Lab Report # 9

Temple University physics

Refraction, Reflection, and Lenses

Unperturbed light propagates in a straight line, but the direction of propagation changes abruptly when the light encounters a reflective surface; something we take for granted when we look into the mirror. Light can also change direction when passing from one medium to another; a phenomenon known as refraction. Refraction occurs because, as the light passes into a different medium, the speed at which it can propagate is altered. To understand these phenomena, we will think of the light as idealized narrow beams called rays. Modeling the light as rays is a simple but accurate way to trace light propagation through an optical system (e.g. a mirror, a magnifying glass, or a microscope).

Learning Goals for this Laboratory:

· Become familiar with ray optics terminology

· Practice drawing ray diagrams for determining image-object relationships

· Understand how air water interfaces refract light by applying Snell’s Law

· Observe total internal reflection and understand the conditions under which it occurs

· Understand how to apply the lens equation to the human eye

Part I. Reflection

1. Hold a large shiny metal spoon at arm’s length with the concave side facing you. You should see your image in the spoon.

Question 1. Is your image upright or inverted as seen in the concave side of the spoon?

2. Now flip the spoon around so that the convex side is facing you and again hold it at arm’s length and look for your image. Flip back and forth between the convex and concave sides and observe how the images are different. For your report make a data table for Part I to compare the two images, recording in each case whether the image is upright or inverted and whether the size is magnified or reduced or neither in comparison to the original object (remember you are the object).

3. It is not always easy to determine whether and image is virtual or real without using the mirror equation. Use the mirror equation to determine whether your image is real or virtual in the two cases (convex and concave). Assume the focal distance is 1 cm and estimate the object distance (your face is the object in this situation). Be sure to use the correct signs, refer to the sign conventions for mirrors if necessary.) Include the results of your calculation in your data table for Part I.

4. With the concave side facing you, observe and record what happens to your image as you move the spoon closer and closer. Be sure to note how the size of the image changes as well as whether it remains inverted or becomes upright.

5. Look for the inflection point very close to the spoon where the image flips. It may be easier to see this point if you use your finger as the object and see how its image changes as you move it closer: at some point you should see your finger reflected upright and at about normal size.

Question 2. Show that this inflection point is the focal point of the concave mirror. To do this, use the mirror equation and the magnification equation noting that a positive magnification indicates an upright image.

Part II. Refraction

1. Place a straight object such as a pencil, chopstick, or ruler, into a clear container and fill the container with water until the object is about halfway submerged. A wine glass, glass bowl, or clear flower vase work well.

2. Observe the straight object from all sides, recording your observations for the data section of your lab report. Be sure to note whether the object appears to be bent or not when you are observing it from each position.

Question 3. Why does the object appear to be straight when viewed from directly above, but bent when you move your head slightly to either side? Use Snell’s Law to support your answer. Assume the index of refraction of air is 1 and that of water is 1.33. Include sketches where helpful.

3. Kneel down and observe the water’s surface from below. From a low enough vantage point, the surface of the water is mirror-like. Can you see this total internal reflection of the light from the object?

Question 4. Why don’t you see total internal reflection on the top surface of the water? Use Snell’s law and a sample calculation to support your answer.

Part III. Lenses – The Human Eye

Lenses make use of refraction to focus or spread out light rays.. The human eye is essentially a lens (the cornea) and a screen (the retina). See the diagram below showing the parts of the eye. Though most of the refraction occurs at the cornea, the eye actually has a lens inside to fine-tune the amount of refraction in order to focus on objects at different distances, a process called accommodation. In this virtual lab, we’ll make a simple model of the eye and see how it accommodates, then we’ll look at how corrective lenses work.

1. Open the web-based simulation https://ricktu288.github.io/ray-optics/simulator/

2. Draw a lens on the workspace by clicking on the glasses menu and selecting the ideal lens. Then click and drag to make the lens a few inches long on your screen. The lens appears as a gray line and we are viewing it from the side.

3. To the left of the lens, draw a beam of parallel rays by selecting “beam” from the menu and clicking and dragging on the workspace as you did for the lens. You can reposition or resize either the ray or the lens by clicking on their center or their ends, respectively. You should now have a beam of rays passing through the lens and converging to a point like this:

Question 5. Click on the lens. Notice that you can set its focal length to be negative or positive. What is the difference between such negative and positive lenses?

4. For the data section of your report, record how the path of the rays differs between the positive lens and negative lens.

5. Let’s add a retina and see how images form on it. Reset the focal length of the lens back to +100 units; the units are arbitrary so let’s call them “mm.” Select “blocker” form the menu and draw the blocker at the focal point of our beam of light, 100 mm from the lens. The blocker represents the retina, the location where images form when the eye is properly focused. Also add a ruler so we can measure distances. Now your setup should look something like this:

Now we have the eye properly focused on a distant object represented by a beam of parallel rays. In other words, an image of the distant object is present on the retina, so the eye sees the object clearly.

Question 6. Is the image that forms on the retina of the eye a real or virtual image? How do you know?

6. Make a prediction: if the lens and retina stay fixed, where will the light from a nearby object form: on the retina just like the image of the distant object? In front of the retina (inside the eye)? Beyond the retina (outside the eye)?

7. Now test your prediction. We will use a point source to represent a nearby object because light from nearby objects is diverging steeply outward just like that from the point source. Place a point source to the left of the lens at a position 200 mm left of the lens. Where does the image form (i.e. where does the light come to focus)? To help you see where the image forms, move the blocker out of the way. Record for the data section of your report where the light from the nearby object forms in comparison to light from distant objects.

Question 7. In the situation we have modeled, the eye can focus on distant but not nearby objects? Which type of eye dysfunction is this: nearsightedness or farsightedness?

8. As mentioned above, the normal eye can focus on both near and far objects by accommodation: changing the focal length of its lens by making it more rounded in shape as seen here.

 

Accommodate your eye by clicking on the lens and changing its focal length until the light from the point source is focused on the retina.

Record this new lens focal length of the accommodated eye as well as the focal length of the relaxed, unaccommodated eye (100 mm in our model).

Question 8. What is the new focal length of the accommodated eye? How does this compare to the focal length of the relaxed, unaccommodated eye?

9. Return your eye to its relaxed state by changing the focal length to 100 mm so the distant object is in focus. Note that the light from the nearby object is focused behind the retina.

10. Make a prediction: what type of corrective lens can we place in front of the eye in order to correct the farsightedness, bringing the image of the nearby object forward from its current position behind the retina to its proper place on the retina? A positive (converging) lens? A negative (diverging lens)?

11. Test your prediction by placing a second ideal lens to act as a corrective lens 20 mm in front of the existing eye. Adjust the focal length of your corrective lens until you get the image from the point source to land on the retina (you should end up with a value slightly larger than 180). Record the focal length and type (converging or diverging) lens that corrects farsightedness.

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7/22/2020 1:24 PM

 

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