Two wires forming the open ends of a circuit consisting of a bulb and an electric cell are dipped in the following liquids one by one and it is noted whether the bulb in the circuit glows or not. Following are the observations :

An object is placed 15 cm in front of a convex lens of focal length 25 cm. The image distance will be

A short object of length L is placed along the principal axis of a concave mirror slightly away from the focus of the mirror (u ≈ f). What will be the length of the image? (symbols carry their usual meanings)

An object X is placed in air in front of a convex refracting surface. Following four sketches are drawn for depicting the situation (symbols carry their usual meanings).

Which of these could be correct sketches?
a)

b) 

c)

d)

When white light passes through the prism, the light colours bend differently. Arrange the following colours from the one that bends the least to the one that bends the most :
I. Blue
II. Green
III. Yellow
IV. Orange
Select the correct answer from the codes given below :

The optical phenomena, twinkling of stars, is due to :

The sky appears blue due to :