A light incident normally on a plane mirror attached to a galvanometer coil retraces backward as shown in the figure. A current in the coil produces a deflection of \(3.5^\circ\) of the mirror. What is the displacement of the reflected spot of light on a screen placed \(1.5~\text m\) away?
(given \(\tan (7)^\circ =0.1228\))

1. \(15.0~\text{cm}\)
2. \(18.4~\text{cm}\)
3. \(21.2~\text{cm}\)
4. \(12.5~\text{cm}\)

Monochromatic light of wavelength \(589\) nm is incident from the air on a water surface. What is the wavelength of reflected light? The refractive index of water is \(1.33\).
1. \(500\) nm
2. \(545\) nm
3. \(485\) nm
4. \(589\) nm

Light of wavelength \(5000~\mathring{A}\) falls on a plane reflecting surface. For what angle of incidence is the reflected ray normal to the incident ray?
1. \(37^{\circ}\)
2. \(45^{\circ}\)
3. \(90^{\circ}\)
4. \(0^{\circ}\)