not transmit light.
4.2. Color for Opaque Objects
Why does an apple look red and a leaf green? When white light strikes the surface of the apple, only red light is reflected. All the other colors are absorbed by the apple. Your eyes only see the red light, so you say the apple is red. The leaf appears green because only green light is reflected from the leaf. White objects reflect all the colors of light which your eye interprets as white. Black objects absorb all the colors of light. This is why a black t-shirt gets hot in the Sun. The black material absorbs all the energy of the light. The energy from the light is converted to heat.
4.3. Color with Transparent or Translucent Objects
The color of a transparent or translucent object is determined by the color of light it transmits. A green filter will only allow green light to pass through while all the other colors are absorbed. A color filter is a transparent or translucent material such as the lenses of your sunglasses. An object can appear to be a different color than it actually is if it is viewed through a color filter. For example, a red apple normally appears red because it reflects red light. However, when this red apple is viewed through a green filter, the red light is absorbed and the red apple will now appear black. Its green leaf will still look green because the green light can pass through the lens.
4.4. Combining Colors of Light
Primary colors of light – blue, green and red combine to produce white light. Secondary colors of light are produced by combining two different primary colors known as color addition. For example, red and blue combine to produce magenta, red and green produce yellow, while green and blue produce cyan. The colors that you see on your television set are produced by color addition.
4.5. Colors of Pigments
The primary colors of pigments are magenta, yellow, and cyan. Any of these two primary pigments can be combined to produce a secondary pigment color. For example, magenta and cyan when combined make blue. However, when the three primary colors are combined in equal amounts, all the colors of light are absorbed resulting in a black pigment. The colors you see on a printed image are produced by combining different color pigments.

4. Pause and Interact

5.1. Review
Use the whiteboard tools to answer the questions.
5.2. Transmission of Light
Drag each item to the box that describes how light interacts with it.

5. Interactions of Light

6.1. Reflection of Light
You can see your reflection in any shiny surface, from a metal spoon to a puddle of water. Sometimes the reflection is sharp and clear, like the reflection from a mirror. This is called regular reflection. At other times the image is fuzzy, like the reflection in moving water. This is called diffuse reflection. In a regular reflection, parallel light rays hit a smooth surface and are all reflected at the same angle. While in a diffuse reflection the light rays are reflected in different angles.
6.2. Refraction of Light
Light is refracted, or bent, when it passes from one substance to another. In a vacuum and in air, light travels at about 300,000 km/s. If the light travels from air to another material, such as water, the speed of light decreases. As light passes from air to water, instead of continuing in a straight line, the light ray is bent downward, or refracted. This causes an object in water to appear higher and the water to appear shallower than it really is. A material’s index of refraction, represented by n, is a measure of how much light bends when it enters that material. The higher the index of refraction the more light bends. For example, glass will bend light more than water because glass has a higher index of refraction.
6.3. Diffraction of Light
You can hear someone who is speaking to you from another room because sound diffracts or spreads out around openings or corners. It is harder to see the diffraction of light because light travels in a straight line. Diffraction of light can only be seen when light passes through very small openings. The molecules that make up the atmosphere can sometimes act like these tiny openings. Sunlight behind a cloud is diffracted by the air molecules in the cloud, and produces a halo around the cloud.
6.4. Polarized Light
Light from most sources, such as the Sun or a lamp, travels outward in all directions. Light that travels in all directions is called incoherent light. Light that travels in only one direction is called coherent or polarized light. A special filter can be used to polarize light, which only allows light traveling in one direction to pass through. This is how polarizing sunglasses reduce the amount of light you see on a sunny day. However, two polarizing filters placed at right angles to each other will not let any light to pass through. A laser produces a special kind of polarized light that has high energy.

6. Pause and Interact

7.1. Review
Use the whiteboard tools to fill in the table below. Describe each interaction of light and give a real life example of each.
7.2. Interactions of Light
Identification: Select the best answer(s) and then click on the check button.

7. Reflection and Mirrors

8.1. Plane Mirrors
A plane mirror is typically formed from a smooth glass surface with a silver painted back. A regular reflection is formed when light reflects off a plane mirror. When you look in a plane mirror, the image you see is a virtual image. Virtual images don’t exist unless someone is there to see it. In the plane mirror, your image appears to be behind the mirror but is not actually there. The virtual image formed by a plane mirror is upright, the same size as the object, but reversed left to right.
8.2. Curved Mirrors
Curved mirrors have an optical axis. The optical axis is an imaginary line that divides the mirror in half. The point on the optical axis where distant light rays meet when reflected, is called the focal point. The distance from the focal point to the mirror is called the focal length, f. The distance from the object to the mirror is the object distance. The distance from the image to the mirror is the image distance.
8.3. Ray Diagram of a Mirror
A ray diagram is used to locate an image. Where is the image of the hammer? A line is drawn from the top of the hammer to the mirror, parallel to the optical axis. The reflected ray is drawn from the mirror through the focal point. Another ray is drawn from the top of the hammer, through the focal point, to the mirror. The reflected ray is drawn from the mirror and is parallel to the optical axis. The lines will meet at a point showing the location of the image.
8.4. Concave Mirrors
A concave mirror has a surface that curves inward. Two kinds of images can be formed when light reflects off a concave mirror. If the object is farther away than the focal point, a real image is produced. A real image can be projected on a screen. The image produced is upside down and can be smaller or larger than the object. When the object is at the focal point, no image is seen. A virtual image is formed when the object is between the focal point and the mirror. The rays of light never actually meet, but appear to meet behind the mirror. The virtual image is upright and larger than the object.
8.5. Convex Mirrors
A convex mirror has a surface that curves outward. The rays reflected from a convex mirror will never meet. The reflected rays appear to meet behind the mirror, producing a virtual image. The image is upright and smaller than the object. Convex mirrors are useful because they allow you to see a larger field of view.

