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Important Questions for Class 12
Physics
Chapter 9 – Ray optics and Optical Instruments
Very Short Answer Questions 1 Mark
1. A person standing before a concave mirror cannot see his image, unless
he is beyond the centre of curvature. Why?
Ans: Let a man stand beyond focus i.e., between focus and centre of curvature,
then the image formed will be real and inverted and is formed beyond C (beyond
him). Thus, he cannot see the image.
But if he stands beyond C, the image will be formed between focus and centre of
curvature which is in front of him and thus he will be able to see his reflected
image.
2. For what angle of incidence, the lateral shift produced by a parallel sided
glass plate is maximum?
Ans: We know that lateral shift d is given as,
t d sin (i r) cos r
For lateral shift d to be maximum, sin (i r) must be maximum i.e., i r must
be minimum. This happens wheni 90
.
t d sin(90 r) cos r
D t
Then, we can say that lateral shift is maximum.
3. You read a newspaper, because of the light it reflects. Then why do you
not see even a faint image of yourself in the newspaper?
Ans: We know that image is formed due to regular reflection of light.
However, when we read a newspaper, there is diffused (irregular) reflection of
light, thus we are not able to see even a faint image of ourselves on the
newspaper.
4. A substance has critical angle of 45
for yellow light, then what is its
refractive index?
Ans: We know that refractive index is given as below
sin C
Substituting the values, we have
sin 45 (^) 1 / 2
5. An object is placed between the pole and focus of a concave mirror
produces a virtual and enlarged image. Justify using mirror formula.
Ans: We know that the mirror formula is as given below,
v u f
uf v u f
Now magnification,
v m u
uf
u f m u
f m u f
8. In a telescope the focal length of the objective and the eye piece are
60 cm and 5 cm respectively. What is (1) its magnification power? (2) the
tube length?
Ans:
- We know that magnification
o
e
f 60 M 12 f 5
- The tube length we be L fo fe 60 5 65cm.
9. Show the variation of uand vin case of a convex mirror.
Ans: The variation of u and v in a convex mirror is as shown below.
10. Two lenses having focal length f 1 and f 2 are placed coaxially at a
distance x from each other. What is the focal length of the combination?
Ans: The formula for the focal length of the combination of two lenses when
placed coaxially at a distance xfrom each other is as given below:
1 2 1 2
1 1 1 x
F f f f f
11. Does short-sightedness (myopia) or long-sightedness (hypermetropia)
imply necessarily that the eye has partially lost its ability of
accommodation? If not, what might cause these defects of vision?
Ans: Myopia and hypermetropia are common eye defects.
A myopic or hypermetropic person need not necessarily suffer a partial loss in
their eyes’ ability of accommodation.
Myopia occurs when the eye-balls engage in elongation from the front to the
back whereas hypermetropia occurs when the eye-balls shorten themselves.
On the other hand, when the eye-lens completely loses its ability of adjusting
itself, then the defect is called presbyopia.
Short Answer Questions 2 Mark
1. What are optical fibres? Give their one use.
Ans: Optical fibres are thin and long strands of fine quality glass or quartz
coated with a thin layer of material with refractive index less than that of the
strands.
They work on the principle of total internal reflection and thus, they avoid any
loss in transfer of information.
Uses
Optical fibres are often used in medical investigations i.e., one can examine the
inside view of stomach and intestine by a method called endoscopy.
2. How do the focal lengths of a lens change with increase in the wavelength
of the light?
Ans: We know that
1 2
f R R
5. A thin converging lens has focal length (f) when illuminated by violet
light. State with reason how the focal length of the lens will change if violet
light is replaced by red light.
Ans: We know that,
1 2
(n 1) f R R
Since, n for violet is more that n for red colour, and since m f, we can say
that the focal length of the lens will decrease when violet light is replaced by red
light.
6. Thin prism of angle 60 gives a deviation of 30. What is the refractive
index of material of the prism?
