12. A glass plate 1 cm thick, of refractive index 1.50, is placed between a point source of light of wave length 6000 Å and a
screen. The distance from the source to the screen is 4 cm.
How many waves are there between the source and the
screen?

7

Explanation:

The light travels a total of 4 cm to the screen, of that, 3 cm is in air and 1 cm is in the glass plate.

The total number of wavelengths of light between the source and screen is just the number of wavelengths in air plus the number in the glass.

To determine the number of wavelengths in air, divide the thickness of air (3 cm) by the wavelength of the light (6000 Angstroms), converting units as needed.

The refractive index of the glass is 1.5. That means that the velocity of propagation of the light in the glass is 2/3 of what it is in air, and so the wavelength of the light in glass is 2/3 of what it is in air. So, divide the thickness of glass (1 cm) by the wavelength of the light in glass (6000 * 2/3).

Add the two values for the final answer

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