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## SECTION A 1. A beam of light of wavelength 720 nm in air enters water (refractive index = $\frac{4}{3}$). Its wavelength in water will be: * (A) 540 nm * (B) 480 nm * (C) 420 nm * (D) 720 nm 2. A capacitor of capacitance C has reactance X in an ac circuit. If the capac...

## SECTION A 1. A beam of light of wavelength 720 nm in air enters water (refractive index = $\frac{4}{3}$). Its wavelength in water will be: * (A) 540 nm * (B) 480 nm * (C) 420 nm * (D) 720 nm 2. A capacitor of capacitance C has reactance X in an ac circuit. If the capacitance and the frequency of the applied voltage are doubled, the new reactance will become: * (A) 4X * (B) 2X * (C) $\frac{X}{2}$ * (D) $\frac{X}{4}$ 3. Four point charges Q each, are held at the four corners of a square of side l. The amount of work done in bringing a charge Q from infinity to the centre of the square will be: * (A) $\frac{Q^2}{\pi \varepsilon_0 l}$ * (B) $\frac{\sqrt{2}Q^2}{\pi \varepsilon_0 l}$ * (C) $\frac{Q^2}{2\pi \varepsilon_0 l}$ * (D) Zero 4. A metal sheet is inserted between the plates of a parallel plate capacitor of capacitance C. If the sheet partly occupies the space between the plates, the capacitance: * (A) remains C * (B) becomes greater than C * (C) becomes less than C * (D) becomes zero 5. Four resistors, each of resistance R and a key K are connected as shown in the figure. The equivalent resistance between the points A and B when the key K is open, will be: * (A) 4R * (B) $\infty$ * (C) $\frac{R}{4}$ * (D) $\frac{4R}{3}$ The image also contains calculations and diagrams. The calculations appear near the resistors diagram. The calculations include terms like $\frac{kQQ}{\sqrt{2}l} + \frac{kQQ}{\sqrt{2}l}$. The diagram includes a circle with four resistors connected in a square, and key K.