Ans: We know that the refractive index of a thin prism is as follows
sin A m 2 n A sin 2
Substituting the given values, we have
sin 2 sin 45 n (^60) sin sin 2
n 1.41 , which is the refractive index of the given thin prism.
7. Although the surfaces of a goggle lens are curved it does not have any
power. Why?
Ans: Since the two surfaces of a goggle lens are parallel i.e., one surface convex
and the other concave, the resultant power of the two surfaces is zero as powers
on both surfaces are equal but opposite in sign.
p p 1 p 2 p ( p) 0
8. A ray of light is incident normally on one face of the prism of apex angle
30 and refractive index^2. Find the angle of deviation for the^ ray of light.
Ans: Let us assume that the ray PQ falls normally on AB.
Then, it goes straight to AC without any refraction (QR) as shown in the figure.
Also given that
n 2 ;
A 30
^ ;
i 30
Applying Snell’s law for face AC,
sin r n sin 30
sin r 2
r 45
Now angle of deviation
r i
, is the angle of deviation.
9. Following data was recorded for values of object distance and
corresponding values of image distance in the experiment on study of real
- u 30 and v 37 is incorrect.
Here, when the object is placed between f 20cm and 2f 40cm, the
image is obtained before 2f 40cm.
Clearly, we can conclude that observation (3) is incorrect because both object
and the image here lie between f and 2 f.
10. Birds flying high in the air appear to be higher than in reality. Explain
why?
Ans: Birds fly in air, which is a rarer medium when compared to the ground,
which is denser.
The light from the birds when viewed will undergo refraction towards the
normal. Thus, the birds appear to fly at a higher point. i.e.,
Apparent height > Real height
11. What is the focal length of a convex lens of focal length 30cm in contact
with a concave lens of focal length 20cm? Is the system a converging or a
diverging lens? Ignore thickness of the lenses.
Ans: Given that,
Focal length of the convex lens,f 1 30 cm
Focal length of the concave lens,f 2 20 cm
Focal length of the system of lenses f
Then the equivalent focal length of a system of two lenses in contact is given as:
1 2
f f f
f 30 20 60 60
f 60cm
Hence, the focal length of the combination of lenses is 60 cm. The negative sign
indicates that the system of lenses acts as a diverging lens.
12. The image of a small electric bulb fixed on the wall of a room is to be
obtained on the opposite wall 3m away by means of a large convex lens.
What is the maximum possible focal length of the lens required for the
purpose?
Ans: Given that,
Distance between the object and the image,d 3 m
Maximum focal length of the convex lens fmax
For real images, the maximum focal length is given as:
max
d 3 f 0.75m 4 4
Hence, for the required purpose, the maximum possible focal length of the
convex lens is 0.75 m.
13. A screen is placed 90cm from an object. The image of the object on the
screen is formed by a convex lens at two different locations separated by
20cm. Determine the focal length of the lens.
Ans: Given that,
Distance between the image (screen) and the object, D 90 cm
Distance between two locations of the convex lens,d 20 cm
2
f 100cm P 1 10
Hence, the far point of the person is 100cm. He might have a normal near point
of 25 cm.
When he uses the spectacles, the objects placed at infinity produce virtual
images at 100 cm.
He uses the ability of accommodation of the eye-lens to see the objects placed
between 100 cmand 25 cm.
During old age, the person uses reading glasses of power, p 2D.
The ability of accommodation is lost in old age. This defect is called presbyopia.
As a result, he is unable to see clearly the objects placed at 25 cm
16. A person looking at a person wearing a shirt with a pattern comprising
vertical and horizontal lines is able to see the vertical lines more distinctly
than the horizontal ones. What is this defect due to? How is such a defect of
vision corrected?
Ans: In the given case, the person is able to see vertical lines more distinctly
than horizontal lines.
This means that the refracting system (cornea and eye-lens) of the eye is not
working effectively in different planes. This defect is called astigmatism.
The person's eye has enough curvature in the vertical plane. However, the
curvature in the horizontal plane is insufficient.
