Short answer-Class 10 -Science- Physics-Chapter 10 – Light – Reflection and Refraction

Class 10 -Science-Short answer- Physics-Chapter 10 – Light – Reflection and Refraction

Q1. (a) For the same angle of incidence 45°, the angle of refraction in two transparent media; I and II is 20° and 30° respectively. Out of I and II, which medium is optically denser and why?

(b) Light enters from air to diamond which has refractive index of 2.42. Calculate the speed of light in diamond, if speed of light in air is 3 x10⁸ ms^-1

Ans:

For the same angle of incidence. The refracted ray in medium I deviates less than in medium II. Hence medium I is more denser than medium II.

Q2. A student focused the image of a candle flame on a white screen by placing the flame at various distances from a convex lens. He noted his observations:

a. From the above table, find the focal length of lens without using lens formula.

b. Which set of observations is incorrect and why?

c. In which case the size of object and image will be same? Give reason for your answer.

Ans : 

a. Focal length of the lens without using the lens formula: f=15cm.

b. Incorrect observations: u=15cm and v=70cm violate the expected behaviour of a convex lens.

c. Size of object and image will be the same when object distance u=2f. In the given data, this occurs when u=v=30cm.

Q3. One half of a convex lens is covered with a black paper. a. Show the formation of image of an object placed at 2Fp of such covered lens with the help of ray diagram. Mention the position and nature of image.

b. Draw the fay diagram for same object at same position in front of the same lens, but now uncovered. Will there be any difference in the image obtained in the two cases? Give reason for your answer.

Ans:

a. If the lower half of the lens is covered even then it will form a complete real, inverted image of same size at C2 (2F2 ) with reduced intensity of image.

b. There will be no change in the nature and position of the object except in later case the image will be brighter.

Q4. State the laws of refraction of light. Explain the term ‘absolute refractive index of a medium’ and write an expression to relate it with the speed of light in vacuum.

Ans : Laws of refraction: a. The incident ray, refracted ray and normal to the point of incidence, all lie in the same plane. b. The ratio of sine of incident angle and sine of angle of refraction for a given pair of medium is constant. 

Sin i/ sin r =Constant

Absolute refractive index of a medium is the ratio of speed of light in air or vacuum and speed of light in the medium.

Absolute refractive index = speed of life in air or vacuum/speed of light in medium speed of life in air vacuum

Q5. (a) Calculate the distance at which an object should be placed in front of a convex lens of focal length 10 cm to obtain a virtual image of double its size. (b) In the above given case, find the magnification, if image formed is real. Express it in terms of relation between v and u

 Ans :

Given  f = +10cm, u=?

For virtual image

m=+2

m=v/u or v/u=2

v=2u à(1)

(1/v)-(1/u)=1/f  à (2)

substituting (1) and (2)

(1/2u)-(1/u)=1/10

-1/2u =1/10

u=-5cm

For real image,    f = 10cm, m = -2

v/u = -2, v=-2u

(1/v)-(1/u)=1/f

(1/-2u)-(1/u) = 1/10

-3/2u = 1/10

U= -15cm