Page 95 - Physics - XII

Q3. A converging lens is used to form an image on a screen. When the upper half of the lens is
covered by an opaque screen
(a) half the image will disappear. (b) complete image will disappear.
(c) intensity of image will decreases. (d) intensity of image will increase.

Q4. A convex lens is dipped in a liquid whose refractive index is equal to the refractive index of the
lens. Then its focal length will
(a) become zero. (b) become infi nite.
(c) become small, but non-zero. (d) remain unchanged.
Q5. A concave mirror of focal length f is placed at a distance d from a convex lens of focal length
1
f . A beam of light coming from infi nity and falling on this convex lens – concave mirror
2
combination returns to infi nity. The distance d must equal
(a) f + f (b) –f + f (c) 2f + f (d) –2f + f 2
2
1
2
1
1
2
1
Q6. Two lenses of focal lengths 20 cm and –40 cm are held in contact. The image of an object at
infi nity will be formed by the combination at
(a) 10 cm (b) 20 cm (c) 40 cm (d) infi nity
Q7. Focal length of a convex lens of refractive index 1.5 is 2 cm. Focal length of lens when
immersed in a liquid of refractive index of 1.25 will be
(a) 10 cm (b) 7.5 cm (c) 5 cm (d) 2.5 cm
Q8. A student measures the focal length of a convex lens by putting an object pin at a distance ‘u’
from the lens and measuring the distance ‘v’ of the image pin. The graph between ‘u’ and ‘v’
plotted by the student should look like

(a) (b)
v(cm) v(cm)
u(cm) u(cm)

(c) (d)

v(cm) v(cm)
u(cm) u(cm)

Q9. A tank is fi lled with water to a height of 12.5 cm. The apparent depth of a needle lying at the
bottom of the tank is measured by a microscope to be 9.4 cm. If water is replaced by a liquid
of refractive index 1.63 up to the same height, by what distance would the microscope have to
be moved to focus on the needle again?

(a) 1.00 cm (b) 2.37 cm (c) 1.73 cm (d) 3.93 cm
Q10. A ray of light passes through an equilateral prism (refractive index 1.5) such that angle of
incidence is equal to angle of emergence and the latter is equal to 3/4 of the angle of prism.
th
The angle of deviation is
(a) 60° (b) 30° (c) 45° (d) 120°

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