Question

The mirror of length 2l makes 10 revolutions per minute about the axis crossing its mid-point O and perpendicular to the plane of the figure There is a light source in point A and an observer at point B of the circle of radius R drawn around centre O (∠AOB = 90°) What is the proportion `"R"/ℓ` if the observer B first sees the light source when the angle of mirror ψ = 15°?

Options

• `sqrt2`

• `1/sqrt2`

• 2`sqrt2`

• `1/(2sqrt2)`

Answer 1

`sqrt2`

Explanation:

Sine rule

`"R"/(sin45°) = 1/(sin 30°)`;

`"R"/"l" = (sin45°)/(sin 30°)`

= `sqrt2`