Assertion:  Thin films such a soap bubble or a thin layer of oil on water show beautiful colours when illuminated by white light.

Reason:  It happens due to the interference of light reflected from upper surface of the thin film.

• If both the assertion and reason are true and reason is a correct explanation of assertion.

• If both assertion and reason are true but assertion is not a correct explanation of the assertion.

• If both the assertion is true but the reason is false.

• If both assertion and reason are false.

For liquid to rise in a capillary tube, the angle of contact should be

• acute

• obtuse

• right

• none of these

153.

Which of the following physical wuantity do not have same dimensions?

• pressure and stress

• tension and surface tension

• strain and angle

• energy and work

The length of a needle floating on water is 2.5 cm. Calculate the added force (due to surface tension) required to pull the needle out of water. (Surface tension of water = 7.2$\times$ 10^-2 N/m²)

• 3.6 $\times$ 10^-3 N

• 3.4 $\times$ 10^-10 N

• 3.0 $\times$ 10^-5 N

• 3.8 $\times$ 10^-11 N

A capillary tube of radius r is immersed in water and water rises in it to a height H. Mass of water in the capillary tube is m. If the capillary of radius 2r is taken and dipped in water, the mass of water that will rise in the capillary tube will be

• m

• 2 m

• m/2

• 4 m

The work done in increasing the size of a soap film from 10 cm x 6 cm to 10 cm x 11 cm is 3 x 10^-4 J. The surface tension of the film is

• 1.5 × 10^-2 N/m

• 3 × 10^-2 N/m

• 6.0 × 10^-2 N/m

• 11 × 10^-2 N/m

The maximum force in addition to the weight required to pull a wire 5.0 cm long from the surface of the water at temperature 20°C, is 728 dyne. The surface tension of water is

• 7.28 N/cm

• 7.28 dyne/cm

• 72.8 dyne/cm

• 7.28 × 10² dyne/cm

If work done in increasing the size of a soap film from 10 cm x 6 cm to 10 cm x 11 cm is 2x10^-4 J, then surface tension is

• 2x10^-2 Nm^-1

• 2x10^-4 Nm^-1

• 2x10^-6 Nm^-1

• 2x10^-8 Nm^-1

At which of the following temperatures, the value of surface tension of water is minimum

• 4º C

• 25º C

• 50º C

• 75º C

Work done in splitting a drop of water of 1 mm radius into 64 droplets is (surface tension of water = 72x 10^-3 J/m²)

• 2.0 x 10^-6 J

• 2.7 x 10^-6 J

• 4 x 10^-6 J

• 5.4 x 10^-6 J