JEE Advance - Physics (2016 - Paper 2 Offline - No. 9)

A small object is placed 50 cm to the left of a thin convex lens of focal length 30 cm. A convex spherical mirror of radius of curvature 100 cm is placed to the right of the lens at a distance of 50 cm. The mirror is tilted such that the axis of the mirror is at an angle $$\theta$$ = 30$$^\circ$$ to the axis of the lens, as shown in the figure.

JEE Advanced 2016 Paper 2 Offline Physics - Geometrical Optics Question 43 English
If the origin of the coordinate system is taken to be at the centre of the lens, the coordinates (in cm) of the point (x, y) at which the image is formed are
(125/3, 25/$$\sqrt 3 $$)
(50$$-$$25$$\sqrt 3 $$, 25)
(0, 0)
(25, 25$$\sqrt 3 $$)

Explanation

For convex lens, u = $$-$$50 cm, f = 30 cm

$$\therefore$$ $${1 \over v} = {1 \over f} + {1 \over u} = {1 \over {30}} - {1 \over {50}} = {{5 - 3} \over {150}} = {2 \over {150}}$$ or v = 75 cm

JEE Advanced 2016 Paper 2 Offline Physics - Geometrical Optics Question 43 English Explanation 1

Image formed by convex lens acts as a virtual object for mirror.

If we consider that the mirror is not tilted, then for mirror,

$${u_1} = 75 - 50 = 25$$ cm, $${f_1} = {R \over 2} = {{100\,cm} \over 2} = 50$$ cm

$$\therefore$$ $${1 \over {{v_1}}} = {1 \over {{f_1}}} - {1 \over {{u_1}}} = {1 \over {50}} - {1 \over {25}} = {{ - 1} \over {50}}$$ or $${v_1} = 50$$ cm

So, the co-ordinates of final image formed (I2) if the mirror is not tilted are (0 cm, 0 cm)

JEE Advanced 2016 Paper 2 Offline Physics - Geometrical Optics Question 43 English Explanation 2

If mirror is not tilted then this ray starts from the object, refracts through the lens without deviation, incident normally on the mirror and finally comes to the image I2 after reflection from the mirror. If the mirror is tilted by an angle $$\theta$$ then this ray is incident on the mirror at an angle of reflection $$\theta$$ and finally forms a new image at I'2 (see figure). Thus, the axis on which image lies makes an angle 2$$\theta$$ = 60$$^\circ$$ with the principal axis of the lens. Also, the image distance will remain same upto the first order of approximation (image distance will remain exactly same in case of plane mirror). Thus, (x, y) coordinates of the image are

$$x = 50 - 50\cos 60^\circ = 25$$ cm,

$$y = 50\sin 60^\circ = 25\sqrt 3 $$ cm.

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