Question

Two parallel rays are incident on a thin lens of refractive index 1.5 as shown in figure and radius of curvature of its surfaces are 30 cm & 60 cm . Origin of coordinate axes is at the optical centre of the lens.Then

• A. focal length of lens is 120 cm
• B. light rays get reflected (TIR) from first surface of the lens
• C. y-coordinate of the point at which light rays focus is -4 $\pi$cm
• D. y-coordinate of the point at which light rays focus is 4 $\pi$cm

Answer 1

Answer: (A), (C)

focal length of lens is 120 cm
y-coordinate of the point at which light rays focus is -4 $\pi$cm

We use the lens maker's formula,
$\frac{1}{f}=(\mu -1)(\frac{1}{R_1}-\frac{1}{R_2})$

$\mu =1.5,R_1=-60,R_2=-30$

$f=120$

Since, a lens converges a parallel beam of light on its focus, the y coordinate of the lens is,
$y=-f\theta$

$y=120\times \frac{6\pi }{180}$

Thus, $y=-4\pi$