8. Pause and Interact

9.1. Review
Use the whiteboard tools to fill in the chart below with real, virtual or both.
9.2. Reflection and Mirrors
Click on the Terms button, then click and drag each term to the correct box. Use the reset button to clear the terms and start over. Use the gear button to customize the draggable terms.

9. Refraction and lenses

10.1. Lenses
A lens is a curved piece of a transparent material such as glass or plastic. A lens is similar to a mirror except light is refracted, not reflected. A lens has an optical axis which divides the lens in half. The point on the optical axis where distant light rays meet when refracted, is called the focal point. The distance from the focal point to the lens is called the focal length, f. There is a focal point on both sides of the lens.
10.2. Convex Lenses
A convex lens is thicker in the middle and tapers at the ends. What kind of image will you see when you view an insect through a convex lens? A light ray is drawn from the insect’s head, parallel to the optical axis. It is refracted when it meets the lens and continues through the focal point. Another light ray is drawn through the focal point and is refracted by the lens. The light continues outward, parallel to the optical axis. The two rays meet at a point and form an image. The image is upside down. When the object is beyond the focal point, the image is always real and upside down. If you move the object closer to the lens, between the focal point and lens, the image will be virtual and upright.
10.3. Concave Lenses
Concave lenses are thinner at the center than at the edges. A concave lens can only produce virtual images because light passing through the lens will bend away from the optical axis. The image of the insect through the concave lens is virtual and upright. It will seem to appear on the same side of the lens as the object.
10.4. Light and the Human Eye
Light enters your eye through the cornea and then the pupil. A lens behind the pupil forms an upside down image on the lining of the back of your eyeball, called the retina. The retina is made of two types of specialized cells, rods and cones. Rods respond to light, while cones respond to color. These cells send the image information along the optic nerve to the brain. The brain interprets the information and you see the image. If your eyeball is too long or short, the image on the retina is out of focus. Glasses and contact lenses correct these types of vision problems.
10.5. Uses of light in technology
Telescopes use lenses and mirrors to collect and focus light from distant objects such as planets and stars. There are many types of telescopes that allow us to see into outer space. The simplest telescope, a refracting telescope, uses two lenses. The first lens, the objective, gathers and focuses the light. The second lens, the eyepiece, enlarges the image. A reflecting telescope includes a mirror to help gather light. Microscopes also use a combination of lenses to magnify objects. An objective lens and an eyepiece lens are used to magnify an object to produce a real and enlarged image. Optical fibers are long thin pieces of glass or plastic and are used to transmit light. Light travels along the fiber and only leaves at the other end. Optical fibers are used to send information between telephones or computers.

10. Pause and Interact

11.1. Review
Use the whiteboard tools to fill in the chart below with real, virtual, or both.
11.2. Reflections and Lenses
Click on the Terms button, then click and drag each term to the correct box. Use the reset button to clear the terms and start over. Use the gear button to customize the draggable terms.

11. Vocabulary Review

12.1. Light and Optics Vocabulary Matching
A lens is a curved piece of a transparent material such as glass or plastic. A lens is similar to a mirror except light is refracted, not reflected. A lens has an optical axis which divides the lens in half. The point on the optical axis where distant light rays meet when refracted, is called the focal point. The distance from the focal point to the lens is called the focal length, f. There is a focal point on both sides of the lens.

12. Virtual Investigation

13.1. Mirrors and Lenses
What happens to the image’s size and placement as an object is moved closer to a mirror or lens? Is there a relationship between an object and its image? In this virtual investigation you will choose a type of mirror or lens and investigate the properties of the image formed. When the object in front of the lens or mirror is moved, the size and placement of the image will be displayed. The following variables are used to represent distances and heights: is the distance from object to mirror/lens, is the distance from the image to the mirror/lens, is the height of the object, and is the height of the image.All parallel light rays that strike a mirror converge to one point called the focal point. The distance from the mirror to this point is called the focal length. The focal length depends on the shape of the mirror. In optics not all distances are represented by positive numbers. The focal length of a convex mirror is a negative distance. This negative distance actually represents a point on the “wrong side”, in back, of the mirror. An image can also be on this “wrong side”. For any mirror, if an image distance is negative, then the image is located behind the mirror. Lenses have the same properties. A concave lens also has a negative focal length. For any lens, if an image distance is negative, the image is located in front of the lens.Don’t forget the following information when deciding if an image is real or virtual.All real images are upside down. Real images are in front of the mirror or on the far side of a lens.All virtual images are upright. Virtual images are behind the mirror or in front of the lens.

13. Assessment

14.1. Light
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