Question: Q. 5. Define the power of a lens. Write its S.I. unit.
R [CBSE Comptt. 2018]
Show Answer
Solution:
Ans. The power of a lensequals to the reciprocal of its focal length (in metre).
Also accept
$$ P=\frac{1}{f(\text { metre })} $$
Do not deduct mark if student does not write the word metre.
(Alternatively,
Power of a lens is the ability of conversion/ diversion of the rays incident on the lens.)
SI Unit : Diopter (D)
1
[CBSE Marking Scheme 2018]
(AI) Q. 6. (i) What is the relation between critical angle and refractive index of a material ?
(ii) Does critical angle depend on the colour of light? Explain.
R [O.D. 2013, 2009]
Ans. (i) Refractive index $(n)=\frac{1}{\sin C}$
$1 / 2$
where, $C$ is the critical angle.
(ii) Yes, Since, refractive index depends upon the wavelength of light, the critical angle for a given pair of media is different for different wavelengths (colours) of light.