Hence, sharp images of the vertical lines are formed on the retina, but horizontal
lines appear blurred. This defect can be corrected by using cylindrical lenses.
17. A small telescope has an objective lens of focal length 140cm and an
eyepiece of focal length 5.0cm. What is the magnifying power of the
telescope for viewing distant objects when
a) the telescope is in normal adjustment (i.e., when the final image is at
infinity)?
Ans: Given that,
Focal length of the objective lens,fo 140 cm
Focal length of the eyepiece,f (^) e 5 cm
Least distance of distinct vision,d 25 cm
When the telescope is in normal adjustment, its magnifying power is given as:
o
e
f m f
m 28 5
Thus, the magnifying power is 28.
b) the final image is formed at the least distance of distinct vision 25cm?
Ans: When the final image is formed at d, the magnifying power of the
telescope is given as:
e
f (^) o fe m 1 f d
m 1 5 25
m 28 1 0.2
m 28 1.2 33.
Thus, the magnifying power is 33.6.
18. Light incident normally on a plane mirror attached to a galvanometer
coil retraces backwards as shown in figure below. A current in the coil
produces a deflection of 3.5 in the mirror. What is the displacement of the
reflected spot of light on a screen placed 1.5 maway?
Ans: Given that,
R 0.15m
Thus, the radius of curvature is R 0.15m.
2. Show that the limiting value of the angle of prism is twice its critical
angle. Hence define critical angle.
Ans: We know that,
Angle of the prism is given by A r 1 r 2.
In a case of a triangular prism wherei 1 i 2 90
, angle of refraction is given by
r 1 r 2 C.
where, Cis the critical angle.
Clearly,
A r 1 r 2
A C C
A 2C
Therefore, the angle of incidence for which angle of refraction is 90 , is called
the critical angle.
3. Draw a labelled diagram of telescope when the image is formed at the
least distance of distinct vision? Hence derive the expression for its
magnifying power.
Ans: We know that,
magnifying power
angle subtended by the image at the eye
angle subtended by the object at the eye
tan MP tan
(Since angles are very small)
A B tan B E
and
A B
tan B O
A B A B
MP
B E B O
o
e
B O f MP B E v
o
e
f MP v
......(i)
For eye piece,
e
v v f
e e
D v f
Multiply by D,
e e
D D
v f
e e
D D
v f
e
e e e
1 1 1 1 f 1 v f D f D
Substituting in (i),
o e
e
f f MP 1 f D
r 1 r 2 A ......(4)
Substituting equation (4) in equation (1)
i e A
Or
A i e
5. Draw a ray diagram to illustrate image formation by a Newtonian type
reflecting telescope. Hence state two advantages of it over refracting type
telescopes.
Ans: The ray diagram of the Newtonian type reflecting telescope is as shown
below.
Advantages
The image formed in a reflecting type telescope is free from chromatic
aberrations.
The image formed is very bright due to its large light gathering power.
6. The magnifying power of an astronomical telescope in the normal
adjustment position is 100. The distance between the objective and the eye
piece is 101 cm. Calculate the focal length of the objective and the eye piece.
Ans: Given that,
f (^) o f (^) e101cm ......(1)
o
e
f M 100 f
f (^) o 100fe ......(2)
Substituting equation (2) in equation (1),
f (^) e 100f (^) e 101
101f (^) e 101
fe 1cm
Substituting fe in equation (2),
f (^) o 100 1
f (^) o100cm
Thus, the focal length of the eye-piece is 1cm whereas the focal length of the
objective is 100cm.
7. A convex lens made up of refractive index n 1 is kept in a medium of
refractive index n 2. Parallel rays of light are incident on the lens. Complete
the path of rays of light emerging from the convex lens if
a)n 1 n 2
Ans: When n 1 n 2 , the lens behaves as a convex lens.
b)n 1 n 2
Ans: When n 1 n 2 , the lens behaves as a plane plate and thus, no refraction
takes place.
