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PSAA Curriculum. Unit Physical Science Systems. Problem Area. Energy and Power Systems. Optics. Lesson. How are the shadows created?. With an overhead projector allow students to make shadows on the projector screen. How are the shadows created?. Learning Objectives.
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PSAA Curriculum Unit Physical Science Systems
Problem Area Energy and Power Systems
Optics Lesson
How are the shadows created? • With an overhead projector allow students to make shadows on the projector screen. • How are the shadows created?
Learning Objectives • Describe how light travels and state the speed at which it travels in a vacuum. • Describe the processes by which the direction of a light ray can be changed. • Compare the light rays that emerge from a convex lens and concave lens and explain what is meant by the focal length of a lens. • Describe the difference between a real image and a virtual image and explain the properties of images from plane mirrors.
Concave lens Converge Convex lens Diverge Focal length Focus Light rays Lux Photons Real image Reflection Refraction Virtual image Terms
How do light rays travel and at what speed do light waves travel in a vacuum? • The path of light can be plotted using straight lines called light rays. • Light rays are not real, they are simply plotting lines. • Light rays are always straight unless something happens to bend them. • The speed of light is far too fast to measure by ordinary means.
Light Rays (cont.) • Danish Astronomer Vlaus Roemer studied eclipses of the moons of Jupiter and was able to figure out how long light takes to travel to the earth. • In a vacuum, light travels at a speed of 300 million meters per second. • At this speed, a light beam could get from Los Angeles to Atlanta in less than a hundredth of a second.
Albert Einstein published a paper in 1905 explaining what happens when light is emitted from a hot object, such as the filament of a light bulb. Einstein stated that the energy of the light comes in tiny packages, which are now called photons. Light Rays (cont.)
Light Rays (cont.) • The human eye can adjust to many different brightnesses of light. • Photographers, architects, factory managers, and theatre technicians all need ways to measure the brightness of light. • The SI unit of brightness is the lux. • One lux is about the brightness of the light in a theatre lobby. • Inside the theatre, the light is no brighter than a few hundredths of a lux, and brightness gets up to about 50,000 lux in the sunshine.
Light Rays (cont.) • The energy in a lux depends on the color of the light. • If the light is red or violet, a lot more energy is needed to produce one lux than if the light is green from the middle of the visible light spectrum. • Light rays come out of a small source of light and go in all directions. • The farther one is from the lap, the more the rays are spread out. • The farther apart the rays, the dimmer the light.
High-Tech Image Processing is Used to Analyze Agricultural Products
How can the direction of a light ray be changed? • Eye glasses, mirrors, telescopes and microscopes can change the light that enters your eyes to tell you what is going on in the world around you, for specific purposes. • Optional devices work because light rays are straight unless something bends them. • In order to make devices that use light, technicians had to learn how to bend the light rays.
How can the direction of a light ray be changed? (cont.) • One way to bend light rays is to use a mirror. • The reflection from a flat mirror obeys the rule that the angle of incidence is equal to the angle of reflection. • Angle of incidence refers to the angle of the wave hitting the mirror and angle of reflected refers tot he angle of the reflected wave coming off the mirror.
How can the direction of a light ray be changed? (cont.) • Another way to bend light is by refraction. • Refraction is changing the direction of light rays by sending them through a new medium. • A change in direction occurs of the wave speeds up or slows down as it moves from one medium to another.
How can the direction of a light ray be changed? (cont.) • To find out which way the wave bends, draw the normal line at the boundary of the medium. • If the wave of light speeds up as it enters, it bends away from the “normal” line. • If the speed slows down, it bends toward the normal. • Light rays can also be bent by shining them on a rough surface. • Since the surface is rough, rays bounce off in every direction, scattering the rays.
How do light rays react passing through convex lenses and concave lenses? What is focal length of a lens? • A lens is like a series of prisms arranged in a circle. • A lens that causes light rays to come together or converge, is called a convex lens. • The convex lens is thicker in the middle than at the edges. • The point at which the light entering the lens converges is called the focus.
Convex and Concave Lenses • When the incoming rays are all parallel, the distance from the lens is called the focal length. • The stars, like any source, produce rays that diverge, or move apart. • As a light source comes closer to a convex lens, the light rays are no longer parallel. • Light rays now become divergent as they enter the lens and slightly convergent when they leave the lens.
Convex and Concave Lenses (cont.) • Divergence of rays increases as the light source gets closer to the lens as the convergence of the rays leaving the convex lens decreases. • If the source of light is closer to the lens than the focal length, the lens cannot bring the rays together or even make then parallel as they leave the convex lens.
Convex and Concave Lenses (cont.) • Not all lenses bring light rays together. • A concave lens, which is thinner in the middle than at the edges, causes light rays to diverge. • You may use a concave lens to spread the light from a flashlight so that the beam covers a larger area.
Convex and Concave Lenses (cont.) • A concave lens cannot focus light. • The rays are more divergent when they come out of the lens than they were when they went into the lens. • The rays coming out act as if they are all diverging from a single point. • This point occurs in front of the lens and is called the virtual focus.
What is the difference between a real and virtual image? What properties do images possess from plane mirrors? • You point your camera at a building to take a picture. • The building is bathed in sunlight. • Much of that light is reflected from the building, and a very tiny amount of the reflected light gets to the lens of the camera. • The lens uses the light to form a picture on the film that is an image of the building. • The building is composed of a large number of separate points.
Real and Virtual Images • Each point reflects the light that comes to it, scattering the light in all directions. • Each point is like a point source of light. • The camera focuses the light from each point on the film. • Each point of the building is represented by a point of light on the film. • The collection of points of light, focused by the lens, is known as a real image.
Real and Virtual Images (cont.) • When you use a convex lens as a magnifying glass, you form a completely different kind of image. • The object is placed close to the lens at a distance less than the focal length of the lens. • Under these conditions, the rays from a point on the object do come to a focus.
Real and Virtual Images (cont.) • The rays coming out of the lens are divergent, although less divergent than when they entered the lens. • When some of these rays enter your eyes, they seem to be diverging from some point beyond the object.
Real and Virtual Images (cont.) • What you see is an enlarged image of the object, located behind the object. • This kind of image, formed only because the light seems to diverge from it, is called a virtual image.
Review/Summary • How do light rays travel and at what speed do light waves travel in a vacuum? • How can the direction of a light ray be changed?
Review/Summary • How do light rays react passing through convex lenses and concave lenses? • What is focal length of a lens? • What is the difference between a real and virtual image? • What properties do images possess from plane mirrrors?
