Physics Optics Pre Lab - Pre lab answers PDF

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Lab 3: How can microscopes magnify objects? Exploring Light and Lenses Introduction Optics—the sub-field of physics focussing on the study of light—is important to many areas of biology and engineering, including optometry, ecology, photography, botany, neurobiology, and molecular biology. The use of optics in biology has evolved from the simple light microscope used by Darwin to the complex, livecell, high resolution microscopes used in current cutting-edge research. In engineering and ecology, the use of binoculars, cameras, and surveying scopes have followed suit. In this two-week lab, we will be exploring the behavior of light and lenses. Microscopes and telescopes usually employ at least two lenses together— but as we are only beginning to learn about light, we will be exploring single-lens systems. In this week’s lab, your task will be to design an experiment that can help you determine the relationship between the focal length, image distance, and object distance of a device in which the object-to-image distance is held a fixed length, L. This pre-lab is designed to help introduce you to the ideas of geometric optics if you have not yet encountered it in class. Plan on spending roughly 45 minutes working on this.

Getting Set Up This pre-lab makes use of the PhET optics simulation. • Visit the site https://phet.colorado.edu/en/simulation/bending-light • Click play on the simulation thumbnail. This runs on HTML5 and will play straight on your browser • Select the “prisms” icon

How does light travel? 1. Set up the virtual laser to emit multiple beams of light. Select the semi-circular lens and place it with its flat-side against the laser:

Describe what you see in words. Draw a picture of what you see. I see all the beams meeting at a certain focal length. As if all entering the same angle but the ones on top bending later as well as the ones on the bottom.

2. What happens to the light coming out of the laser when the lens is removed? Draw what you see. The light beams go back to going straight and do not bend or meet at any focal point.

3. One of these set-ups is called a “Parallel ray” arrangement and the other is called a “Point source” arrangement. Which is which? Why?

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The parallel ray is definite the beams that do not meet while the point source is the one with the lens where the do meet at a focal point.

Focussing Light 1. Set up the laser to emit “parallel” rays again. Place the circular lens in the path of the light. What happens to the the rays of light because of this lens? The rays of light meet at a shorter focal length, but they do converge and meet. This lens is called a bi-convex lens, meaning that it bulges out on both sides. 2. Let’s explore what happens if we use other optical elements besides the bi-convex lens. Still using the parallel ray set-up, try replacing the bi-convex lens with the concave lens (one side is pushed inwards). Try mixing and matching these elements. You should see that some arrangements spread the light out as it passes through the lens and other arrangements bring the light together as it passes through the lens. The “spreading out” of the lig ht is referred to as diverging. The “bringing together” of the light is referred to as converging. In the space below, sketch some of the combinations which produce diverging and converging light rays. Converging Diverging

3. Now look at the rays passing through a single lens. When does the diverging/converging happen for light traveling through the lenses? Pick a lens, set it up with the laser emitting parallel rays, and examine how the incoming parallel rays are changed as they enter, travel through, and then leave the lens. Draw what you see in the space below. These affects occur after they leave the lens. But more specifically at the angle of curvature of the lens. Which is the focal length/ point. 4. This bending of light is called refraction. From what you can see, where do you think refraction occurs? Refraction occurs as soon as the light exists the lens.

Effect of rays emitted by a light source—Parallel rays or Point source? For incoming parallel rays, light that passes through a bi-convex lens (or the semi-circular lens) focuses together at the ‘focal point’ of the lens. We say that these “parallel incident rays converge on the focal point of the lens.” The distance from the center of the lens to this ‘focal point’ is called the focal length of the lens. 1. Look at the focal length of your lens. Does this focal length change when the lens is brought closer to/farther from the parallel ray light source? Do you think it should change? Why or why not? Draw what you see in the space below and carefully label the focal length, f, of your lens. It does not change with respect to the lens distance. Only changes with respect to the light beam source but that is what should happen. Since the beam only converge after they exit the lens it does not matter where the lens is, they will always be the same distance away from the lens.

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2. What do you see when you try using the ‘diverging’ arrangements? Where do you think the focal point is for the concave, ‘diverging’ lens? Draw your ideas in the space below.

The focal point should be the angle of the concave curve 3. By convention, we say that the converging lenses have positive focal lengths and that the diverging lenses have negative focal lengths. How do you reconcile this convention with the diagrams/pictures that you have drawn for the converging and diverging lenses? Because the focal lengths are of the curvature of the lens since the concave diverging lens is inwards then the length should be negative and vice versa for the converging lens positive. 4. Now set up the light box as a ‘Point source.’ When light rays from a point source interact with a lens, the place where the rays overlap is called the image location1. Using the circular, converging lens, move the lens closer to/farther from the point source and observe the image location relative to the lens position. Do you see a pattern? If so, describe the pattern in the space below. Yes, the image moves relative with the lens. 5. Now let’s compare the parallel ray light source to the point source. Using the circular converging lens, place the lens at a known distance from the light source. With the light source emitting parallel rays, note the focal point and focal length of the lens. Now add the semi-circular lens to create a point source. How did the light rays shift as they pass through the circular lens? Where is the image formed with respect to the focal length of the lens? Draw the point source (“after”) picture below and carefully label the image location as well as the focal length of the lens.

If you made the flat side of the semi circle the side where the beams go though first then the beams spread apart and act like a diverging lens. 6. What happens when you perform this same investigation for the circular converging lens (comparing parallel rays to point source light) at a CLOSER distance to the light source? What happens at a FARTHER distance? Draw what you observe for both in the space below. Write a sentence or two to summarize the relationship that you observe between the distance of the lens from the light source (the object distance, o), the distance of the image from the lens (the image distance, i), and the focal length, f, of the lens.

As it gets closer to the focal point then the beams of light do not change much. It isnt until the lens is moved farther away that the beams then begin to converge again.

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