When light enters from air into a medium, its speed changes. Which of the following properties of light remains constant during t…

Question

When light enters from air into a medium, its speed changes.

Which of the following properties of light remains constant during this transition?

1. A. Wavelength
2. B. Frequency (Correct answer)
3. C. Speed
4. D. Amplitude

Correct Answer

Option B — Frequency

Detailed Solution & Explanation

The correct answer is Frequency.

Key Points

• When light passes from one medium into another, its speed and wavelength change, but its frequency stays constant.
• Frequency is fixed by the source that emitted the light; the receiving medium cannot alter how many wave crests are generated per second.
• Since v = fλ, if v falls on entering a denser medium and f is unchanged, then λ must fall in proportion.
• Amplitude may also change through partial reflection and absorption, so it is not conserved either.

Additional Information

• The refractive index n = c/v, where c is the speed of light in vacuum (3 × 10⁸ m/s) — so a higher refractive index means a slower speed in that medium.
• Because frequency is unchanged, the colour of light does not change on refraction; colour is determined by frequency, not wavelength in a given medium.
• Snell's law: n₁ sin θ₁ = n₂ sin θ₂, describing the bending at an interface.
• Light bends towards the normal entering a denser medium and away from it entering a rarer one; beyond the critical angle it undergoes total internal reflection, the principle behind optical fibres.

प्रश्न (हिन्दी में)

जब प्रकाश वायु से किसी माध्यम में प्रवेश करता है, तो उसकी गति बदल जाती है। इस संक्रमण के दौरान प्रकाश का निम्नलिखित में से कौन सा गुण स्थिर रहता है?

1. A. तरंगदैर्ध्य
2. B. आवृत्ति
3. C. गति
4. D. आयाम