A lens having focal length f and aperture of diameter d forms an image of intensity I. Aperture of diameter $${d \over 2}$$ in central region of lens is covered by a black paper. Focal length of lens and intensity of image now will be respectively
A
$$f\,$$ and $${I \over 4}$$
B
$${{3f} \over 4}$$ and $${I \over 2}$$
C
$$f$$ and $${{3I} \over 4}$$
D
$${f \over 2}$$ and $${I \over 2}$$
2
AIPMT 2008
MCQ (Single Correct Answer)
+4
-1
A boy is trying to start a fire by focusing sunlight on a piece of paper using an equiconvex lens of focal length 10 cm. The diameter of the sun is 1.39 $$ \times $$ 109 m and its mean distance from the earth is 1.5 $$ \times $$ 1011 m. What is the diameter of the sun's image on the paper ?
A
6.5 $$ \times $$10$$-$$5 m
B
12.4 $$ \times $$10$$-$$4 m
C
9.2 $$ \times $$10$$-$$4 m
D
6.5 $$ \times $$10$$-$$4 m
3
AIPMT 2008
MCQ (Single Correct Answer)
+4
-1
Two thin lenses of focal lengths f1 and f2 are in contact and coaxial. The power of the combination is
A
$${{{f_1} + {f_2}} \over 2}$$
B
$${{{f_1} + {f_2}} \over {{f_1}{f_2}}}$$
C
$$\sqrt {{{{f_1}} \over {{f_2}}}} $$
D
$$\sqrt {{{{f_2}} \over {{f_1}}}} $$
4
AIPMT 2007
MCQ (Single Correct Answer)
+4
-1
A small coin is resting on the bottom of a beaker filled with liquid . A ray of light from the coin travels upto the surface of the liquid and moves along its surface. How fast is the light travelling in the liquid ?
