SAMPLE MARKING SCHEME

OPTICS AND E.M.WAVES

2010

SET1

Q.1

Which part of electromagnetic spectrum has largest penetrating power ? 1

A.1

Rays.

Q.2

A glass lens of refractive index 1.45 disappears when immersed in a liquid. What isthe value of refractive index of the liquid ? 1

A.2

1.45

Q.3

State the conditions for the phenomenon of total internal reflection to occur. 1

A.3(i) Refraction should take place from denser to rarer medium (ii) Angle of

incidence should be greater than the critical angle. ½+½ 1

Q.4

Arrange the following electromagnetic radiations in ascending order of theirfrequencies : 2(i) Microwave(ii) Radio wave(iii) X-rays(iv) Gamma raysWrite two uses of any one of these 2

A.4Radio waves < Microwaves < X-rays < Gamma rays 1

Two uses of any one of these. ½+½

Q.5

The radii of curvature of the faces of a double convex lens are 10 cm and 15 cm.If focal length of the lens is 12 cm, find the refractive index of the material of thelens. 2

A.5

Q.6

(a) The bluish colour predominates in clear sky. 2(b) Violet colour is seen at the bottom of the spectrum when white light is dispersedby a prism.State reasons to explain these observations.

A.6

Q.7

What is an unpolarized light ? Explain with the help of suitable ray diagram howan unpolarized light can be polarized by reflection from a transparent medium.Write the expression for Brewster angle in terms of the refractive index of densermedium. 3

A.7

Q.8

State Huygens’s principle. Show, with the help of a, suitable diagram, how thisprinciple is used to obtain the diffraction pattern by a single slit.Draw a plot of intensity distribution and explain clearly why the secondary maximabecome weaker with increasing order (n) of the secondary maxima. 5

A.8

SET 2 &SET 3

Q.9

A converging lens is kept coaxially in contact with a diverging lens – both the lensesbeing of equal focal lengths. What is the focal length of the combination ? 1

A.9

Q.10

Name the part of electromagnetic spectrum whose wavelength lies in the range of10–10 m. Give its one use. 1

A.10

Q.11

When light travels from a rarer to a denser medium, the speed decreases. Does thisdecrease in speed imply a decrease in the energy carried by the light wave ? Justifyyour answer. 1

A.11

Q.12

Draw a ray diagram to show the working of a compound microscope. Deduce anexpression for the total magnification when the final image is formed at the nearpoint. 2

A.12

Q.13

In a compound microscope, an object is placed at a distance of 1.5 cm from theobjective of focal length 1.25 cm. If the eye piece has a focal length of 5 cm and thefinal image is formed at the near point, estimate the magnifying power of themicroscope. 2

Q.14

Draw a neat labelled ray diagram of an astronomical telescope in normaladjustment. Explain briefly its working.(ii) An astronomical telescope uses two lenses of powers 10 D and 1 D. Whatis its magnifying power in normal adjustment ? 3

A.14

Q.15

(i) Draw a neat labelled ray diagram of a compound microscope. Explain brieflyits working.(ii) Why must both the objective and the eye-piece of a compound microscopehave short focal lengths ? 3

A.15

Q.16

In Young’s double slit experiment, the two slits 0.15 mm apart are illuminatedby monochromatic light of wavelength 450 nm. The screen is 1.0 m away from theslits.(a) Find the distance of the second (i) bright fringe, (ii) dark fringe from the centralmaximum.(b) How will the fringe pattern change if the screen is moved away from theslits ? 3

A.16

Q.17

An illuminated object and a screen are placed 90 cm apart. Determine the focallength and nature of the lens required to produce a clear image on the screen, twicethe size of the object. 3

A.17

Q.18

How does an unpolarised light get polarised when passed through a polaroid?Two polaroids are set in crossed positions. A third polaroid is placed between thetwo making an angle with the pass axis of the first polaroid. Write the expressionfor the intensity of light transmitted from the second polaroid. In what orientationswill the transmitted intensity be (i) minimum and (ii) maximum ? 5

A. 18

2013

Q.1

A 1 A + Dm = 2i

Q.2

Q.2 (a) i) Ray of light shoul d travel from denser to rarer medium.

ii) Angle of incidence should be more than the critical angle. (b) = sin i c

where ic is the critical angle

Q.3

Power of the combination P= P1+P2 = -1D Nature: Diverging

Q.4

a) In doubl e slit experiment, the pattern on the screen is actually a super position of si ngl e slit defraction from each slit and double slit int erference pattern. As a r esult, t here appears a broader diffracti on peak i n whi ch t here occur several fri nges of s mall er widt hs due t o doubl e slit int erference. b) Di st ance of first secondary maxi mum from centre of the screen

x = Therefore spacing between first secondary maxima on the screen f or two gi ven wavel engt hs = = × 10 - 9 = = 6. 75× 10- 5 m a D 23 x 1 2 23aD 4 10 2 2 5 .13 590 596 4 10 6 5 .45

Q.5

Q.6

Tot al magnification when image is formed at infinity

m= m0 me = (b) Myopia Hypermetropia

1. Distant object arriving at the eye lens get converged at a point in front of the retina

1. Eye lens focuses the incoming light behind retina

2. The eye ball is elongated 2. The eye ball is shortened

3. Person cannot see distant objects clearly.

3. Person cannot see nearby objects clearly.

Verifi cati on of Snell’s law Fr om figure sin i = = sin r = = = = (b) Yes, (i) Refl ecti on and refraction arise through i nt eraction of inci dent light wi t h t he at omi c constituent s of matt er. At oms may be vi ewed as oscillators, whi ch t ake up t he frequency of t he ext ernal agency (li ght) causi ng f orced oscillations. The frequency of li ght emi tted by a charged oscillat or equal s i t s frequency of oscillation. Thus, t he frequency of scattered li ght equal s the frequency of inci dent light. [ Any ot her correct explanati on] (ii) No. Energy carried by a wave

depends on t he amplit ude of the wave, not on t he speed of wave propagati on. AC BC AC t v 1 AC AE ACt v 2 ri sin sin 21 vv