# Find the distance d that places the final image at infinity.

1. May 12, 2004

### justagirl

Anyone know how to solve the following problems? Thanks!

1.) An object is placed 12 cm to the left of a diverging lens of focal length
-6cm. A converging lens of focal length 12 cm is placed a distance of d to the right of the diverging lens. Find the distance d that places the final image at infinity.

2.) Light of wavelength 460 nm falls on a diffraction grating with 5000 slits per cm. What is the required ditance from the slits to a screen if the spacing between the first and second dark fringes is to the 0.7m?

2. May 13, 2004

### Staff: Mentor

The first problem is a straightforward application of the lens equation:
$$\frac{1}{f} = \frac{1}{o} + \frac{1}{i}$$

The second problem requires the equation for diffraction:
$$d sin\theta = m \lambda$$

To learn what these equations mean and how to use them, consult your text.