Question

The above images are that of a specialized slide projector. Slides are small transparencies mounted in sturdy frames ideally suited to magnification and projection, since they have a very high resolution and a high image quality. There is a tray where the slides are to be put into a particular orientation so that the viewers can see the enlarged erect images of the transparent slides. This means that the slides will have to be inserted upside down in the projector tray. To show her students the images of insects that she investigated in the lab, Mrs. Iyer brought a slide projector. Her slide projector produced a 500 times enlarged and inverted image of a slide on a screen 10 m away.

(a) Based on the text and data given in the above paragraph, what kind of lens must the slide projector have?

(b) If v is the symbol used for image distance and u for object distance then with one reason state what will be the sign for $\frac{v}{u}$ in the given case?

(c) A slide projector has a convex lens with a focal length of 20 cm. The slide is placed upside down 21 cm from the lens. How far away should the screen be placed from the slide projector’s lens so that the slide is in focus?
[4 Marks]

Answer 1

Solutions:
(a) Convex Lens [1 Mark]

(b) Negative as the image is real and inverted. [1 Mark]

(c) $\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$ [1 Mark]
$\frac{1}{20}=\frac{1}{v}-\frac{1}{-20}$
$\frac{1}{v}=\frac{1}{20}-\frac{1}{21}$
$\frac{1}{v}=\frac{21-20}{420}$
$\frac{1}{v}=\frac{1}{420}$
v = 420 cm [1 Mark]