Question 240: Ray Optics

Question

To find the focal length of a convex mirror, a student records the following data :

ObjectPin
ConvexLens
ConvexMirror
ImagePin

22.2 cm
32.2 cm
45.8 cm
71.2 cm

The focal length of the convex lens is f1 and that of mirror is f2. Then taking index correction to be negligibly small, f1 and f2 are close to :

Accepted Answer

For lens :
u
1
= $$-$$ (32.2 $$-$$ 22.2) cm
= $$-$$ 10 cm
v
1
= (71.2 $$-$$ 32.2) cm
= 39 cm
$$ 	herefore $$   $${1 \over {{f_1}}}$$ = $${1 \over {{v_1}}} - {1 \over {{u_1}}}$$
= $${1 \over {39}}$$ + $${1 \over {10}}$$
= $${{49} \over {390}}$$
$$ 	herefore $$   f
1
= 7.8 cm
For mirror :
R = (71.2 $$-$$ 45.8) cm
= 25.4 cm
$$ 	herefore $$   f
2
= $${R \over 2}$$ =  $${{25.4} \over 2}$$ = 12.7 cm

Answer

f1 = 12.7 cm    f2 = 7.8 cm

Answer

f1 = 7.8 cm    f2 = 12.7 cm

Answer

f1 = 7.8 cm    f2 = 25.4 cm

Answer

f1 = 15.6 cm    f2 = 25.4 cm

Ray Optics

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