When light passes from air to glass, then
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → its wavelength and velocity both decrease.
On refraction the frequency does not change, because it is set by the source. Glass is optically denser than air, so the speed of light falls.
\( \displaystyle v = \frac{c}{n} \;\Rightarrow\; v_{\mathrm{glass}} \lt c \quad (n \gt 1) \)
\( \displaystyle \lambda = \frac{v}{\nu} \;\Rightarrow\; \lambda_{\mathrm{glass}} = \frac{\lambda_{\mathrm{air}}}{n} \)
With \( \nu \) fixed, \( \lambda \propto v \), so the wavelength decreases along with the velocity.
NCERT (2026-27):
Refraction of a Plane Wave – “… The above equation implies that when a wave gets refracted into a denser medium (v1 > v2) the wavelength and the speed of propagation decrease but the frequency \( \nu\ (= v/\lambda) \) remains the same.”