Answers

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Hints:
1. When rays of lights parallel to the principal axis coming from infinity after refraction actual meet at a point on the principal axis , this point is known as principal  focus.
2. i) Wider field of view as it is convex outward,so it cover larger area. ii) It always form virtual,erect and diminished image.

3. 

REAL IMAGES	VIRTUAL IMAGES
1 These images are formed due to actual intersection of light after reflection or refraction.	1 These images are formed due to apparent intersection of light after reflection or refraction.
2 They can be obtain on the screen.	2 They cannot be obtain on the screen.
3 These are always inverted.	3 These are always erect.

4. Change in speed of light when it travels from one medium to another.Speed increases from denser to rarer and speed decreases from rarer to denser.
5.At right angle to surface (i e 90⁰) and angle of incidence zero degree.
6.It is the ratio of sine of angle of incidence to the sine of angle of refraction.

          i e     Sin i / sin r = constant (refractive index).

7. increases
8.yes , When refractive index of the medium is different.
9. see answer 1.
10.Power of the lens is ability of the lens to converse or diverse the light rays.              or
It is also defined as the reciprocal of the focal length.
11.    1D = 1m^-1
12.Diverging lens , Concave lens.
13.Linear magnification is the ratio of height of the image to the height of object.      m=h^'/h 

It has no units.
14.When rays of lights parallel to the principal axis coming from infinity after reflection actual meet at a point on the principal axis due to shape of concave mirror curved inward(reflecting surface) so it has a real focus.
15. No
16. Intensity of the image formed is decreased.
17. Choose a well-illuminated distant object.
    Hold the lens between object and the screen.
    Adjust the position of the screen such that a clear image form on it.
    Measure the distance between lens and screen it is the rough focal length of the lens.
18.Infinity
19. By Snell's law 

n₁₂=sin i/sin r

n₂₁=sin r/sin i

Now  dividing n₁₂ by n₂₁ 

So , n₁₂/n₂₁=1

n₁₂=1/n₂₁

20. Negative sign indicates that image is inverted,half the size of object and lens is convex lens.