Two charges of equal magnitude '$$q$$' are placed in air at a distance '$$2 r$$' apart and third charge '$$-2 \mathrm{q}$$' is placed at mid point. The potential energy of the system is $$\left(\varepsilon_0=\right.$$ permittivity of free space)

The size of the real image produced by a convex lens of focal length $$F$$ is '$$m$$' times the size of the object. The image distance from the lens is

A double slit experiment is immersed in water of refractive index 1.33. The slit separation is $$1 \mathrm{~mm}$$, distance between slit and screen is $$1.33 \mathrm{~m}$$ The slits are illuminated by a light of wavelength $$6300 \mathop A\limits^o$$. The fringe width is

Potential difference between the points P and Q is nearly