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

• -20 cm

• -10 cm

• +20 cm

• +10 cm

Draw a ray diagram to show image formation when the concave mirror produces a real, inverted and magnified image of the object.

Explain two advantages of a reflecting telescope over a refracting telescope.

In an optics experiment, with the position of the object fixed, a student varies the position of a convex lens and for each position, the screen is adjusted to get a clear image of the object. A graph between the object distance u and the image distance v, from the lens, is plotted using the same scale for the two axes. A straight line passing through the origin and making an angle of 45^o with the x-axis meets the experimental curve at P. The coordinates of P will be

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