# Ray Optics and Optical Instruments VVI Questions

Q.1 Differentiate between primary and secondary rainbow.

## Ans:-Primary Rainbow:-

(i) In this type of rainbow red colour is present at outer edge and violet colour at inner edge.

(ii) It has angular width of 2°.

(iii) It its formation light rays get refracted two times and is internally reflected once.

(iv) Intensity of this rainbow is much brighter.

(v) The middle part of this rainbow is enlighted and the upper part is not so clear.

## Secondary Rainbow:-

(i) In this type of rainbow violet colour is present at outer edge and red colour at inner edge.

(ii) It has angular width of 3°.

(iii) It its formation light rays get refracted two times and is internally reflected two times.

(iv) Intensity of this rainbow are not so bright.

(v) The upper part of this rainbow is enlighted and lower part is not so clear.

## Q.2. Write is the material of optical fibre core ?

Ans:- Glass or quartz.

## Q.3. Why red signals are used as danger sign ? Explain.

Ans:- According to Rayleigh’s, the intensity of scattered light is inversely propertional to the fourth power of wavelength. In the visible spectrum, red colour has the largest wavelength, it is scattered the least. Even in foggy conditions, such a signal covers large distances without any appreciable loss of intensity due to scattering.  Therefore, red coloured signals are preferred.

## Q.4. What is total internal reflection ?

Ans:- The phenomenon in which a ray of light travelling at an angle of incidence greater than the critical angle from denser to rarer medium is totally reflected back into the denser medium is called total internal reflection.

## Q.5. Define critical angle and write its conditions.

Ans:- Critical angle for a pair of given media in contact can be defined as, “the angle of incidence in denser medium for which angle of  refraction  in rarer medium is 90°”.The value of critical angle depends on the nature of two media in contact.

Conditions for critical angle:-

(i) Ray of light should be goes from denser of rarer medium.

(ii) Refraction angle is always 90°.

## Ans:- Magnification-

(i) Magnification is the ratio of image size to object size.

(ii) The range of magnification is infinity.

## Magnifying power-

(i) Magnifying power is the ratio of visiual angle suntended by the image(β) and visiual angle suntended by the object (α).(ii) The value of magnifying power lies between D/F and (1+D/F).