If we put a cardboard `(say 20 cm xx 20 cm)` between a light source and our eyes, we can't see the light. But when we put the same cardboard between a sound source and our ear, we hear the sound almost clearly, Explain.

Two plane mirrors makes an angle of 120^(@) with each other. The distance between the two images of a point source formed in them is 20 cm . Determine the distance from the light source of the point where the mirrors touch, the light source lies on the bisector of the angle formed by the mirrors.

Figure shows a metal rod PQ resting on the smooth rails AB and positioned between the poles of a permanent magnet. The rails, the rod, and the magnetic field are in three mutual perpendicular directions. A galvanometer G connects the rails through a switch K. Length of the rod =15 cm, B =0.50 T, resistance of the closed loop containing the rod 9.0 m Omega . Assume the field to be uniform. (a) Suppose K is open and the rod is moved with a speed of 12 "cm s"^(-1) in the direction shown. Give the polarity and magnitude of the induced emf. (b) Is there an excess charge built up at the ends of the rods when K is open ? What if K is closed ? (c) With K open and the rod moving uniformly, there is no net force on the electrons in the rod PQ even though they do experience magnetic force due to the motion of the rod. Explain. (d) What is the retarding force on the rod when K is closed ? How much power is required (by an external agent) to keep the rod moving at the same speed (= 12 cm s^(-1) ) when K is closed ? How much power is required when K is open? (f)How much power is dissipated as heat in the closed circuit ? What is the source of this power ? (g) What is the induced emf in the moving rod if the magnetic field is parallel to the rails instead of being perpendicular ?

In the figure shown S is a point monochromatic light source of frequency 6xx10^(14)Hz .M is a concave mirror of radius of curvature 20 cm and L is a thin converging lens of focal length 3.75 cm AB is the principal axis of M and L. Light reflected from the mirror and refracted from the lens in succession reaches the screen. An interference pattern is obtained on the screen by this arrangement. Q. Distance between two coherent sources whichmakes interference pattern on the screen is

Explain why (or how): (a) in a sound wave a displacement node is a pressure antinode and vice versa. (b) bats can ascertain distances, directions, nature, and sizes of the obstacles without any ''eyes''. (c ) a violin note and sitar note may have the same frequency. yet we can distinguish between the two notes. (d) solids can support both longitudinal and transverse waves, but only longitudinal waves can propagate in gases, and (e ) the shape of a pulse gets distorted during propagation in a dispersive medium.

We would like to prepare a scale whose length does not change with temperature. It is proposed to prepare a unit scale of this type whose length remains, say 10 cm. We can use a bimetallic strip made of brass and iron each of different length whose length (both components) would change in such a way that difference between their lengths remain constant. If alpha_("iron")=1.2xx10^(-5)//K and alpha_("brass")=1.8xx10^(-5)//K what should we take as length of each strip ?

When a situation demands our immediate action, it takes some time before we really respond. Reaction time is the time a person takes to observe, think and act. For example, if a person is driving and suddenly a boy appears on the road, then the time elapsed before he slams the brakes of the car is the reaction time. Reaction time depends on complexity of the situation and on an individual. You can measure your reaction time by a simple experiment. Take a ruler and ask your friend to drop it vertically through the gap between your thumb and forefinger . After you catch it, find the distance d travelled by the ruler. In a particular case, d was found to be 21.0 cm. Estimate reaction time.

The area of cross-section of the two vertical arms of a hydraulic press are 1 cm ^(2) and 10 cm^(2) respectively. A force of 10N applied. As shown in the figure, to a tight fitting light piston in the thinner arm balances a force F applied to the corresponding piston in the thicker arm. Assuming that the levels of water in both the arms are the same, we can conclude:-