PHYS1402 – General Physics II

Image Formation with Thin Lenses

Materials

Optics Bench 100 mm lens

Light Source -150 mm lens

Screen Post-it note

White paper

Introduction

In this lab we examine several different examples of image formation as predicted by the thin lens formula, . (1) Our apparatus will consist of an optics bench, lenses in lens holders, a screen to project images, and a light source.

Part 1 – The Image of an Object at ∞

A distant object such as the sun, a distant building, or mountains off in the distance can be treated as though it is infinitely far away.

Q1. If an object is infinitely far away, explain why the rays coming from that object incident on a lens can be treated as parallel. (Drawing a picture may help.)

Q2. If the rays from a distant object incident on a lens are parallel, then where will the image be formed?

Q3. Draw a ray diagram showing the formation of the image.

With lights in the room off, handle the 100 mm converging lens by the edges and form the image a distant object such as the window on a white piece of paper. Measure the distance from the lens to the image.

d_i = ___________

Q4. Use your image distance to estimate the focal length of the lens

f =

This is a common method for quickly determining the focal length of a lens, although it is not particularly accurate.

Q5. Find the % difference between the focal length of the lens that you estimated in Q4. and the value listed on the lens.

Q6. Comment on any reasonable sources of error that might occur in determining the focal length of the lens in this manner.

Part 2 – The Image of an Object at the Focal Point

Place the light source at one end of the optical bench, place the lens squarely in the holder, and place the holder on the mount on the optic bench a distance of 15 cm from the light source at a height where the center of the lens is even with the light source. With the light source lit, slowly slide the mount holding the light source forward. Find the position of the light source at which you just can no longer find the image. Measure the distance from the light source to the lens. This will be the focal length.

f =

Q1) For each estimate of the focal length, calculate the percent difference between your two estimates and with the nominal value of 100 mm.

Q2) You may have noticed that you have rather large percent differences. Discuss what difficulties in these two measurements of the focal length might lead to large errors.

Part 3 - Real Image Formation with a Converging Lens

Mount a card on a second lens holder behind the first lens which will act as a screen to allow us to see the images. In the space below, calculate the location of the image and the magnification for each of the following object distances: i) 40 cm, ii) 20 cm, iii) 6 cm.

For part i) and ii) find the image and compare your measured value of the object distance to the predicted. Measure the height of the object and the height of the image and compute the magnification. If the object is inverted be sure to make the magnification negative.

i) d_i = ii) d_i =

%difference = %difference =

h_o = h_o =

h_i = h_i =

M = M =

For part iii) use the screen to examine the light coming from the lens.

Q1: Is the light diverging or converging?

Q2: If the light is diverging, where do you have to look to find the image?

Remove the screen to view the image for part iii). Look through the lens towards the light source and see if you can find the image. Use the parallax method demonstrated by your instructor to locate the image.

iii) d_i =

% difference =

Q3: How important is an accurate value of the focal length in predicting the location of the image?

Q4: Draw ray diagrams showing the formation of the image for parts i) and iii)

Part 3 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 of the rays. We will explore that idea. Answer the following question first before attempting the experiment.

Q1: 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.

Q2: Describe the appearance of the image after the Post-it-Note was placed on the lens.

Q3: Does the full image form or only part of it?

Q4: If the full image formed how is it different? Try moving the Post-it-Note on and off to compare.

Q5: Summarize your observations by completing the following. When the lens was half covered, __________ of the image was formed, but its ___________ was decreased.

Part 4 Image formation by a concave lens

Place a -15 cm focal length lens in the holder 30 cm in front of the object. Use the thin lens equation to predict the image location and magnification. Use the screen to examine the light passing through the lens.

Q1: Is the light converging or diverging?

Q2: Where do you have to look to see the image?

Use the parallax method to find the location of the image. Compute the percent difference between the measured and calculated image locations.

d_i =

%difference =

Q4: Draw a ray diagram showing the image formation.