Question

When light travels from one medium to another medium which are separated by a sharp boundary, the characteristic which does not change is:
A.Velocity
B.Wavelength
C.Frequency
D.Amplitude

Answer 1

Hint: Use the expression showing relation between frequency and the energy of a photon. If there is change in the energy of the photon, then only the frequency will change and if there is no change in energy, then the frequency remains constant. You can also use the relation between velocity, frequency and wavelength. This relation shows that the change in one parameter leads to the change in another parameter while the third parameter remains constant.
Formula used:
$E=h\nu$
$v=\nu \lambda$

Complete answer:
We know, relation between frequency and energy of photon is given by,
$E=h\nu$
Where, $\nu$ is the frequency
               E is the energy of photon
When light travels from one medium to another, energy remains constant. Thus, from the above equation we can say, frequency also remains constant.
We also know, the relation between velocity, frequency and wavelength is given by,
$v=\nu \lambda$
Where, v is the velocity of the light
               $\lambda$ is the wavelength
When the light travels from one medium to another, the velocity of light changes. To compensate this change, wavelength changes too or vice versa. Due to these compensatory changes, frequency remains constant. Thus, the wavelength and velocity of light changes whole travelling from one medium to another and the frequency does not change.

Hence, the correct answer is option C i.e. frequency.

Note:
To answer this question, you should be familiar with all the terms mentioned. Speed of light is maximum in vacuum and then the air. It depends on the refractive index of the medium. A wave of light of single frequency is called Monochromatic light. Frequency is the parameter of source which is emitting light and not the medium it is travelling through.