C27: An interference pattern is formed on a screen by shining a planar wave on a double-slit arrangement (left). If we cover one slit with a glass plate (right), the phases of the two emerging waves will be different because the wavelength is shorter in glass than in air. If the phase difference is 180°, how is the interference pattern, shown left, altered?

1. The pattern vanishes.

2. The bright spots lie closer together.

3. The bright spots are farther apart.

4. There are no changes.

5. Bright and dark spots are interchanged.