Question

A ray of monochromatic light travelling in air is incident on a glass slab and is partly reflected and partly refracted. Both the reflected and refracted lights will have :

• A. same wavelength
• B. same frequency
• C. same intensity
• D. same speed

Hint:

A useful mnemonic for optical transitions: "Frequency is Fundamental." It never changes during reflection, refraction, or interference, as it solely depends on the source oscillator.

Answer 1

The Correct Option is B

Concept:
• When light undergoes reflection, it stays in the same medium, so its speed and wavelength remain constant.

• When light undergoes refraction, it enters a different medium with a different optical density (refractive index), causing its speed and wavelength to change.

• Frequency is a fundamental characteristic of the light source and relates to the energy of the photon (\( E = h\nu \)).

• Therefore, frequency does not change when light travels across different optical media or undergoes boundary phenomena like reflection and refraction.

Step 1: Analyze the changing properties (Speed, Wavelength, Intensity)
The reflected ray remains in air, so it travels at speed \( c \) with wavelength \( \lambda \).
The refracted ray enters the glass slab (an optically denser medium), so its speed decreases to \( v = \frac{c}{\mu} \).
Because \( v = f \lambda \) and speed decreases, its wavelength also decreases to \( \lambda' = \frac{\lambda}{\mu} \).
The incident energy is split between the reflected and refracted rays, so their individual intensities are lower than the original ray and generally unequal.

Step 2: Analyze the invariant property (Frequency)
The frequency of a wave corresponds to the number of oscillations per second generated by the source.
It is independent of the medium's properties.
Since both the reflected and refracted rays originate from the same incident monochromatic light ray, they must share the exact same frequency.

Step 3: Conclusion
Since wavelength, speed, and intensity differ between the two rays, but frequency remains constant, option (B) is the correct choice.