Group Activity

Light Source Activity

Light Source Activity

The goal of this activity is to determine how images are formed and learn a bit about lenses along the way. The activity is broken into two parts. In the first part of the guided activity you will observe how a light source and a lens can be used to create an image. In the second part you will observe how the position of the object with respect to the focal point of the lens influences the image formation. Any time you need help with the simulation click on the green "Show Help" button. Check the "Screen" check box in the parameters area. You should see a table lamp that is switched on. Click the "No rays" radio button in the parameters area. You should see circle on the black screen. Set the curvature radius to 0.8 m, the refractive index to 1.53, and the diameter to 0.8 m if they are not already there. Discussion Discuss what you observe about the circle of light on the screen. Record a qualitative description of the circle of light on the screen. Move the refractive index slider until you find the smallest circle on the black screen. Discussion Discuss what you observe about the circle of light on the screen. Record the refractive index and a qualitative description of the circle of light on the screen. Test all the options for Rays (ie Marginal rays, Principal rays, and Many rays). Discussion Discuss what you observe about the rays of light at the screen. Record your observations for all the different options. Click on the "No rays" radio button in the parameters area. Check the second point check box and do not move the second light source. Discussion Discuss what you observe about the circles of light on the screen. Record a qualitative description of the light on the black screen. Test all the options for Rays with the second point check box checked. Discussion Discuss what you observe about the rays of light on the screen. Record your observations for all the different options. Move the refractive index slider back to the default of 1.53. Try moving the light sources around and playing with the refractive index. Discussion Discuss what you observe about the rays of light on the screen.

Special thanks to the Physics Education Technology PhET Team

Two Light Source Question

• Check the second point checkbox and move the second table lamp so that is a different distance from the lens. Move the screen until one of circles of light has its smallest diameter. Test all the options for Rays (left side checkboxes on the simulation). What answer best describes the difference between the rays for the two spots at the screen.
  • the rays for the largest diameter spot cross at the screen.
    • Incorrect! Confirm that the lamps are different distances from the lens with a ruler.
  • the rays for the smallest diameter spot cross at the screen
    • Absolutely Correct!
  • the largest diameter spot is the brightest spot on the screen.
    • Incorrect!
  • the rays for both spots cross at the screen.
    • Incorrect! Confirm that the lamps are different distances from the lens with a ruler.

Tip: Click the "Ruler" checkbox to use a ruler. Confirm that the lamps are different distances from the lens with the ruler.

Real Object Activity

Real Object Activity

You have completed the first part of the activity where you investigated imaging light sources on a screen. In this part, you will investigate the properties of images created by a lens and a real object. The concepts of ray tracing used in this activity are the same as with a light source. This may not be obvious at first. For a light source it is easy to see that light rays begin at the light source such as a light bulb or a laser. When we think about real objects like pencils, faces, etc we need to consider light from outside sources because these objects are not direct sources of light. Hopefully, you are comfortable with light rays traveling in straight lines in all directions from a light source. Some of the light from a light source reflects off a real object and the light typically reflects in all directions. Therefore, we usually do not draw the outside light source when dealing with real objects. We make a mental note that there must be an outside light source, and start our light rays traveling in straight lines from the real object in all directions. Begin this part of the activity by un-checking the Screen check box and clicking the No Rays radio button. Move the object around between the left-hand-side and the left focal point (marked with an X). Record "Outside the focal point" in the Object position column of the data table below (or in your notes). Discussion Discuss your observations about the image orientation and image type (Virtual image check box un-checked). Record a summary of your observations about the image orientation and image type with the Virtual image check box un-checked (If you are having difficulty finding an image you may want to increase the refractive index or decrease the curvature radius.). Discussion Discuss your observations about the image orientation and image type (Virtual image check box checked). Record a summary of your observations about the image orientation and image type with the Virtual image check box checked. Click the No Rays radio button and the second point check box. Test all the options for Rays. Discussion Discuss your observations about the rays at the image. Record a summary of your observations about the rays at the image. Move the second point up and down the object. Discussion Discuss your observations about the rays at the image. Record a summary of your observations about the rays at the image. Move the object around between the focal point and the lens. Record "Inside the focal point" in the Object position column of the data table below (or in your notes). Discussion Discuss your observations about the image orientation and image type (Virtual image check box checked). Record a summary of your observations about the image orientation and image type with the Virtual image check box checked (If you are having difficulty finding an image you may want to decrease the refractive index or increase the curvature radius.). Discussion Discuss your observations about the image orientation and image type (Virtual image check box un-checked). Record a summary of your observations about the image orientation and image type with the Virtual image check box un-checked.

Example data table:

Object Position	Image Orientation	Image Type

Reflection

Reflection

For real images, what did you always observe about the rays at the image when you changed the refractive index, curvature radius, and/or the diameter? The rays always cross or converge at the image independent of the refractive index, the curvature radius, the diameter of the lens.