The radius of the convex surface of planoconvex lens is 20 cm and the refractive index of the material of the lens is 1.5. The focal length is

• 30cm

• 50 cm

• 30 cm

• 40 cm

An experiment is performed to find the refractive index of glass using a travelling microscope. In this experiment distance are measured by 

Two lenses of power -15D and +5D are in contact with each other. The focal length of the combination is

The angle of incidence at which reflected light totally polarized for reflection from air to glass (refractive index n), is

If θ1 and θ2 be the apparent angles of dip observed in two vertical planes at right angles to each other, then the true angle of dip θ is given by

A beam of light from a source L is incident normally on a plane mirror fixed at a certain distance x from the source. The beam is reflected back as a spot on a scale placed just above the source L. When the mirror is rotated through a small angle θ, the spot of the light is found to move through a distance y on the scale. The angle θ is given by

A thin prism having refracting angle 10° is made of glass of refractive index 1.42. This prism is combined with another thin prism of a glass of refractive index 1.7. This combination produces dispersion without deviation. The refracting angle of second prism should be