As a monochromatic light ray passes from air
into water, two characteristics of the ray that will
not change are
(1) wavelength and period
(2) frequency and period
(3) wavelength and speed
(4) frequency and speed
The Correct Answer and Explanation is :
The correct answer is (4) frequency and speed.
Explanation:
When a monochromatic light ray passes from air into water, certain properties of the light wave change, while others remain constant. The key properties to consider are wavelength, frequency, period, and speed.
- Frequency: The frequency of a wave is determined by the source of the wave and does not change when the wave moves from one medium to another. In this case, the light ray enters from air into water, but its frequency remains the same because it is dependent on the light source.
- Speed: The speed of light is different in different media due to the change in the refractive index. In air, the speed of light is approximately 3.0 x 10^8 m/s, but when it enters water (which has a higher refractive index), the speed decreases. Thus, the speed of light decreases as it passes from air into water.
- Wavelength: The wavelength of a wave is given by the equation ( \lambda = \frac{v}{f} ), where ( \lambda ) is the wavelength, ( v ) is the speed of the wave, and ( f ) is the frequency. Since the frequency remains constant (as explained earlier), and the speed of light decreases in water, the wavelength must decrease as well. Therefore, the wavelength changes when the light moves from air to water.
- Period: The period of the wave is the inverse of the frequency (( T = \frac{1}{f} )). Since the frequency does not change when light enters the water, the period remains the same as well.
Thus, the frequency and the period of the light ray do not change as it moves from air to water, while the speed and the wavelength do change. Therefore, option (4) is the correct answer.