Question 20

  A converging lens is used to project a real image onto a screen, as in Figure 26.28b. A piece of black tape is then placed over the upper half of the lens. Will only the lower half of the image be visible on the screen? Justify your answer by drawing rays from various points on the object to the corresponding points on the image.

SOLUTION:
The whole image will be seen. To see this, draw one ray from the top of the image through the center. Then draw another ray from the top of the image through the focus of the lens, emerging paraxially on the real side. These two lines criss-cross below the axis, and this is where the image of the top of the object is; the image is inverted. Finally, draw similar lines from the bottom of the object. These lines meet nearer to the axis, and this is where the image of the bottom of the object is. Clearly, then, the entire image can be seen because you can see both the top and the bottom. However, because fewer rays reach the image, the intensity of the image will be less than without the tape.