A . Objective questions with only one correct answer –
1 . Rays of light from the sun falls on a biconvex lens of focal length f and the circular image of the Sun of radius r is formed on the focal plane of the lens . Then ,
- (a) area of image is π r² and area is directly proportional of f .
- (b) area of image is π r² and area is directly proportional to r².
- (c) intensity of image increases if f is increased.
- (d) if lower half of the lens is covered with black paper , area will become half .
2 . A ray of light travelling in water is incident on its surface open to air . The angle of incidence is θ , which is less than the critical angle . Then , there will be –
- (a) only a reflected ray and no refracted ray .
- (b) only a refracted ray and no reflected ray .
- (c) a reflected ray and a refracted ray and the angle between them would be less than 180° – 2 θ .
- (d) a reflected ray and a refracted ray and the angle between them would be greater than 180° – 2 θ .
3 . A convex lens is in contact with a concave lens . The magnitude of the ratio of their focal lengths is 2/3 . Their equivalent focal length is 3o cm . What are their individual focal lengths ?
- (a) – 15 , 10
- (b) – 10 , 15
- (c) 75 , 50
- (d) – 75 , 50
4 . A container is filled with water ( μ = 1.33 ) upto a height of 33.25 cm . A convex mirror is placed 15 cm above the water level and the image of an object is placed at the bottom is formed 25 cm below the water level . Focal length of the mirror is –
- (a) 15 cm
- (b) 20 cm
- (c) – 18.31 cm
- (d) 10 cm
5 . A short linear object of length b lies along the axis of a concave mirror of focal length f at a distance u from the pole of the mirror . The size of the image is approximately equal to
- (a) b ( u – f / f )½
- (b) b ( b / b – f )½
- (c) b ( u – f / f )
- (d) b ( f / u – f ) ²
6 . A thin prism P1 with angle 4° and made from glass of refractive index 1.54 is combined with another thin prism P2 made from glass of refractive index 1.72 to produce dispersion without deviation . The angle of the prism P2 is –
- (a) 5.33°
- (b) 4°
- (c) 3°
- (d) 2.6°
7 . A ray enters a glass slab at an angle α from air where the refractive index varies along the slab as μ = μ0– k r² , where r is the distance measured along the normal and μ0 , k are positive constants . If the glass slab is sufficiently thick , how far along the normal will the ray go before it is reflected back ?
- (a) μ0 – sin θ / k
- (b) μ0 + sin θ / k
- (c) √ ( μ0 – sin θ / k )
- (d) √ ( μ0+ sin θ / k )
8 . A bird is flying up at an angle sin-1 ( 3/5 ) with the horizontal. A fish in a pond looks at the bird when the bird is vertically above it . The angle at which the bird appears to fly , to the fish is – ( nwater= 4/3 )
- (a) sin-1 ( 3/5 )
- (b) sin-1 ( 4/5 )
- (c) 45°
- (d) sin-1 ( 9/16 )
9 . A glass sphere of radius 5.0 cm and refractive index 1.6 is used to construct a paperweight by slicing through the sphere on a plane that is 2.0 cm from the centre of the sphere . Moreover , this plane is perpendicular to a radius of the sphere that passes through the centre of the circle formed by the intersection of the plane and the sphere . The paperweight is placed on a table and viewed from directly above by an observer who is 8.0 cm from the tabletop. When viewed through the paperweight , how far away does the table top appear to the observer ?
- (a) 7.42 cm
- (b) 6.5 cm
- (c) 3 cm
- (d) 4.44 cm
10 . A fish rises up vertically in a pond with a speed of 4 cm/s and notices a bird which is diving downwards towards the fish. The speed of the bird appears to be 16 cm/s , to the fish . If the refractive index of the water is 4/3 , the actual velocity of the bird is –
- (a) 4 cm/s
- (b) 9 cm/s
- (c) 16 cm/s
- (d) 6.2 cm/s