PHYS 1407 – Conceptual Physics II
Image Formation by Lenses
Leader: ___________________ Recorder: __________________________
Skeptic: ___________________ Encourager: ________________________
Laptop
100 mm FL lens
-150 mm FL lens
Masking Tape
Post-it Note
Flashlight
Introduction
In this lab we will explore some of the features of images formed by lenses.
Part 1 – A real image formed by a converging lens
Set up the light source on one end of the optical bench so that the side with the arrows faces the other end of the bench. Place the screen so that it is at the other end of the bench. Place the 100 mm focal length lens so that it is 30 cm from the light source. Mark a point 10 cm behind (on the opposite side of the light source) with a small piece of masking tape. This shows the location of the focal point of the lens. Move the screen until you find the image.
Q1) How do you know you’ve found the image?
Q2) Is the image real or virtual? How do you know?
Q3) Is the image located at the focal point of the lens that you marked previously or at a different location?
Measure the distance from the center of the lens to the location of the image. This distance is called the image distance.
Q4) Record the image distance.
Move the lens so that now it is 20 cm from the light source. Measure 10 cm behind the lens again and place the piece of masking tape to mark the location of the lens’s focal point. Move the screen so that you find the image again. Measure the image distance again.
Q5) Is the image real or virtual? Explain.
Q6) Is the image at the focal point of the lens that you marked previously or at a different point?
Q7) When you moved the object closer to the lens, did the image distance increase, stay the same or decrease?
P8) If you moved the lens so that it will be 15 cm in front of the object, will the image distance increase again, stay the same or decrease?
Test your prediction by moving the lens to 15 cm from the object, finding the image and measuring the image distance.
Q9) Was you prediction correct? If not explain why it was wrong.
Part 2 - What Rays are Needed to
Form the Image?
It was stated in class that the rays chosen when we draw a ray diagram are only chosen for the ease of drawing the diagram. The image is formed from any and all of the rays. We will explore that idea. Answer the following question first before attempting the experiment.
P10) If we place a Post-it-Note so that it covers half of the object, what will the image look like?
Place the 100 mm focal length lens 20 cm from the object. Find the image on the screen. Place a Post-it Note so that it covers half of the object.
Q11) Describe the appearance of the image after the Post-it-Note was placed over half of the object.
Q12) Does the full image form or only part of it?
Make the following prediction before making the observation.
P13) If we place a Post-it-Note so that it covers half of the lens, what will the image look like?
Place the 100 mm focal length lens in the holder at a distance of 20 cm from the object. Find the image on the screen. Place a Post-it Note so that it covers half of the lens.
Q14) Describe the appearance of the image after the Post-it-Note was placed on the lens.
Q15) Does the full image form or only part of it?
Q16) If the full image formed how is it different? Try moving the Post-it-Note on and off to compare.
Part 3 – Virtual image formed with a converging lens
Place the 100 mm fl lens so that it is 5 cm from the object. Move the screen close to the lens and then slide it backwards.
Q18) Describe the light transmitted through the lens.
Look through the lens towards the object.
Q19) Describe what you see.
Q20) Are you viewing the object or the image of the object. Cite specific observational evidence to support your answer.
Q21) Are you viewing a real or virtual image? Explain your answer.
When you have a converging lens, images change from real to virtual when the distance between the object and the lens, the object distance, equals the focal length of the lens.
S22) If the object distance is greater than the focal length of the lens a ________ image is formed. As the object distance is decreased, the image distance is ________. If the object distance is less than the focal length a _________ image is formed.
Part 4 – Image formed by a diverging lens
Replace the 100 mm lens with the -150 mm lens. Place the lens 5 cm from the object. Use the screen to view the light transmitted by the lens.
Q23) Describe the appearance of the light.
Move the lens to 10 cm from the object and view the transmitted light with the screen again.
Q24) Describe the appearance of the light.
Move the lens to 20 cm from the object and view the transmitted light with the screen again.
Q25) Describe the appearance of the light.
Move the lens to 30 cm from the object and view the transmitted light with the screen again.
Q26) Describe the appearance of the light.
Q27) When a diverging lens is used, regardless of object distance, what does the transmitted light do?
Look through the lens towards the object.
Q28) Describe what you see.
Q29) Are you viewing the object or the image of the object. Cite specific observational evidence to support your answer.
Q30) How does the size of the image compare to the size of the object in this case?
Q31) Are you viewing a real or virtual image? Explain your answer.
Move the lens to different object distances and view the image.
Q32) For each case is a real or virtual image formed?
Q33) For each case how did the size of the image compare to the size of the object?
S34) A diverging lens always forms a _______ image. The image is always ______ in size than the object.