Question

When a ray of light moves from one medium to another medium,

• A. its frequency alone changes
• B. its frequency alone remains unchanged
• C. its wavelength remains unchanged
• D. both its frequency and wavelength change
• E. its velocity remains constant

Hint:

Frequency never changes during refraction.

Answer 1

The Correct Option is B

Step 1: Understanding Refraction:
When a wave, such as light, crosses the boundary from one medium into another, it undergoes refraction. During this process, some of its properties change while others remain constant. The key properties are frequency (\(f\)), wavelength (\(\lambda\)), and velocity (\(v\)), which are related by the wave equation \(v = f\lambda\).
Step 2: Analyzing Wave Properties at an Interface:
Frequency (f): The frequency of a wave is determined by its source. It represents the number of wave cycles passing a point per second. For a wave to be continuous across a boundary, the number of wave fronts arriving at the boundary per second must equal the number of wave fronts leaving it. If the frequency changed, wave fronts would either have to be created or destroyed at the boundary, which is not physically possible. Therefore, the frequency of the light wave remains constant.
Velocity (v): The speed of light is a property of the medium through which it travels. It is determined by the medium's refractive index (\(n\)) via the relation \(v = c/n\). Since the light is moving to a different medium, the refractive index changes, and therefore the velocity of light changes.
Wavelength (\(\lambda\)): Since frequency (\(f\)) is constant and velocity (\(v\)) changes, the wavelength (\(\lambda\)) must also change to satisfy the equation \(v = f\lambda\). The new wavelength will be \(\lambda' = v'/f\).
Step 3: Evaluating the Options:
(A) its frequency remains unchanged - This is correct.
(B) its frequency alone changes - This is incorrect. Speed and wavelength also change.
(C) its wavelength remains unchanged - This is incorrect. It changes as the speed changes.
(D) both its frequency and wavelength change - This is incorrect. Frequency remains constant.
(E) its velocity remains constant - This is incorrect. It changes due to the change in medium.
Step 4: Final Answer:
When a ray of light moves from one medium to another, its frequency remains unchanged.