Page 92 - Physics - XII

Precautions

1. Parallax should be removed tip to tip.
2. Principal axis of the mirror should vertical while needle should be kept horizontal.
3. The distance should be measured from the centre of the mirror and not from the surface of water.

4. Concave mirror used should be of large radius of curvature.

Sources of Error

1. The needle may not be held horizontal.
2. Parallax may not be removed properly.

VIVA VOCE

Q1. What is the unit of refractive index?
Ans. It is the ratio of similar quantities and hence have no unit.
Q2. On what factors does the refractive index of a medium depend?

Ans. It depends upon: (i) wavelength of light (ii) surrounding medium (iii) temperature.
Q3. What is angle of deviation?
Ans. The angle between the incident ray and emergent ray is called angle of deviation.

Q4. Defi ne angle of minimum deviation.
Ans. The angle of deviation changes with angle of incidence. At a particular value of angle of incidence,
it is minimum. This angle of deviation is called angle of minimum deviation.

Q5. State the laws of refraction.
Ans. First Law: The incident ray, the refracted ray, and the normal at the point of incidence all lie on the
same plane.
Second Law: The ratio of the sine of the angle of incidence to the sine of angle of refraction is
constant for a given pair of media, i.e.
sini = a constant = µ (Refractive index)
sin r

This law is known as Snell’s law.

Q6. How refractive index is related to the wavelength of light?
Ans. The refractive index m is related to the wavelength λ of the light by Cauchy's formula as given
below,
b
ma
2
where a and b are constant for a given material.

Q7. Can refractive index be defi ned in terms of velocity of light?
Ans. Yes. Refractive index of a medium is defi ned as the ratio of velocity of light in vacuum to the
velocity of light in that medium.

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