2018-10-01_Physics_For_You

respect to the pass axis of P 1. A monochromatic

source S of unpolarised light of intensity I 0 is

kept in front of the polaroid sheet P 1 as shown

in the gure. Determine the intensities of light

as observed by the observers O 1 , O 2 and O 3 as

shown.

SOLUTIONS

1. No, the speed of light in glass (i.e., v) depends upon
   the colour of light (i.e., O).

As P = c/v, v

c

=

μ

, where c is the speed of light in a
vacuum.
According to Cauchy's formula.
P = a + b/O^2 , where a and b are constant and their values
depend upon the nature of the substance.

us, v c
ab

=

• /λ^2
  As Or > Ov, vv < vr i,e., violet component of white light
  travels slower than the red component.

2. Because of total internal reections, the diver will
   not see anything above water. Instead, he would see the
   reection of something on the sides and/or bottom of
   the pool.


3. Because of their smaller wavelength, microwaves
   are not bent by objects of normal dimensions. So they
   can be used to beam signal in a particular direction.


4. As^1111110
   Ff 12 fFff

=+, =−= or F = f

(as f 1 = f and f 2 = –f )

5. Only those waves can be polarised which are
   transverse in nature. us, light waves, which are
   transverse in nature, can be polarised whereas sound
   waves, which are longitudinal cannot be polarised.


6. Given that focal length of convex lens, f 1 = + 25 cm and
   focal length of concave lens, f 2 = –20 cm
   Equivalent focal length,
   1111
   25

1

20

1

Ff 12 f 100

= +=+

−

=−

? F = –100 cm

Power of the combination, P
F

==

−

=−

11

1

1

()mm

D

e focal length of the combination= 1 m = 100 cm.

e system will be diverging in nature as the focal length

is negative.

7. From mirror formula,^111
   vfu

=−

Now for a concave mirror, f < 0 and for an object is

placed on the le side of the mirror, the object distance

u < 0

? 2 f < u < f or^1

2

11

fuf

>>

or −<−<−

1

2

11

fuf

or^11

2

1111

fffuff

−<−<−^ or

1

2

1

0

fv

<<

is implies that v < 0 so that image is formed on le.

Also the above inequality implies

2 f > v

or |2f | < |v| [' 2 f and v are negative]

i.e., the real image is formed beyond 2f.

8. (a) Microwaves are used in radar. Because of their
   small wavelengths, microwaves are not diracted or
   bent by objects of normal dimensions. So these waves
   can be used to beam signal in a particular direction.
   (b) X-rays are used to photograph internal parts of a
   body, X-rays have high penetrating power.
   (c) Infrared rays are used for taking photographs of the
   sky during night and foggy conditions. is is because
   these radiations are readily absorbed by water molecules
   present in most materials. Aer absorption they heat up
   the materials and their surroundings.


9. (a) Gamma rays lie between 10–11 m-10–14 m.
   ese rays are used in radiotherapy to treat certain
   cancers and tumors.
   (b) Infrared waves lie between 10–4 m-10–6 m. ese
   waves are used in taking photographs during conditions
   of fog, smoke etc as these waves are scattered less than
   visible rays.


10. Fringe width, β=Dλ
    d
    When D and d are kept xed, β
    β

λ

11 λ

=

or λ λβ

(^1) β
1 630 81
72
==×.
.
==^5103
72
708 75
.

. nm
11. (a) From Snell’s law, we have :

sin()

sin()

i

r

=μ

At A, i = 60°; P =^3

Now, sin()

sin()

r

i

=

μ