81.

The albedo of a planet is indicative of its

• transmission coefficient

• reflection coefficient

• absorption coefficient

• adsorption coefficient

Three observers A, B, C measure the speed of light coming from a source to be v_A, v_B and v_C. The observer A moves away from the source at the same speed. The observer B stays stationary. The surrounding space is vacuum everywhere. Then

• v_A > v_B > v_C

• v_A < v_B < v_C

• v_A = v_B = v_C

• v_A = v_B > v_C

Lumen is the unit of

• luminous flux

• luminosity

• illuminance

• quantity of light

The refractive index of water is 1.33. The direction in which a man under water should look to see the setting sun is

• 49° to the horizontal

• 90° with the vertical

• 49° to the vertical

• along the horizontal

A fish, looking up through the water sees the outside world contained in a circular horizon. If the refractive index of water is 4/3 and the fish is 12 cm below the surface of water, the radius of the circle in centimetre is

• $\frac{12 \times 3}{\sqrt{5}}$

• $12 \times 3 \times \sqrt{5}$

• $\frac{12 \times 3}{\sqrt{7}}$

• $12 \times 3 \times \sqrt{17}$

A given ray of light suffers minimum deviation in an equilateral prism P. Additional prisms Q and R of identical

shape and material are now added to P, as shown in the figure. The ray will suffer

• same deviation

• greater deviation

• total internal reflection

• no deviation

The aperture of the objective lens of a telescope is made large so as to

• increase the resolving power of the telescope

• increase the magnifying power of the telescope

• to focus on distant objects

• make image aberrationless

A lamp hanging 4 metres above the table is lowered by 1 metre. The illumination on the table

• decreased by 25%

• increased by 25%

• decreased by 66.7%

• increased by 77.7%

If a ray of light in a denser medium enters into a rarer medium at an angle of incidence i, the angle of reflection and refraction are respectively r and r'. If the reflected and refracted rays are at right angles to each other, the critical angle for the given pair of media is

• sin^-1 (tan r')

• sin^-1 (tan r)

• tan^-1 (sin i)

• cot (tan i)

A thread is tied slightly loose to a wire frame as in figure and the frame is dipped into a soap solution and taken out. The frame is completely covered with the film. When the portion A is punctured with a pin, the thread

• becomes concave towards A

• becomes convex towards A

• either (a) or (b) depending on the size of A with respect to B

• remains in the initial position